RAB/83/016 RER/83/001 Field Document

February 1989

THE PARALIC REALM

GEOLOGICAL, BIOLOGICAL AND ECONOMIC EXPRESSIONS OF CONFINEMENT

Report prepared for the Mediterranean Regional Aquaculture Project

by

O. Guelorget and J.-P. Perthuisot

FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS Rome, 1989

This report was prepared during the course of the project identified on the title page. The conclusions and recommendations given in the report are those considered appropriate at the time of its preparation. They may be modified in the light of further knowledge gained at subsequent stages of the project.

The designations employed and the presentation of the material in this document do not imply the expression of any opinion whatsoever on the part of the United Nations or the Food and Agriculture Organization of the United Nations concerning the legal or constitutional status of any country, territory or sea area, or concerning the delimitation of frontiers.

ABSTRACT

The paralic aquatic milieux which are situated between the marine and continental domains are extremely different as far as their size, morphology and genesis are concerned. Furthermore, the regional climatic and hydrographic conditions, along with the local hydrological patterns, induce a great variety and variability of the physical and chemical parameters and of the sedimentary deposits.

On the contrary, the biological populations are characterized by species strictly bound to that kind of milieu; their common qualitative and quantitative zonal organization is independent of salinity and/or salinity gradients, and they are relatively stable in spite of variations of the milieu. These original biological features allow the paralic milieux to be considered together as an autonomous ecological domain: it is proposed to call it the "paralic domain".

The parameter which appears largely to control the distribution of organisms and the features of living populations may be described as the time of renewal of the elements of marine origin at any given point, and is referred to as "confinement" (by comparison with the sea). A confinement scale is proposed from biological data, which essentially concerns the seaward part of the paralic domain, where thalassoid species still persist (Near paralic). Further from the sea, the Far paralic is characterized by the appearance of freshwater, or on the contrary, evaporitic associations, and changes gradually towards the continental domain.

In the light of these new ideas, it appears that, far from being marginal, the paralic domain has played an important role in the history of the biosphere and lithosphere of our planet.

Lastly, given the high organic productivity and original sedimentological features pf the paralic domain, it appears that paralic basins are a considerable source of economic richness. Confinement may be in many cases the key to a rational exploitation of their economic potentialities.

FOREWORD

The work of synthesis presented here is a first stage in the combined studies of two naturalists, a biologist and a geologist working together on paralic milieux over several years. This, study has led to a proposal of a new global concept of these very often little known milieux which may perhaps seem farfetched and over-ambitious. An attempt has been made to clarify all the geological, biological and economic implications, even the most audacious. In the process, many disciplines and specialities outside the authors' usual field of study have been touched upon, and it is possible that there may be errors. Further, this work is not intended to be exhaustive and is essentially the result of experience based - but not exclusively on the study of Mediterranean, lagoons. Thus other bio-geographical fields and certain milieux such as estuaries and delta channels will be mentioned in passing, but the authors' limited experience of these environments, and bibliographic data, suggest that the pattern proposed here is applicable to them. The aim is not to impose any new dogma, but on the contrary, to stimulate new trends of thought in each speciality so that better scientific knowledge of the Paralic Domain may be acquired, and its economic value better developed.

The fortuitous meeting in Corsica in the autumn of 1978 would have been without result, and the Paralic Domain Study Group would no doubt never have been created without the financial and practical help of the Compagnie Française des Pétroles, and the presence of André Maurin to whom grateful thanks is extended at the beginning of this text. He is associated with the very "theory" of the Paralic Domain along with Guy François Frisoni whose amiable competence in the field of phytoplankton was of considerable help.

It is not possible to mention here the names of the many people who helped in the various studies which constitute the basis of this work and the authors hereby sincerely thank them all. But special appreciation is expressed to Jacqueline Gaudin and the team at the Antenne Graphique du CNRS at the Laboratory of Geology who contributed to the organization of this work, and to Professor André Jauzein who has kindly contributed the preface to this sixteenth volume of the collection which he initiated in 1967 and which will no doubt be the last published before his departure for a well-deserved retirement.

N.B. Certain commentaries or digressions which would have been inappropriate within the body of the text have been included in Appendix 3.

TABLE OF CONTENTS

PageABSTRACT iii

FOREWORD v

INTRODUCTION 1

1. MORPHOLOGICAL, GEOCHEMICAL AND SEDIMENT DIVERSITY OF PARALIC MILIEUX 2

1.1 Morphological and genetic diversity of paralic basins 2 1.2 Hydrochemical gradient 3

1.2.1 Mediterranean examples 3 1.2.2 Examples outside the Mediterranean 4 1.2.3 Extreme examples 6 1.2.4 Conclusion 7

1.3 The sedimentological diversity of paralic milieux 8 1.4 Conclusion 9

2. THE BIOLOGICAL UNITY OF PARALIC MILIEUX 9

2.1 The existence of strictly paralic species 9 2.2 Biological zoning of paralic milieux 11

2.2.1 The distribution of benthic species 11 2.2.2 Biological gradients 13

2.3 The influence of salinity upon marine species 13 2.4 The stability of paralic communities 14 2.5 Conclusion 15

3. CONFINEMENT: THE FUNDAMENTAL PARAMETER OF THE PARALIC DOMAIN 16

3.1 The reasons for a mistaken approach 16 3.2 The search for the fundamental parameter of the paralic domain 17 3.3 General organization of the paralic domain 20 3.4 The establishment of a confinement scale 21 3.5 The position of several Mediterranean basins on the confinement

scale 23

3.5.1 Bahiret el Biban 23 3.5.2 Mar Chica of Nador 23 3.5.3 Lagoon of Logarou 24 3.5.4 Lagoons of Tsoukalio and Rodia 24 3.5.5 Lagoons of Diana and Urbino 24 3.5.6 Lagoon of Biguglia 24 3.5.7 Lake Melah 24 3.5.8 Palavas lagoons 25 3.5.9 Bermuda Triangle (Santa Pola, Spain) 25

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3.6 Adjustment of other biological compartments into the confinement scale 25

3.6.1 The phytoplanktonic communities of paralic milieux 25 3.6.2 The ichthyofauna 27

3.7 Quantitative biological gradient 31

3.7.1 Phytoplanktonic biomass and biomass of the benthic macrofauna 31 3.7.2 Production of soft bottom pelecypoda 32

3.8 Qualitative and quantitative biological zoning of the paralic macrobenthos on hard substrate

35

3.8.1 Biological zoning 35 3.8.2 Biological gradients 36

3.9 Conclusions regarding the confinement parameter 37

4. HYPOTHESES CONCERNING THE INFLUENCE OF CONFINEMENT ON PARALIC COMMUNITIES

37

4.1 Confinement in time, stabilization of the gradient, the role of depth 37 4.2 Intracontinental "paralic" basins 38 4.3 Importance of trace elements in the biological expression of confinement 39 4.4 Conclusions regarding the influence of confinement 40

5. THE SCIENTIFIC IMPORTANCE OF THE PARALIC DOMAIN 41

5.1 From a biological and paleontological point of view 42 5.2 From a geological point of view 43

6. THE ECONOMIC IMPORTANCE OF THE PARALIC DOMAIN 44

6.1 Mineral production 44 6.2 Food production 45

6.2.1 Gathering of "sea food" 45 6.2.2 Halieutic production 45 6.2.3 Aquaculture 46

6.3 Future prospects 46

6.3.1 Biomass 46 6.3.2 A strategy for the development of the paralic domain 47

7. CONCLUSIONS 47

BIBLIOGRAPHY 49

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LIST OF TABLES

1. Taxonomic resources of the phytoplankton in various Mediterranean paralic basins 53

2. Number of species and diversity of the ichthyological populations of the lagoons of the eastern plain of Corsica 53

3. "Sedentary" species: percentage of captures 54 4. Quantity of molluscs in the Etang du Prévost 54 5. Average values of density and biomass, annual production of the main

Pelecypod Molluscs at different stations in the Etang du Prévost 55 6. Some heavy metal concentrations measured in the tissues of marine and/or

paralic species 56 7. Comparison of the number of species in the Caspian fauna and the

Mediterranean fauna 57 8. Monthly variations of the total lagunar algal biomass. Etang du Prévost 58 9. Monthly variations of the lagunar algal biomass for the four principal

types. Etang du Prévost 58 10. Present-day stratigraphic classification of the Paleogene formations in the

Paris Basin (Pomerol, 1973), and equivalences with the classification of Furon and Soyer (1947) 59

11. Composition of total thalossoid fauna for each of the Paleogene layers of the Paris Basin 59

LIST OF FIGURES

CHART OF PARALIC SITES STUDIED BY THE AUTHORS 61

1. Map of isohalines in the Etang d'Urbino 62 2. Salinity levels (in ‰) in the Etang du Prévost 63 3. Total concentrations (in g/l) in the surface waters of the Bahiret el Biban 64 4. Salinity levels (‰) in the Etang de Biguglia 65 5. Salinity levels of the Dybsø Fjord 66 6. Macrophytobenthic associations in the Dybsø Fjord 66 7. Salinity levels (‰) in the Kysing Fjord 67 8. Mangrove around the Grand Cul de Sac Marin 67 9. Two paralic basins in the Guadeloupe Mangrove and their surface water

concentration levels 68 10. Map of Cl concentration in the waters of the Khour el Aadid 68 11. The Bocana de Virrila and its total concentration levels 69 12. The salt marsh of Salin-de-Giraud 69 13. The Baltic Sea and its salinity levels 70

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14. Plant associations in the salt marsh of Salin-de-Giraud 71 15. Quantitative variations of benthic macrofauna in the Etang de Prévost

and the Bahiret el Biban 72 16. Mean salinities relating to decrease of molluscan species in the North Sea-

Baltic transition area and difference of relative diversity of fossilizable fauna along Texas coast 73

17. Diagram of the geochemical and biological organization of the paralic domain 74

18. Diagram of the dominant sedimentological features of the paralic domain 76 19. A diagrammatic representation of the biological zoning in the pattern of

the Mediterranean paralic ecosystem 77 20. Diagrammatic map of confinement zones in the Bahiret el Biban 78 21. Diagrammatic map of confinement zones in the Nador lagoon 78 22. Biological zoning in the lagoons of the Louros delta 79 23. Biological zoning in the lagoons of the eastern plain of southern Corsica 80 24. Biological zoning in the Etang de Biguglia 81 25. Biological zoning in "Lake" Melah 81 26. Biological zoning in the Palavas lagoons 82 27. Biological zoning in the Bermuda Triangle (Santa Pola, Spain) 83 28. Relative quantities of "migrant" and "sedentary" species in the ichthyofauna of

the lagoons of Diana and Urbino 84 29. A A diagrammatic representation of the biological zoning defining the

scale of confinement in the model of the Mediterranean paralic ecosystem 85 B Variations showing the phytoplanktonic and benthic biomass in relation

to the confinement scale 85 30. Annual malacological production of the Etang du Prévost 86 31. Seasonal evolution of the malacological production of the Etang du Prévost 87 32. Situation of the stations and levels of total malacological production

in the different parts of the Etang du Prévost 88 33. The biological gradients of the hard substrate macrobenthofauna in the

Etang du Prévost 89 34. Comparative production of some terrestrial and aquatic systems 90 35. The potentialities of the Near paralic in the genesis of hydrocarbons

in relation to confinement 91 36. A comparison between demersal halieutic production and lagunar production

along the Mediterranean coastline 92 37. Map of aragonite content of the superficial sediments in the agoon of Logarou 93 38. Situation of observation stations in the lagoons of Belle Plaine and

Manche à Eau 94 39. Abiotic parameters 94 40. Analysis of abiotic data into principal components 95 41. Multidimensional positioning of the stations according to similarities

of benthic populations 95 42. The Caspian Sea, bathymetric map 96 43. Diagrammatic map of surface currents in the Caspian Sea 97 44. Salinity levels in the Caspian Sea 97

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45. Benthic biomass distribution in the Caspian Sea 98 46. Distribution of Nereis succinea biomass in the northern basin of the Caspian

Sea 98 47. Diagrammatic map of confinement zones in the Caspian Sea 99 48. Diagrammatic section of the Caspian Sea and zoning linked to "bathymetric

confinement" 100 49. Map of total algal biomass distribution in the Etang du Prévost 101 50. Monthly variations of total and specific algal biomass in the central zone of the

Etang du Prévost 102 51. Biological zoning of the intertidal zone on rocky surface at Port Aransas 103 52. Extent of the different layers of the Paleogene in the Paris Basin 104 53. Evolution of confinement in the Paris Basin during the Paleogene 105 54. Situation of principal Stampian fossil deposits in the Paris Basin 106 55. An attempt to reconstitute the confinement zones of the Lower Stampian

in the Paris Basin 107 56. An attempt to reconstitute the confinement zones of the Upper Stampian

in the Paris Basin 107 57. Diagrammatic paleogeographical map of the Stampian in N.W.France 107 58. An attempt at paleogeographical reconstitution of the Stampian in

N.W.France according to paleofauna 108 59. The Baltic Sea and its principal basins 108 60. Density of benthic macrofauna in the Baltic 109 61. Biomass of benthic macrofauna in the Baltic 110 62. Diversity of benthic macrofauna species in the Baltic 111 63. Increase in percentage of the total number of individuals with the increase in

the number of taxa, in different parts of the Baltic 112 64. Distribution of Ophiura albida and Asterias rubens in the Kattegat and at the

entrance of the Baltic Sea 112 65. Map of confinement zones in the Baltic Sea 113 66. Currents and principal tributaries of the Nador lagoon 114 67. Bathymetry and granulometry of the sediments in the Nador lagoon 114 68. Salinities and localization of the zone of maximal phytoplanktonic biomass 114 69. Confinement zones in the Nador lagoon 115 70. "Positive" socio-economic aspects in the immediate surroundings of the Nador

lagoon 115 71. Water and energy sources, means of access, around the Nador lagoon 115 72. "Negative" socio-economic aspects in the immediate surroundings of the

Nador lagoon 116 73. Proposition for short-term aquacultural development for the Nador lagoon 116 74. Longer-term development of aquacultural activities in the Nador lagoon, after

modification of communication with the sea 116

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Appendix 1: CONVERSION TABLE OF DIFFERENT PARAMETERS 117

Appendix 2: LIST OF THE PRINCIPAL COMMON MACROBENTHIC SPECIES IN THE MEDITERRANEAN PARALIC DOMAIN 118

Appendix 3: COMMENTS AND DIGRESSIONS 121

Appendix 4: USE OF THE NOTION OF CONFINEMENT IN AN ATTEMPT TO RECONSTITUTE PALEOMILIEUX IN THE PALEOGENE OF THE PARIS BASIN 134

Appendix 5: THE BALTIC SEA 139

Appendix 6: USE OF THE NOTION OF CONFINEMENT FOR AN AQUACULTURAL DEVELOPMENT SCHEME IN A PARALIC MILIEU EXAMPLE OF THE NADOR LAGOON (MOROCCO) 142

Appendix 7: ONE LAST WORD ON GEOLOGY 144

INTRODUCTION

It is common practice to divide hydrobiosphere and hydrosphere into a maritime domain and a continental domain each possessing its own features, its original populations, its sedimentary processes and its distinct geochemical characteristics.

Bordering these two great domains, there exist a number of milieux apparently very different from one another described by an extensive terminology: saltwater lake, lagoon, brackish lagoon, bay (Arcachon), sea marsh, mangrove swamp, salt marsh, polder, estuary, ria, fjord, aba, delta, tidal water splash, coastal sebkha, melaha, etc. These milieux, very rich from an economic point of view, have been and still are the object of many applied studies, either with a view toward their exploitation or their development. For example (Amanieu, 1973), for the year 1971, the production of small-scale fishing alone in the Etangs de Thau and Berre is superior (33 million francs) to the output of the combined French coastal trawlers (30 million francs). Moreover, the value of the conchological production of the Etang de Thau alone (15 million francs) is almost equal so that of the production of the entire sardine fleet (18 million francs).

The generic terms used to qualify these milieux are as imprecise as they are unsatisfactory and show the perplexity of the authors faced with the "mixed" aspect of these milieux and their diversified geochemistry: one can find in literature (biological as well as geological) terms such as: transition milieux, mixing zones, intermediary milieux, variable salinity milieux, coastal fringing zones, brackish milieux. Here the term paralic 1/ is used, created by Nauman in 1854 to qualify the coal beds in which a certain marine influence can be detected; the term was used by Perthuisot in 1975 to qualify certain evaporitic basins whose salts are essentially of marine origin. In this context, the adjective "paralic" applied to an area, a basin or a milieu simply means that it possesses a certain relationship with the sea.

One can wonder whether the morphological variety and geochemical diversity of paralic milieux found today in nature entail an equivalent diversity in their populations and their biological dynamics.

1/ From the Greek "para" near, "halos" salt (and, by extension, the sea)

1. MORPHOLOGICAL, GEOCHEMICAL AND SEDIMENTARY DIVERSITY OF PARALIC MILIEUX

1.1 MORPHOLOGICAL AND GENETIC DIVERSITY OF PARALIC BASINS

The catalogue of natural paralic basins is not definite or homogeneous, and varies from one author to another. Moreover, the shape and size of the basin, and those of the channel communicating with the sea, depend on various factors which all belong to a sometimes complex geological history. It is not aimed to give yet another classification but rather to single out the main types.

The "estuary" type of lakes, situated more or less perpendicularly to the coast correspond to portions of valleys (either recent or fossil) flooded by the sea. To this type belong rias, fjords, deep creeks, etc. In some cases the river mouths are cleared by fluviatile currents and/or tides, while in other cases the mouth may be partially or totally obstructed by a barrier of sedimentary or other origin.

The lakes of the "lagunar" (sensu lato) type are generally more isodiametrical (circular in shape), or stretch parallel to the coast. Among them can be mentioned:

"Lagoons" (sensu stricto) which correspond to the isolation of certain portions of the maritime domain by one or several recent soft coastal sounds. There usually remains a communication link with the sea, more or less permanent thanks to one or several "graus" (more or less fixed, sometimes maintained by man). Generally, in this type of basin the water depth is low in relation to the surface area.

In arid countries intermittent stretches of water may alternate with "coastal" sebkhas (Perthuisot, 1975).

Delta areas often have lagoons whose formation is aided by a massive inflow of detrital debris, the progradation of sedimentary bodies and coastal transit. In this category are situated brackish lagoons (for example in Palavas), delta lagoons, coral lagoons, etc.

"Bahira" (small sea in Arabic), diversely shaped, correspond to land-locked continental depressions of complex and various origin (river, wind, tectonic, etc.) developed or completed during the Quaternary era and flooded by the sea during the Holocene transgression. They communicate with the sea by permanent passages. The water depth varies but may be considerable in relation to the surface area.

To this type belong the Bahiret el Biban (Medhioub, 1979) in the southeast of Tunisia,the Etang de Diana in Corsica, etc.

Recent tectonic movements (subsidence and/or uplift) occurring in the vicinity of the waterline may lead to the isolation of a paralic stretch of water: in this case alone the term "tectonic lagoon" can be used. Examples are few; however one could mention the saltwater lake of Guemsah, Gulf of Suez (Ibrahim et al., 1982).

It is obvious that the above-mentioned types represent only the different extremes of typology. There exist a great number of intermediary types: thus the lagoon of Nador in Morocco or the Etang d'Urbino in Corsica fall between the "lagoon" type and the "bahira" type.

Lastly, there exist artificial stretches of paralic water corresponding to the development of natural paralic zones (salt marshes for example) or harbour structures.

1.2 HYDROCHEMICAL GRADIENT

1.2.1 Mediterranean Examples

(a) Three standard examples: the Etang du Prevost (France), the Etang d'Urbino (Corsica) and the Bahiret el Biban (Tunisia)

The hydrochemical diversity of the paralic milieux appears when considering only the examples taken around the Mediterranean: the Etang du Prévost on the Languedoc coast (Guelorget and Michel, 1976), the Etang d'Urbino in western Corsica (Frisoni et al. 1982) and the Bahiret el Biban in southeast Tunisia (Medhioub, 1979; Medhioub and Perthuisot, 1977, 1981).

These three basins of very different sizes correspond to different morphologies and origins. The Etang du Prévost (3.8 km2 , 1 m depth) is a lagoon in the strict sense of the word, i.e., a stretch of water progressively separated from the sea by a sandbar and still communicating with it through 2a "grau" (communication link). The Bahiret el Biban (230 km2, 6.5 m depth) is a Wiirmian continental depression flooded by Flandrian transgression; it communicates with the Gulf of Gabes through a series of channels the larger of which is 2a former cluse (transverse valley). The Etang d'Urbino (7.9 km2, 9 m depth) is a continental depression of a complex origin covered by and then separated from the sea by a sandbar intermittently sectioned by a grau.

If the Etang d'Urbino maintains a salinity close to that of the sea (Fig. 1), the other two basins are very different, one tending towards hypohaline (Fig. 2), the other hyperhaline (Fig. 3). In both cases, there exists a salinity gradient, variable according to the seasons, progressing from the channel communicating with the sea to the continental margins of the basins. The salinity gradient is negative in the Etang du Prévost and positive in the Bahiret el Biban. Naturally these differences in direction and value of the salinity gradients are related to the intensity of the exchange with the sea on the one hand, and on the other, with the total quantity of freshwater received by each basin (direct rainfall, continental inflows as opposed to evaporation). Moreover, the outermost edges are subject to local variations: inflows from tributaries, exchanges with supratidal waters.

(b) Etang de Biguglia (Fig. 4)

This lagoon on the western Corsican plain lies parallel to the sea, from which it is separated by a thin sandbar. The communication link with the sea is situated to the north at the end of a long narrow channel. To the south this lagoon communicates with the mouth of a small coastal river, the Golo, through a system of canals. It has a surface area of 1 450 ha, and a depth of about 1 m. Its average salinity varies between 5 and 26 ‰; however, values below 1 ‰ may be observed in the southern zone which is subject to continental inflows. Near-normal values are to be found in the northern part of the basin where the influence of the sea is constant (Burelli et al., 1979).

(c) Others

The study of other Mediterranean basins, unnecessary to describe here in detail, confirms the generality of salinity gradients in paralic milieux. The following can be mentioned:

− Lagoon of Nador (Mar Chica) (Frisoni et al., 1982)

− Bermuda Triangle (Spain) (Perthuisot et al., 1983)

− Lagoon of the Louros Delta: Tsoukalio, Rodia, Logarou (Greece) (Frisoni et al., 1982)

− Lake Melah (Algeria) (Guelorget et al., in preparation)

1.2.2 Examples Outside the Mediterranean

Several further examples of paralic milieux, in very different climatic contexts, are discussed below:

(a) Danish fjords

The lagoon of the Dybsø Fjord was described by Muus (1967) in an outstanding study dealing with the entire system of Danish lagoons and estuaries. The Dybsø Fjord is a lagoon opening onto the Baltic Sea in the south of the Danish Sjaelland; it has a surface area of about 1 700 ha and an average depth of 1.50 m. The fjord communicates with the sea through a narrow channel, 3-4 m deep, opening to the west onto the lagoon. In spite of several small tributaries, there is no salinity gradient, with values remaining relatively stable around an average of 10 ‰ (Figs. 5 and 6)

The Kysing Fjord is an estuary of 186 ha situated on the east coast of Jutland. This fjord, also studied by Muus (1967), constitutes a basin lying eastwest, with an average depth of 0.60 m; however, along the western shore, there is a channel whose depth in places reaches 2 m.

The channel linking the Kysing Fjord with the sea is narrow and winding with an average depth of around 4.50 m.

The bed of the fjord may be divided into three distinct zones:

− A sandy zone, devoid of vegetation: this zone regularly dries out during low tides;

− Further west, a central sand-mud zone settled by Zostera and Ruppia water plants;

− Finally, at the western extremity of the fjord, a muddy zone where in summer Ulva lactuca covers all that part of the fjord.

The waters of the River Odder Å flow into the Kysing Fjord at its western extremity. This freshwater supply results in a salinity gradient varying from 0 ‰ at the mouth of the river to 24 ‰ at the channel. However, the major part of the fjord has a salinity of around 19-21 ‰ (Fig. 7).

(b) Mangrove swamps in Guadeloupe

The "Manche à Eau" and "Belle Plaine" are two mangrove swamps in Guadeloupe (Gaujous, 1981). These two paralic basins, situated at the end of the "Grand Cul de Sac Marin" which separates Basse Terre and Grande Terre, are where mangroves reach their maximum development (Fig. 8). The "Manehe à Eau" has a surface area of 26 ha. Its depth, between 1.50 and 2 m, is uniform for the whole stretch of water. The lagoon bed is bare and consists of a sandy mud extremely rich in organic matter. The lagoon communicates with the sea by a system of branching channels

flowing into the Rivière Salée which in its turn flows into the lagoon. The freshwater comes all round the lagoon through diffuse trickling. This hydrological context results in a salinity gradient averaging between 30 and 25 ‰ (Fig. 9).

The lagoon of "Belle Plaine" has a surface area of 18 ha. It is much more open to the sea than the "Manehe à Eau"; communicating directly with the lagoon through two short channels. Unlike the "Manche à Eau", the freshwater supplies are concentrated in the "Belle Plaine" channel which drains the mangrove zones and beyond for more than 3 km. This sizable continental water supply flows in at a central point on the eastern shore. Consequently, the wide opening of the lagoon to the sea results in a uniform salinity (30 ‰) almost throughout the basin, except at the very mouth of the "Belle Plaine" channel, where the salinity falls drastically to levels below 10 ‰ (Fig. 9).

The average depth of this lagoon is between 1.50 and 2 m and the sand/mud bed is devoid of vegetation.

(c) A lagoon in the Persian Gulf

The Khour el Aadid with a surface area of around 100 km2 is

situated between the Emirates of Qatar and Abu Dhabi. It is a Flandrian "bahira" progressively reduced by a sandy spit which leaves only a long narrow channel between the Persian Gulf and the first of a succession of three basins. The rarity of rain results in a salinity gradient with levels of salts reaching over 110 g/l in the distal basin which, moreover, is totally covered by a thick layer of Cyanophyceae (Perthuisot and Jauzein, 1978) (Fig. 10).

1.2.3 Extreme Examples

When paralic basins stretch inland far from the open sea, or when climatic conditions become extreme, the tendencies described above are intensified.

(a) Bocana de Virrila (Fig. 11)

The Bocana de Virrila in Peru is an interesting case, although little known. It is a relict estuary in a particularly arid region, reaching inland over a distance of 20 km with a maximum width of 2 km (Morris and Dickey, 1957). The water concentration increases from the mouth up to the continental extremity of the stretch of water and it is possible to define two milieux separated by shallows. In the part closest to the sea, there is a "penesaline" environment, and in the distal part, which is very shallow a "saline" or evaporitic environment where gypsum then halite are precipitated: here, concentrations reach over 400 g/l. Moreover, the lower layers of water are more concentrated than the surface waters, which reveal a certain stratification 1/.

At present the Bocana de Virrila seems to be the only paralic stretch of water in direct communication with the sea where halite can precipitate (Perthuisot, 1980).

(b) Example of salt-marsh : Salin-de-Giraud (Camargue )

Artificial salt marshes present an organization similar to that of the Bocana de Virrila (Fig. 12) (Perthuisot, 1983) but here, the salinity gradient is artificially established by specialists who adjust the water transfers from one condenser to the other according to weather conditions.

(c) The Baltic Sea

Finally, an example totally opposed to those already studied should be mentioned: that of the Baltic Sea and its annexes (Gulf of Bothnia, Gulf of Finland). Here, freshwater supplies are numerous and very little evaporation takes place: this leads to a concentration gradient showing waters with a salt content of below 1 g/l, and it is possible to pass from the waters of the North Sea, that is, from a marine environment, albeit with a lower-than-average salt content, to an almost freshwater milieu (lacustrian) with no transition (Fig. 13).

1.2.4 Conclusion

The few above-mentioned examples show the diversity of morphological, hydrological and climatic conditions which lead to extreme diversity and variability of salinity scales and geochemical gradients in paralic milieux. However, a general characteristic of these salinity fields seems to emerge: that is their longitudinal organization from the communication link (on the zone of marine supply) towards the continental margins. This organization, particularly obvious in very elongated basins, supplied by the sea at one of their ends, is less schematically obvious in the more isodiametric basins where it assumes more concentric shapes. It should also be noted that, locally, longitudinal gradients may be accompanied by vertical gradients which tend to establish a true stratification of the waters (a phenomenon well known in the Black Sea, for example) with appearance of clines (haline, thermic, etc.).

This wide geochemical diversity - which does not affect salinity alone, but all the mineral components of water - is the most striking aspect of paralic milieux: it has led many authors to propose classifications based on salinities. Their complete description will be found in Petit (1953), Ancona (1954), Aguesse (1957), Petit and Schachter (1959), Segerstråle (1959),Mars (1966), Amanieu (1967), Arnaud and Raimbault (1969), and Marazanof (1972).

The Venice Symposium (1958) adopted a fairly simple classification known as the "Venice System", which defines a series of standard waters characterized by an average salinity varying from 0.5 ‰ for limnic waters to over 40 ‰ for hyperhaline waters. (This type of classification is discussed later.) Geochemical gradients (essentially salinity, but also other scales such as ionic concentrations, or hydrochemical parameters) depend not only upon the hydrodynamic parameters of each basin, but also local climatic and hydrographic characteristics.

1.3 THE SEDIMENTOLOGICAL DIVERSITY OF PARALIC MILIEUX

This diversity is apparent by simply considering the difference between estuaries where sedimentation is low and the deltas which are characterized by rapid deposit of large quantities of detrital matter. In the "lagoon" type milieux, sedimentation may vary considerably from one basin to another, according to local conditions: in the Etang du Prevost, for example, the detrital fraction of the sand class predominates along the maritime shore; the opposite occurs in the Bahiret el Biban because of wind-borne deposits on the continental margin. Therefore, their qualitative importance, the nature and the granulometry of the detrital phase, depend upon local or regional transport agents, the importance of the winnowing of the peripheral zones of the basins, the lithological characteristics of the adjacent region, etc.

The biodetrital phase is often considerable, especially in the zones closest to the sea.

The biochemical carbonated phase is strongly modifiedby bioclimatic conditions; sedimentation of organic matter is also extremely variable according to its origin (autochthonous or allochthonous) and the conditions of its conservation (presence of reducing milieux or brines, etc.).

It is in the extreme paralic milieux that the best examples of sedimentary diversity are seen, since, depending on local and regional physiography either evaporites or on the contrary very coarse detrital debris can be found.

It may even be that both types of sediment lie side by side in the same paralic environment; for example, the Camargue with its salt marshes, but also its quasi-lacustrian basins.

In general, the sedimentology of paralic milieux is a result of two types of organization which are more or less superimposed:

− a longitudinal organization in parallel with geochemical gradients, in some cases reinforced by the distribution of affluents;

− a concentric organization depending mainly on depth; this too may be reinforced or counteracted by the disposition of the tributaries.

According to local conditions either organization can prevail, but all types of intermediary situations exist between purely longitudinal organization and purely concentric ones (Perthuisot, 1975, 1980; Busson and Perthuisot, 1977).

1.4 CONCLUSION

Thus, to the geochemical diversity of the paralic milieux can be added the extreme variability of the sedimentary deposits they contain: both are examples of the variability of climatic hydrological and morphological conditions.

However, it is possible to differentiate schematically two types of paralic milieux or areas:

− In the basins or the portions of basins closed to the sea, the biogenic phases 1/ - biodetrital and/or biochemical - prevail; the geochemistry is moderately different from that of sea water.

− In the portions of basins furthest away from the sea, the abiogenic mineral phases become predominant whether one considers the hyperhaline milieux or where evaporites deposit, or the heavily hypohaline quasi freshwater milieux where ferrigenous deposits -associated or not with vegetal debris - constitute practically the totality of the sediment.

1/ This tendency to a stratification of the waters, however shallow, is a common characteristic of the paralic milieux; the lateral co-existence of water masses of varying chemical compositions and therefore varying densities, which tend to overlap when they come into contact, contribute greatly to this stratification.

1/ This term is to be preferred to that of "biogenous", the use of which has become too frequent in the academic world and whose etymology has too much an implication of "spontaneous generation"

2. THE BIOLOGICAL UNITY OF PARALIC MILIEUX

2.1 THE EXISTENCE OF STRICTLY PARALIC SPECIES

The paralic milieux which have been studied and the bibliographical data available underline one fact: there exist species which live and develop only in paralic milieux. Moreover, their distribution is independent of the salinity of the milieu. Examples are numerous, and only a few are mentioned here. Among the most striking is that of

Ruppia spiralis. This monocotyledon groups in communities in the Etang d'Urbino (33 ‰), and is often to be found in the Dutch polders where Den Hartog (1971) calculates that they have a salinity tolerence ranging between 1.5 and 23 ‰. On the other hand, R. spiralis communities are also found in the Bahiret el Biban where salt content exceeds 80 ‰ (Guelorget et al., 1982), or at Salin-de-Giraud at 60-80 ‰. It is moreover remarkable that this monocotyledon is never found in the open sea, although it could obviously find salinity conditions there well compatible with its development. Among molluscs, species such as Hydrobia acuta or its homologue Pirenella conica can be cited, or again Cerastoderma glaucum found only in lagunar milieux - whether hypo- or hyperhaline. Such examples could be enlarged from Protozoa to Tunicata, from blue algae to monocotyledons.

Appendix 2 gives a list (incomplete) of "thalassoid" 1/ species which are usually found in the Mediterranean lagunar milieux, or at least in those basins or portions of basins most influenced by the sea.

In the zones furthest from the sea, there is a more or less clear-cut transition between original communities which will be different from each other depending on whether the evaporitic environment or milieux with a freshwater tendency are being considered:

− in the evaporitic milieux the macrofauna often consists of only 1 or 2 species (such as Artemia salina), as does the microfauna either benthic or planktonic (Bacteria, Cyanophyceae, Dunaliella salina-viridis). However, the importance should be mentioned, at least qualitatively, of the meiofauna (Rotifera, Nematodes, etc.) to be found even within the salt deposits (Basson et al., 1977). One should note that these species occur all over the world, and also inhabit the continental evaporitic milieux which, moreover, are likely to contribute to the explosion of typically continental species (insects, amphibians, etc.) during desalination phases (Perthuisot and Jauzein, 1975).

− in the milieux with a freshwater tendency, as the sea becomes more distant, typically continental species appear (insects, Pulmonata, Oligochaeta, etc.), without it being possible to define precisely the transition into a freshwater milieu.

− in the evaporitic milieux the macrofauna often consists of only 1 or 2 species (such as Artemia salina), as does the microflora either benthic or planktonic (Bacteria, Cyanophyceae, Dunaliella salina-viridis). One should however mention the importance at least qualitative of the microfauna (Rotifera, Nematodes, etc.) to be found even within the salt deposits (Basson et al., 1977). It should be noted that these species occur all over the world and also inhabit the continental evaporitic milieux which, moreover, are likely to contribute to the explosion of typically continental species (insects, amphibians, etc.) during desalination phases (Perthuisot and Jauzein, 1975).

In the milieux with a freshwater tendency, as the sea becomes more distant, typically continental species appear (insects, Pulmonata, Oligochaeta, etc.) without precise definition being possible of the transition into a freshwater milieu.

There are, however, some rare species capable of pas sing from evaporitic milieux into freshwater, such as the common eel, but its presence in these extreme milieux seems only transitory. Nevertheless, this shows a remarkable capacity of osmotic regulation. Some Rotifera seem to be totally insensitive to the concentration of

the milieu since the same species can be found from 1 ‰ up to 350 ‰ (Ruttner-Kolisko, 1971).

Thus, the mere qualitative examination of the flora and fauna of paralic milieux shows their originality in comparison with the continental and maritime domains, particularly owing to the presence of species particular to that milieu, i.e., strictly paralic.

2.2 BIOLOGICAL ZONING OF PARALIC MILIEUX

A notable property of paralic milieux is their biological zoning particularly in the regions not far from the sea.

This characteristic is obvious for benthic communities which sooner or later integrate the minor fluctuations of the milieu (Guelorget et al., 1983). However, it will be seen that the other links in the trophic chains behave in a comparable way • Moreover, a study of the present-day benthos of paralic milieux is interesting from a geologist's point of view for in the paralic series, very often only the toughest portion of the benthic species remains.

2.2.1 The Distribution of Benthic Species

The distribution of species in paralic milieux is generally longitudinal - leaving aside the local anomalies linked to the bathymetry or to the nature of the substratum - and roughly identical whether considering hypohaline or hyperhaline milieux.

(a) Plant communities

The above-mentioned property is obvious with the distribution of macrophytic species. In the Etang de Biguglia for instance, Zostera noltii is confined to the vicinity of the grau, directly under marine influence. Further south, a Ruppia spiralis community has developed, then a Potamogeton pectinatus/Characeae association where Chlorophyceaedevelop (Cladophora vagabunda, Chaetomorpha linum).

A similar distribution can be observed in the lagoons of the Louros delta (Logarou, Tsoukalio, Rodia) where Z. noltii prevails in the zones close to the sandbars; R. spiralis dominates the central zones, sometimes constituting mixed communities with Chara sp. in the zones further from the sea. The lagunar confines are colonized either by little-developed cyanobacterian fields or by Chlorophyceae (Ulva sp., Enteromorpha sp.).

The bottom of the Bahiret el Biban is carpeted with a vast Cymodocea nodosa community, often associated with Caulerpa prolifera. The latter, in the marginal fringes, gives way to a R. spiralis community with fibrous Chlorophyceae (Chaetomorpha linum, Cladophora sp.), which in its turn is superseded by cyanobacterial fields.

In the same way, at Salin-de-Giraud (Fig. 14), the association of R. spiralis with Enteromorpha gr intestinalis which characterizes the first condensers gives way in the more concentrated enclosures, to Lyngbya aestuarii and Microcoleus chonoplastes algal fields, and beyond, to Oscillatoria laetevirens, Phormidium sp., and Spirulina subsalsa (Thomas, 1983).

A very interesting case in that of the Dybsø Fjord where a longitudinal zoning of the benthic macroflora can be noted in spite of the absence of any salinity gradient, which shows that the latter plays no part in the distribution of the macrophytic species in paralic milieux (Fig. 6).

(b) The invertebrate macrofauna

The study of a considerable number of paralic basins shows, whatever the salinity gradient, the following horizontal succession as the distance increases from the communication link with the sea:

− a pelecypod-dominated region, with a few echinoderms, i.e., associations which still have a "stenobionte" (thalassic species) tendency;

− a transitional region still dominated by pelecypods, but from which the echinoderms have disappeared. In this case, the marine influence is too slight to allow an optimal development of species with marine affinities, but still too strong for paralic species to flourish fully. Local variations can however be introduced in cases of "organic pollution". Then the number of pelecypods decreases to the advantage of crustaceans and detrivorous annelids. It is in this zone that the "mixed" species which are also present in the sea are found;

− a third region where strictly paralic species abound (Cerastoderma glaucum, Abra ovata, Nereis diversicolor, Gammarus gr locusta, Sphaeroma hookeri, Chironomidae).

A typical fourth region, common in hyperhaline milieux, less clearly so in hypohaline milieux (particularly in cold climates), is characterized by the presence of cyanobacterial fields or structures (algal fields, stromatoliths) mono- or paucispecific, associated with a small number of animal species (Hydrobia acuta, Sphaeroma rugicauda , Pirenella conica, Ammonia beccarii var. tepida).

Beyond these, are the extreme milieux. There, no zoning is obvious, neither in the almost freshwater milieux, nor in the evaporitic milieux which remain little known. Perhaps the biological zoning continues at the level of the meiofauna but, for instance in a salt marsh, no notable qualitative difference can be observed between the communities in gypsum areas and those of the crystallizers, despite considerable differences in salinity between these two milieux.

2.2.2 Biological Gradients

As far as the benthic communities (macrofauna) are concerned, the distinctive scales (variety of species, density, biomass, production), can be observed to form gradients from the communication link(s) with the sea, towards the edges of the lagoon. Independently of the salinity field and the value of its gradient, the comparison between the Etang du Prévost and the Bahiret el Biban is highly significant in this respect (Fig. 15) (Guelorget and Perthuisot, 1982). Nevertheless, as explained in detail later, in all the cases studied, and according to bibliographical data, the following can be observed progressively from the communication link with the sea:

− a significant decrease in variety of species;

− a progressive increase in the density of the invertebrate macrofauna, followed by a pronounced decrease near the freshwater pole, while the benthic macrofauna disappears completely in the vicinity of the evaporitic pole;

− a progressive decrease in biomass, for the increase in density accompanies a decrease in size (lagunar "dwarfism");

− a drastic decrease in the overall production (calculated from the malacofauna which predominates in paralic milieux) falling from a maximum situated in the zones directly influenced by the sea (Guelorget et al., 1982);

− a parallel can be drawn with specific scales of the phytoplanktonic and ichthyological communities which present similar patterns.

2.3 THE INFLUENCE OF SALINITY UPON MARINE SPECIES

There are cases where marine organisms reputedly stenohaline, such as echinoderms, live in saline concentrations obviously different from those of the sea.

This is the case in the Gulf of Salwa situated between Saudi Arabia and the Qatar peninsula (Basson et al., 1977), where at least three species of Stelleroides (Astropecten phragmosus, A. polyacanthus and Asterina burtoni) "tolerate" salinities in excess of 60 ‰. In the same places live stonebass fish (Epinephelus tauvina), a common gastronomical species of the Persian Gulf, found at over 70 ‰.

Another similar case is the Vonitza Bay, in the Gulf of Amvrakikos, Greece, where a curious association of snakes and sea-urchins (Paracentrotus lividus) have been observed, at salinities which, measured in January and June 1982, do not exceed 5 ‰.

These two examples show that "osmotic regulation" is not a particular problem, even for species considered as not being among the most "stenohaline" (Appendix 1).

2.4 THE STABILITY OF PARALIC COMMUNITES

Another property of paralic ecosystems is their extreme stability, when compared to marine or freshwater ecosystems. It is even possible to say that the more paralic an ecosystem is, and therefore situated in an unstable milieu, the more stable it is itself. This is obvious when one considers the margins of paralic basins (e.g., the Bahiret el Biban), which can be subjected, according to the seasons, to considerable variations in the salt content, and change from a generally hyperhaline milieu into a highly desalinated one. In the case of the Bahiret el Biban, pluri-annual floodings do not affect at all the communities in the most marginal zones.

"Dystrophic crises" ("malaigues" = "bad waters") which affect the lagoons of the Languedoc coastline should also be mentioned: although they destroy instantly great numbers of individuals -especially those which enter the lagoon to feed - they do not destroy the ecosystem which recovers very quickly as soon as the crisis is over.

"Lagooning", i.e., the discharge of effluents rich in organic matter into the lagoons, is yet another feature: the only consequence of this is apparently an increase in the production and the output of the ecosystem, whereas, in the open sea, such a process leads to a desertification of the sea bottom. Accordingly, it can be said that pollution of organic origin has little impact upon paralic milieux.

All things considered, this stability of paralic ecosystems is logical, since the species living in these milieux have hardly any regulatory problems - particularly osmotic ones - since they live in conditions (of salinity for example) which are both varied and variable.

2.5 CONCLUSION

The panorama of the paralic milieux outlined above shows that beyond the morphological, geochemical and sedimentological diversities, these milieux present an

undeniable biological unity: an originality and specificity of the communities, an independence of the qualitative and quantitative biological gradients with respect to salinity fields, and stability of their ecosystems. They are not "mixtures" of flora and fauna, some marine and others freshwater, but original communities having their own logic which is different from the marine or inland water logic•

Therefore a paralic milieu exists as an autonomous entity, distinct from the marine and inland water milieux, having its own structure and its own dynamics 1/.

Further, according to geological and biological data, two sub-systems can be observed:

− the sub-domain closest to the sea (Near paralic) is characterized by a geochemistry little different from that of the sea, by the large quantities of sediment involved in the biogenic phases (carbonates, organic matter), by essentially "thalassoid" communities, and by pronounced biological gradients;

− the sub-domain farthest from the sea (Far paralic) is characterized by a geochemistry radically different from that of the sea with the two poles that are also found in the continental domain: the evaporitic pole and the freshwater pole. The abiogenic sedimentary phases are dominant. The communities are "specialized", original, and the biological gradients not very pronounced (unless proved to be the contrary by future studies).

The limit between these two domains is approximately immediately beyond the "algal fields"; when leaving the sea, this limit also corresponds to the disappearance ofForaminifera (Zaninetti, 1983).

The above-mentioned affirmations are not intended as dogma, but can be regarded as sufficiently innovative to be used in the following sections as the basis for certain deductions.

1/ "Thalassoid" refers to species which both morphologically and genetically are akin to maritime stock without necessarily issuing from the maritime domain, and totally independent of continental stock, "Thalassic" implies species of entirely maritime stock likely to colonize the lagunar regions which are in immediate contact with the sea. Therefore, thalassoid species include both Thalassic and paralic species.

1/ The limit between the marine domain and the paralic domain is not always exactly the shoreline: for example, all seas do not belong to the marine domain. The Baltic, the Black Sea and the Caspian Sea are paralic basins. Moreover, there are zones of the marine domain with abnormal salinities (Salwa Gulf, Vonitza Bay). Therefore, the totality of paralic milieux is different from the totality of "variable salinity" milieux (Plaziat, 1982), as well as from the totality of "marginal-coastal" milieux (Levy, 1970).

3. CONFINEMENT: THE FUNDAMENTAL PARAMETER OF THE PARALIC DOMAIN

There is a natural zoning, biological and geological, in every sizable portion of the hydrosphere. In the marine domain, depth is undoubtedly the leading parameter responsible for zoning. The decisive influence can be only modulated by other parameters. In the continental domain (aquatic or otherwise), latitude, which affects temperature and photoperiodicity, seems to be the fundamental parameter. Is there a dominant parameter in the paralic domain, and if so, what is it?

3.1 THE REASONS FOR A MISTAKEN APPROACH

Until now, salinity has generally been considered as the fundamental ecological parameter of the distribution of the communities in paralic milieux (Kiener, 1978, Remane and Schlieper, 19 56; Petit, 1962; Sacchi, 1967; Vatova, 1963) , and it is on a salinity scale that the usual classification rests: euryhaline, mesohaline, poikilohaline

milieux, etc. A simple observation of the natural surroundings which leads to the considerations mentioned in the previous chapter, shows that salinity (along with the major ionic relations 1/, etc.) as a fundamental ecological parameter of the paralic domain is a mistaken notion.

It is likely that the main reason is the variability, but also the accessibility of this parameter, the presence in paralic milieux of "abnormal salinities" (with relation to the sea which is considered "normal") is an inducement to consider this parameter as paramount. It is true, moreover, that for a given basin, the biological zoning can generally correspond with the local geological zoning (Fig. 16); on a regional scale, if the correspondence is less clearcut, it can be ascribed to local differences and other parameters.

Hyperhaline paralic milieux are rare and to date little studied. Therefore few scientists have studied both hyper- and hypohaline milieux: only comparison between the two, together with the revelation of an identical zoning for both could cast doubts on and lead to the discarding of a mistaken idea (Guelorget and Perthuisot, 1982).

Finally, another error accepted by many authors is the chemical stability of seawater (in composition and concentration) in the course of geological time. This would imply that the marine domain is the fundamental original one, from which most aquatic stocks issue, and that those which "adapt" to "abnormal" milieux must "solve problems of osmotic regulation". Now, there are firm doubts regarding the chemical stability of seawater (Perthuisot, 1980), and on the fundamental or original quality attributed to the marine domain.

3.2 THE SEARCH FOR THE FUNDAMENTAL PARAMETER OF THE PARALIC DOMAIN

It is established that the biological zoning organizes itself roughly according to the distance from the open sea, as do the geochemical gradients (whatever their value). In fact, this is only an estimate. In more detail, it is noted that the weaker the exchange of water with the sea the more "typically paralic" the communities become, i.e., they are composed of eurybiontic species, not numerous, with a strong density, etc. Conversely, the nearer to the zones where the movements of the water allow a regular renovation of the milieu, the richer in species the communities are and the lower the density becomes (Guelorget et al., 1981, 1982 (a) and (b); Perthuisot, 1983).

The most adequate term to qualify the value which organizes the biological zoning of the paralic ecosystems would appear to be "confinement". This term, used in a both empirical and intuitive way in various disciplines of natural science is ambiguous: for example, it is sometimes used to mean "anoxia" (Appendix 3).

This word is used to imply "with relation to the sea"; first, because the lagunar ecosystems are arranged in relation with the communication link with the sea, and second, because of the initial concept that in the smaller, even restricted basins where most of the life-giving elements (mineral salts, trace elements, vitamins, etc.) come from the outside (like oxygen in a nearly closed room); the communities organize themselves, according to the rarity of these elements, rarefaction which they themselves are mostly responsible for. This idea (which will be developed later) is based, for instance, on the amazing capacity of molluscs to fix certain trace elements (Cu, Pb, etc.), within their tissues.

Obviously, in the marine domain, the situation is otherwise, since it can be considered infinite, or at least inexhaustible, if equated to the biomass it contains with the available volumes of water and elements.

Thus, confinement at a given point in a paralic ecosystem would correspond to the lack of "vital elements" of marine origin at that same point. Since this deficiency is essentially linked to the biological activity itself, it is understood that, schematically, confinement is a function of the time it takes water coming from the sea to reach the point in question.

Further, it can be said that the marine domain is the main communication agent between the different fractions of an apparently very fragmentary domain. This is without doubt a determining factor in the dissemination of paralic species, and of the completion of their biological cycles. On the other hand, many marine species, particularly fish, use paralic milieux to feed.

The confinement of a specific zone of a given paralic milieu is the outcome of various factors such as the size of the communication link, the force of the tide, the currentology of the basin, the continental inflows, etc. 1/. It is therefore essentially hydrodynamic. In this connection, it must be mentioned that depth can play a fundamental role when it leads to the presence of a deep stable water mass: it then creates a very large local confinement that can bring about the almost total disappearance of the benthos, and the appearance of facies of a euxinic type (Perthuisot, 1975, 1980).

Finally, when the confinement becomes very widespread (in the Far paralic), its leading role in the dynamics of the communities diminishes, and biological logic tends to depend upon other factors.

What is the relationship between confinement and salinity (or concentration)? Climatic and hydrographic data must be examined: it is clear that in any given basin, salinity at a given point depends both on the renewal of the milieu, hence of the confinement, and on the local freshwater supply (by convention, that algebraic value of the difference between evaporation and freshwater supplies is termed "hydrous deficit"). If the hydrous deficit is positive, the basin is hyperhaline, if the hydrous deficit is negative, the basin is hypohaline. In each basin, hyper- or hypohalinity increases with confinement to a given value (not nil) of the hydrous deficit provided the latter is constant for the whole of the basin being considered.

This proposition can be formalized mathematically with a schematic and theoretical example, starting with the following hypotheses:

− The freshwater supplies of the basin (rain, continental supply) consist of pure water, are constant and are diffused instantly over the entire basin where evaporation is also uniform.

− Depth is low and constant.

− All major ions coming from the sea are preservative (this constitutes a condition incompatible with natural processes, but entails only a minimal error).

− The hydrodynamic conditions of the basins remain stable in time.

In these conditions, it is possible to define the loss of freshwater by unity of volume and unity of time (in an algebraic value), taking ∆ h as that value directly

proportional to the hydrous deficit. If ∆ h is positive, it means an excess of evaporation; if negative, it shows an excess of freshwater supply in the basin. This value is specific in each basin.

Furthermore, one should consider a point p of this ideal paralic basin. At this point, a volume of water Vp contains N major ions of marine origin at a time t. Concentration at that point is:

These N elements were all in the sea at the instant of time to, where they occupied a volume Vo of solution: the concentration of sea water is expressed as:

The time lapse T = t - to is a constant of this point p if o the hydrodynamic conditions remain stable and can be defined as the time of renewal of the milieu at point p which controls the "confinement rate" at that point.

The volume variation from Vo to V

as a result:

Log (V - Vo) = K ∆ h T

V = Voe - K ∆ h T

and

C = Coe K ∆ h T

Deciding a priori upon a given concentration C, T = constant. For example, C = Co can be obtained for ∆ h = 0 or for T = 0, i.e., for basins whose hydrous deficit is nil, or for zero confinement, i.e., the marine domain itself.

This formula is, of course, over-simple and not a natural reality. However, it shows that granted an equal confinement, two points from a same basin or from different basins in similar climatic conditions, have closely related salinities. Conversely, in very different climates, two points of equal confinement have very different salinities. In all cases, as confinement increases, the difference in salinity (or concentration) from the sea increases, except when the hydrous balance is close to zero: in that case, the salinity remains close to that of the sea, whatever the confinement. It is clear that in nature, the more confined a milieu is - hence enclosed within the continental domain - the less likely it is to belong to an environment with a balanced hydrous value (Fig. 17); except if it is a bathymetrie confinement: in the case of stratified basins, the lowest body

of water is characterized by a zero hydrous deficit. Lastly, if the confinement is nil, the point in question belongs to the marine domain.

3.3 GENERAL ORGANIZATION OF THE PARALIC DOMAIN

Figure 17 illustrates the previous and present considerations and ideas on the respective roles of confinement and the local hydrous balance in the organization of the biological and geochemical gradients of the paralic domain.

In this diagram, as the parameters used (confinement, hydrous deficit) cannot be given accurately, various paralic basins have been introduced in an approximative but not an arbitrary way. Their juxtaposition is intended to show the apparent concordance in each basin of the biological gradients and the chemical gradients, and also the total obliquity of the geochemical zoning over the biological zoning on the scale of the entire paralic domain since this biological zoning depends largely on confinement alone.

In spite of present inability to quantify either the confinement or the "hydrous deficit" in natural milieux, this concept seems to account for the situation encountered in the field better than an ecological classification of the paralic milieux according to their salinity.

Figure 18 is an attempt to picture the sedimentology of the paralic domain: its only aim is to confirm the total obliquity of the faciological zoning over the biological zoning.

Thus, to each of the three domains which constitute the hydrobiosphere corresponds a fundamental parameter: depth in the marine domain, latitude in the continental domain, confinement in the paralic domain.

3.4 THE ESTABLISHMENT OF A CONFINEMENT SCALE

As emphasized before, confinement is a complex and abstract value , which cannot be measured in the present state of knowledge. Not having a unit of measurement, a qualitative scale is proposed (comparable for example, to Mohs' hardness scale or Mercalli's scale for the intensity of earthquakes). This attempt cannot rely on either geochemistry or sedimentology: it is therefore necessary to use the peculiar features of the biological zoning, and in this connection, benthos proves convenient: apart from the fact that it is relatively easy to harvest, that it allows an easy direct observation, and in addition to its autochthonous species, it integrates in the short term, minor variations of the milieu, and reflects the latter's global conditions.

In comparing various Mediterranean paralic basins, it is possible to establish 6 degrees of confinement starting from the particular features of the distribution of benthic species on a soft bottom (Fig. 19). This scale, valid for the Mediterranean, concerns only the portion of the paralic domain close to the sea (Near paralic) (Perthuisot and Guelorget, 1982; Guelorget et al., 1982, 1983).

Zone I - This zone falls between degrees 0 and 1. In this zone situated in the immediate vicinity of the communication links with the sea and which is simply a continuation of the marine domain, the macrofauna consists of many strictly thalassic species generally belonging to the fauna of the Well Calibrated Fine Sands (WCFS) biocenosis (Peres and Picard, 1964).

Zone II - This zone falls between degrees 1 and 2: from degree 1 , the penetration into the paralic domain can be considered effective. The benthic macrofauna is characterized by the loss of the sensitive strictly marine species, the most "stenohaline". Molluscs can still be found: Mactra corallina, M. glauca, Tellina tenuis, Donax semistriatus, D.

trunculus, Acanthocardia echinata, Dosinia exoleta, the Polychaeta: Audouinia tentaculata, Magelona papillocornis, Owenia fusiformis , Phyllodoce mucosa , Pectinaria koreni, the crustaceans: Portunus latipes, and the echinoderms: Asterina gibbosa, Holothuria polii, Paracentrotus lividus. Lastly, the presence should be mentioned of Branchiostoma lanceolatum in the sandy facies devoid of "organic pollution".

The phanerogamic macroflora loses the Posidonia oceanica; and small Phanerogams predominate such as Cymodocea nodosa, Zostera noltii and Caulerpa prollfera.

The limit of this zone (degree 2) is marked by the total disappearance of echinoderms.

Zone III - This zone falls between degrees 2 and 3: it is dominated by "mixed species" 1/ Venerupis decussata, V. aurea, Scobicularia plana, Corbula gibba, Loripes lacteus , Gastrana fragilis, Akera bullata, Nephthys hombergii, Armandia cirrosa, Glycera convoluta, Upogebia littoralis) as far as the benthic macrofauna is concerned.

Zone IV - Zone falling between degrees 3 and 4: at degree 3, all thalassic fauna disappears, and the species found are strictly paralic: Abra ovata, Cerastoderma glaucum, Hydrobia acuta, Nereis diversicolor, Gammarus insensibilis, G. aequicauda, Corophium insidiosum).

The macroflora is characterized by the appearance of Ruppia spiralis.

Zone V - Zone falling between degrees 4 and 5: in this zone, the phytoplanktonic production (pinnated diatoma, Cyanophyceae) is at its maximum. The surface of the sediment is in the process of being colonized by the Cyanophyceae. Thus the sedimentation is essentially organic, and the interstitial milieu highly reducing.

The community consists essentially of a vagile fauna with detritivorous crustaceans (Sphaeroma hookeri, S. rugicauda, Corophium insidiosum, Idothea balthica), browsing gastropods (Hydrobia acuta, Pirenella conica), Polychaetes such as Nereis diversicolor, and larvae of Chironomidae in the uppermost stratum of the sediment (a few millimetres). In this zone, and most particularly in very hypohaline milieux, elements of freshwater fauna appear (Tricoptera, Oligochaeta, Odonata). Further, in hyperhaline milieux elements of evaporitic fauna (Artemia salina) can be found.

The plant communities - if any - are dominated either by Potamogeton pectinatus, or by the Characeae. They can also be mixed.

Zone VI - Zone falling between degrees 5 and 6. It represents the passage into Far paralic, either freshwater or evaporitic, and is usually characterized by an almost total colonization of the substratum by Cyanobacteria which form algal fields or stromato-lithic structures. The passage into freshwater is marked by the appearance of strictly freshwater species and the persistence of a few vagile paralic species (Sphaeroma hookeri, Microdeutopus gryllotalpa, Gammarus insensibilis). The sub-evaporitic milieu is characterized by the absence of benthic macrofauna, except browsers which momentarily visit that zone for trophic reasons (browsing of the algal fields).

The far edges of this zone are the outer limit of the Near paralic. Foraminifera disappear, which coincides, around the evaporitic pole, to a considerable extent with the disappearance of stromatolithic structures.

Beyond this, in the Far paralic, the role of confinement seems to decrease, as far as present knowledge indicates.

3.5 THE POSITION OF SEVERAL MEDITERRANEAN BASINS ON THE CONFINEMENT SCALE

According to the confinement scale proposed, the organization of a peri-Mediterranean type lagoon can be illustrated (Fig. 19). This graph shows the zoning of the milieu according to its confinement. With regard to this standard diagram, the lagoons in question can either spread over the entire scale, or occupy only a portion of it.

3.5.1 Bahiret el Biban (Tunisia) (Fig. 20)

Bahiret el Biban, a hyperhaline lagoon, covers the entire confinement scale; however, the largest portion is situated in zones II and III, thus showing low confinement of sizable and regular exchanges with the sea (wide pass, strong tidal currents) (Guelorget et al., 1983).

3.5.2 Mar Chica of Nador (Morocco) (Fig. 21)

This basin, which has a complex history, is little confined at present, because it communicates extensively with the sea. Only a few small marginal areas can suddenly become confined, thus leading to the development of algal fields (Frisoni et al., 1982).

3.5.3 Lagoon of Logarou (Greece) (Fig. 22)

Almost completely situated in zones IV and V, this lagoon -of deltaic formation - hyperhaline in summer, reaches the cyanobacterial zone in its northern part (Frisoni et al., 1982). Only a narrow strip along the lido indicates some influence of the marine domain.

3.5.4 Lagoons of Tsoukalio and Rodia (Greece) (Fig. 22)

These lagoons, close to that of Logarou and belonging to the same deltaic formation, have the same pattern of confinement, but with a negative salinity gradient (Frisoni et al., 1982).

3.5.5 Lagoons of Diana and Urbino (Corsica) (Fig. 23)

These two lagoons are very similar. The marine influence prevails. However, the hydrological originality of Diana has permitted the establishment of a little-confined zone (II) around the sides furthest from the communicating link with the sea. Due to the great depth (9-10 m) in relation to the surface area (500-700 ha), the central zones of these lagoons, little renewed, are submitted to a bathymetric confinement linked to a "hydrological remoteness" from the sea, and above all, to an organic enrichment; the latter entails the appearance of a specific Corbula gibba facies (Burelli et al., 1979; Frisoni et al., 1983).

3.5.6 Lagoon of Biguglia (Corsica) (Fig. 24)

Like the Bahiret el Biban - but with a prevalence of zones IV and V - this lagoon covers the whole of the confinement scale, but towards the freshwater pole (Frisoni et al., 1983).

3.5.7 Lake Melah (Algeria) (Fig. 25)

The whole stretch of water is located in zones IV and V. This high confinement is due to the hydrological isolation (long and narrow channel) on the one hand, and the high bathymetry (6 m) with regard to the surface area (800 ha) on the other. The conjunction of these two characteristics causes a confinement which is much more pronounced than in lagoons of similar size and morphology (Diana, Urbino).

This lagoon offers a particular originality with the presence of algal fields (zone IV) in the vicinity of the communicating link with the sea: those regions contiguous to the embanked channel are isolated from marine currents and undergo a high confinement (Guelorget et al., 1983 ).

3.5.8 Palavas Lagoons (France) (Fig. 26)

This entire lagunar system with few communicating links with the sea (2 graus) is almost entirely situated in zones IV and V, except for the regions of the graus which allow the implementation of thalassic communities.

The prevailing influence of the catchment area leads to the appearance of freshwater zone VI at the periphery of the complex.

3.5.9 Bermuda Triangle (Santa Pola, Spain) (Fig. 27)

This former salt production condenser (Perthuisot et al., 1983) is an annex of hypohaline paralic marsh. More confined and saltier than the latter, the entire basin is situated at the upper end of the confinement scale (II, IV, VI).

3.6 ADJUSTMENT OF OTHER BIOLOGICAL COMPARTMENTS INTO THE CONFINEMENT SCALE

With regard to the biological characterization of benthic macroflora and macrofauna, the biological zoning of these characteristics is sufficiently clear and stable as to define a confinement scale. This is not the case for the other links of the trophic chain which, composed as they are of more mobile species reacting more strongly to the minimum variation in . the milieu, present a less obvious, more fluctuating zoning pattern.

3.6.1 The Phytoplanktonic Communities of Paralic Milieux

Although there are, in most of the paralic basins studied, pronounced and significant gradients concerning the phytoplanktonic biomass and the chlorophyll production, it seems that the phytoplanktonic communities are relatively homogeneous in each basin. This is probably due to the fact that the major part of each basin spreads over a limited number of. confinement zones (usually one or two). Thus only the comparison of a considerable number of paralic basins can trace the outline of a specific zoning of paralic phytoplankton.

(a) Taxonomic resources

Finally, it should be noted that an increase in confinement results in a decrease of the taxonomic resources; however, the basins strongly influenced by continental inflows gain in freshwater taxa (Table 1).

(b) Specific composition of the phytoplanktonic communities

The zones directly under reviving marine influence (I, II, III), are characterized by the presence of typically neritic plankton essentially composed of centric Diatoma - up to more than 25% on relative average - (skeletonema costatum, Melosira sp., Coscinodiscus sp., Chaetoceros sp., Rhizosolenia sp., Thalassiosira sp.) along with pinnated Diatoma (Nitzschia closterium, Thalassiotrix frauenfeldi) and with some Peridinieae (Ceratium furca) and various nannoplanktonic forms (Chlorophyceae, Cryptophyceae, Cyanophyceae).

In the typically paralic zones (IV, V) centric Diatoma are very rare; pinnated Diatoma, although always present, are rarely plentiful (Navicula sp., Gyrosigma sp.,

Amphora sp., Striatella sp.). The community is generally dominated by Peridinieae and the nannoplankton groups.

Among the Peridinieae can be noted the frequent and abundant presence of Exuviella compressa and Prorocentrum scultellum which can represent up to 99% of the community (Lake Melah). These species can also be observed all through the annual cycle in the same basin (Etang de Biguglia (Frisoni, personal communication) 1/.

In the most confined zones of the Near paralic (zone VI), the phytoplanktonic communities are dominated by species which are usually periphytic or benthic. In this connection it should be noted that in these usually very shallow milieux, the ticro-phytobenthos easily regains a state of suspension, that sample-taking can perturb the milieu and affect the element of the phytobenthos all the more as the individuals are affected by lagunar "dwarfism"; lastly, in hyperhaline milieux, the density of the water or the brines practically prevents the existence of free microphytobenthos. The species that would have composed it are in suspension in the milieu.

Beyond zone VI, in the Far paralic, the communities differ according to whether the freshwater pole or the evaporitic pole is considered. In the nearly fresh waters,the milieu is invested by typically freshwater species. In the evaporitic brines, the community consists essentially of Dunaliella salina (or viridis) Chlorophyceae, with salina being the red form giving the colour so characteristic of active salt crystallizers, and viridis the green form of the moderately salty condensers;- to these can also be added the cyanobacterian structures (various blue algae and S and Fe bacteria) (Cornee, 1983).

Thus, as far as phytoplankton is concerned, a zoning similar to and superimposable on that which is presented by the macrobenthos and which defines the confinement scale (see Table 1) has been outlined.

3.6.2 The Ichthyofauna

As with the phytoplankton, the study of various paralic basins shows a common ichthyofaunistic pattern according to the confinement.

The following species are noted:

− Sedentary species (paralic). These are few, present throughout the year, small and shortlived. Their entire biological cycle is completed in a paralic milieu. They have a very high breeding potential and are present in large numbers. As far as Mediterranean basins are concerned, Atherina boyeri , Aphanius fasciatus , Potamoschistus marmoratus, Syngnathus abaster should be mentioned.

− Migrant species, often larger than the sedentary ones, with a longer life-span and whose biological cycle comprises a necessary reproductive phase at sea (or of ill-defined type). Within this group are to be distinguished:

The regularly migratory species ("mixed" species) whose fry and juveniles (0+) enter a paralic milieu to spend a compulsory (trophic) period of their biological cycle (Appendix 3): these include mullet, some Sparidae (Sparus aurata, Diplodus sargus, Pagellus mormyratus), sea bass (Dicentrarchus labrax), sole (Solea vulgaris), plaice (Pleuronectes platessa), eels (Anguilla anguilla). Some adult or juvenile (1+) individuals of these species are liable to return occasionally to the lagoon to feed.

The occasionally or accidentally migratory species (usually thalassic)are sometimes found by chance in a paralic milieu. Among the more frequent are: Labridae,

Gobiidae, garfish (Belone belone), anchovy (Engraulis encrasicolus), red mullet, Rajidae (Myliobatis aquila), Caeculum caeca.

Hereafter all individuals belonging to sedentary species will be referred to as "sedentary", and the fry and juveniles (0+) of the regularly migratory species as "migrant". The term "occasional" will be applied to all the others.

The study of the three largest lagoons of the eastern plain of Corsica (Biguglia, Diana and Urbino) (Frisoni et al., 1983) outlines the relative connection of the ichthyofaunistic communities to the confinement scale, both from a qualitative., and a quantitative point of view. The diagram thus proposed can be applied to all the paralic basins having been similarly studied (Frisoni et al., 1982; Guelorget et al., 1983; Guignard and Zaouli, 1981).

In the Corsican lagoons, migrant and sedentary species constitute more than 90% of the stock collected.

(a) Number of species and diversity

In the three lagoons, the same annual evolution of the number of species can be observed with one peak in spring and one in autumn, each corresponding to the two propitious periods for the "lagoon-ward movement" of the migrant fry and juveniles.

Taking these seasonal variations into consideration, the overall number of species in Biguglia is much lower than that in Diana and Urbino. Moreover, whereas the last two are very homogeneous (Table 2), at Biguglia a decrease in the number of species can be observed between the centre of the lagoon and the grau: this again corresponds to migrant movement and can be shown by the study of the proportion of migrant stock to sedentary stock (M/S). At Urbino, the ratio is very low because of the poor communication with the sea. At Diana, the opening of the basin allows a better stocking with fry. However, as the edges are generally only slightly confined, the migrants spread themselves evenly throughout the basin. At Biguglia (a much more confined lagoon), the migrants, although numerous, are mainly localized in the vicinity of the grau and seldom penetrate the rest of the lagoon. These considerations can also apply to the "occasionals" which are more numerous at Diana than in the two other lagoons (see Table 2).

The diversity of the resident species, measured by the equitability of the Shannon coefficient (ESH) also shows (1981) a cycle with a spring maximum at Biguglia, and a summer one at Diana and Urbino. The minimum diversity occurs in winter, a period when stocking with young fish is low: it therefore characterizes the sedentary populations.

At Biguglia, the rather high minimum diversity can be explained by the presence of a large range of the sedentary ichthyic populations. On the other hand, the relatively low maximum diversity shows that the influence of the migrants upon the resident populations remains moderate (except in the vicinity of the grau), and affects only a small number of species: the migrants do not greatly favour highly confined zones.

At Diana and Urbino, the very low minimum diversities are in keeping with the high homogeneity of both the basins. At Diana, the maximum diversity is high because stocking with young fish is easy, and moreover, as the basin is on the whole little confined, the migrants spread throughout the basin. On the contrary at Urbino there is a low maximum diversity because of the small number of migrant arrivals related to the uncertainty of communication with the sea.

(b) Sedentary species

Table 3 gives the average annual percentages of the captures of the various sedentary species in various zones of the lagoons studied, and the corresponding weights.

Considering only Aphanius and Atherina, it is noted that the former predominates in the vicinity of the graus in the low confinement zones, while Atherina predominates in the more confined zones. This is also the case at Diana whose northern basin, although near the mouth, is on the whole more confined than the southern basin.

The mottled goby seems to behave like the Aphanius as far as its distribution is concerned, except at Urbino where it is evenly distributed, but low in numbers.

The syngnathid, very dependent on the plant communities, is seldom seen in the zones near the mouths.

(c) Migrant species

They are 20 species of fry and juvenile migrants listed in the three lagoons. They can be divided into three groups:

Mullets (5 species) - present in the three lagoons

Sparidae - Diplodus (5 species) Puntazzo (1 species) Boops (1 species) Lithognathus (1 species)

Other species -

Sea-bass (Dicentrarchus labrax) Soles (Solea vulgaris and Solea sp. at Urbino) Eel (Anguilla anguilla) Garfish (Belone belone) Anchovy (Engraulis encrasicolus ) Red mullet (Mullus barbatus)

At Biguglia, the fry belonged mostly to eurybiontic species (garfish, eels, sea-bass). On the other hand, there were very few Sparidae (only 3 species), and no soles, red mullets or anchovies. At Diana and Urbino, the migrant species are more diversified and include more specifically marine species. This is also the case for the "occasionals".

The specific composition of the migrant populations varies according to the season: if the mullets are present all the year round, due to successive arrivals of the different species, the Sparidae and the sea-bass appear mostly in the spring, and the other species in late summer and in autumn.

The distribution of the different species is not uniform. The mullets are concentrated in the grau regions (Diana and Urbino) where they often pass through in shoals. The other species are more evenly distributed. Moreover, the stocking with young fish is very irregular: at Diana, for example, in May 1978, 16% mullets, 59% Sparidae and 25% other species were counted; in May 1979, 97% mullets, 2% Sparidae, 1% other species. Obviously, the state of the grau influences the stocking with fry: at Drbino, it remained practically nil for several years while the grau was closed. Its recent opening and maintenance have allowed a better stocking, and a proportion of migrants more in keeping with the position of this lagoon on the confinement scale.

(d) Diets and coefficient of condition

The species of the paralic milieux are essentially carnivorous or omnivorous, and very few microphagous. Therefore, the fish consume directly very little phytoplankton.

As far as the sedentary species Atherina boyeri is concerned, the highest average weights and sizes of the captured individuals are to be found at Biguglia, taking the year as a whole. Again at Biguglia is noted the highest monthly average coefficient of condition. A. boyeri reaches the maximum coefficient of condition in the spring, i.e., during the reproduction period.

The coefficient of condition is given by the formula:

Kc = W.L-3 where W is the weight in grammes and L the length in centimetres

With regards to mullet (Liza saliens), a regularly migrant species, a similar evolution is found according to seasons; but the individuals reach their maximum coefficient of condition towards the end of summer, before migrating towards the sea. This coefficient is even higher at Biguglia than the two other basins.

Thus, the Biguglia lagoon seems to be more favourable for the growth of individuals, especially sedentary ones, than the other lagoons. This is likely to be related to the heavy phyto-planktonic biomasses which characterize this highly confined lagoon, and which result in the presence of great numbers of primary consumers.

(e) Conclusions

Unlike the relatively fixed benthic communities which settle in stable zones, according to the confinement range of each basin, the ichthyological populations are much more mobile. Therefore, their organization system is of a more global type, and depends upon the totality of the characteristics of each basin and particularly on the relative surface areas occupied by the various zones of confinement, on the type of communication with the sea, etc. Thus, the qualitative and quantitative characteristics of the ichthyological fauna of the paralic milieux account for the originality of each of them, and for the place each occupies on the confinement scale.

Certain data suggest, however, that within the ichthyological population biological gradients exist, which are identical from one basin to another (the Atherina / Aphanius population, for example), which further studies may perhaps specify more accurately.

3.7 QUANTITATIVE BIOLOGICAL GRADIENT

Biological study of the peri-Mediterranean lagoons shows that the quantitative variations of the phytoplanktonic and benthic communities (macrofauna of invertebrates) differ according to confinement.

3.7.1 Phytoplanktonic Biomass and Biomass of the Benthic Macrofauna (soft bottom)

The chlorophyll biomass (expressed in mg of chlorophyll a per m3- fluorimetric analysis) increases regularly from the marine waters (values close to 1) up to zone V. Thus, the lagoons of Diana, Thau, el Biban and Nador, have a biomass of between 1 and 4; the lagoon of Biguglia reaches values of about 20; the lagoon of Mauguio (subject to urban and agricultural pollutions) has biomass values of about 1 000.

Beyond this zone V, the shape of the curve will depend on which pole (freshwater or evaporitic) is approached. In a freshwater milieu, either the persistence of a high biomass (eutrophic milieu), or a decrease of this biomass (oligotrophic milieu) is

noted. In a sub-evaporitic milieu, in all cases a drastic decrease can be observed in the phytoplanktonic biomass. In this extreme zone, the primary link is hardly represented except by the microphytobenthos (algal field).

The biomass of benthic macrofauna (expressed in grammes of dry decalcified weight per m2 ) shows a similar curve. However, this curve shifts in relation to the preceding one: the position of the maximum moves into zone III (Fig. 29). However, as with the primary link, this diagram can show vertical fluctuations (maximum values) according to seasonal, interannual variations of the enrichment of the milieu by organic pollution.

The following values should be kept in mind:

− Around 1 g in zone I, about 10 g in zone II, about 50 g in zone III subject to few organic deposits (el Biban), reaching as much as 500 g in highly enriched zones (Prévost).

Beyond zone III, the curve shows a decrease of the values as far as zone VI where, as with the primary link, two possibilities arise:

− increase of biomass through contamination of the freshwater fauna when in hypohaline aquatic milieu;

− disappearance of benthic macrofauna when moving towards the evaporitic pole, which is the limit of the passage into the terrestrial domain.

Thus, at thelevel of zone IV, the decrease of benthic biomass and the increase of phytoplanktonic biomass are demonstrated by a crossing of the two curves.

Insofar as the majority of the peri-Mediterranean lagoons are located within zones III, IV and V, this phenomenon is shown by a quantitative antagonism in the milieu between benthic macrofauna and phytoplankton. Thus, at the Bahiret el Biban, the confinement gradient results in phytoplanktonic biomass increase, and a benthic biomass decrease (Guelorget et al., 1981). This is also the case at Biguglia, for instance, where the inverse evolution of these two gradients is obvious (Frisoni et al., 1983). On the contrary, Diana and Urbino, not highly confined and less rich in chlorphyll than Biguglia, possess a greater weight of macrofaunistic communities. The single point made by the crossing of the two curves is likely to correspond to a characteristic (as yet not explained) of the organization and the working of the paralic domain. However, considering this single point, it is obvious that the very high primary biomass (essentially phytoplankton and microphytobenthos) shows a large excess with respect to what is consumed by the low biomass benthic populations where the filter-feeders have disappeared and only browsers and detritivorous species remain. Thus the major part of the chlorophyll biomass is unutilized. Besides, within these zones situated at the hydrological limits, the masses of water move extremely slowly, thus creating conditions which contribute to the accumulation of organic matter. The highly "abnormal" salinity of these zones contributes to the stratification of the water, the generalization of the reducing milieux, and the diagenetic preservation of this organic matter. Zones V and VI therefore create sediments liable to become "mother-rocks" for hydrocarbons.

3.7.2 Production of Soft Bottom Pelecypoda - Example of the Etang du Prévost

The evaluation of benthic macrofauna production could only be approached with respect to pelecypods which have been the subject of a particular study in the Etang du Prévost (Guelorget and Michel, 1976; Guelorget et al., 1982).

This restriction to one zoological group is due to the impossibility of assessing the production of all the benthic species present in a lagoon, even though there are fewer species present than in the marine domain. It is impossible, with a reasonably workable sampling system, to collect enough individuals of each species to be able to assess their production. A great number of pelecypods, which are largely dominant in the Near paralic, appear in every sample of sediment and they offer reliable, easily measurable biometrical criteria, which is not the case for other groups, especially Polychaeta. It should also be remembered that pelecypod populations integrate the various factors of the milieu on a more or less long-term basis, owing to their relative immobility dependent on their connection with the bottom; they achieve this much better than the more mobile Polychaeta and crustaceans 1/.

The study of the production curves (Table 4) is based on the study of five species which alone constitute almost the whole of the benthic macrofauna (Venerupis decussata, Scrobicularia plana, Abra ovata, Venerupis aurea, Cerastoderma glaucum) (Guelorget et al., 1980; Guelorget and Mayere, 1981 a and b,- 1983 a and b).

The samples of sediment, taken monthly with a suction pump over an area of 0,5 m2, are sifted through a 1 mm grid. The measurements of the pelecypods (maximum antero-posterior diameter) are taken with callipers to the nearest millimetre. The biomass is measured in dry weight after decalcification. Production is expressed in g/m2/year (Table 5).

Within the malacological fauna, three groups of species can be distinguished:

− "Thalassic" species only found in the sea and in the vicinity of the grau which, because of their density and their biomass, represent only a minor fraction of the total macrofauna. They are not taken into consideration in the present study.

− "Paralic" species (Abra ovata and Cerastoderma glaucum) whose distribution follows the general rule of the increase in density and the decrease in the size in conjunction with the rising gradient of confinement.

− "Mixed" species (Venerupis aurea, V. decussata, Scrobicularia plana) present both in the marine milieu and in the lagunar milieu. Here can be noted a decrease in density and size as the confinement increases, but they have a high or very high biomass, considering the size of the individuals.

After having compared the result obtained by various methods, preference was given to the Bojsen-Jensen method (1919) adapted by Masse (1968) to short time-intervals (a month). This simple method allows the comparison of different milieux and takes migratory phenomena into consideration.

The profiles of annual production (Fig. 30) show the extent of the exchange zone of the mouth from where the total production descends from 890 g/m2/year down to 75 g/m2/year within only 200 m (Station 15). Moving away from this zone, annual output stabilizes or decreases more slowly, for the paralic species begin to take over from the mixed species. Finally, towards the lagunar confines (Station 11), whereas the production of mixed species is practically nil, that of paralic species increases distinctly, but here the prononced "dwarfism" of the individuals reduces the effect of the increase in density, and the yearly production remains moderate (40 g/m2).

This general pattern is valid when considering the seasonal variations in output: particularly the vivified zone of the mouth remains the main producer all year-round (Fig.

31). Winter and spring correspond to the maximum outputs (respectively 60 and 54 g/m2/year on average), for both the mixed species in the vivified zone and the paralic species in the lagunar confines. Production decreases considerably in summer and autumn (respectively 19 and 13 g/m2/year on average) because of the dystrophic crises ("malaigues") which affect mainly the paralic populations which are less soil-dwelling than the mixed species. In autumn, production is minimal, for it is the period of juvenile recruitment for all the pelecypod species, which yield only very little organic matter despite their high density.

The total annual output of the Etang du Prévost represents 171 000 kg of decalcified dry organic matter, which corresponds to a "marketable" quantity of around 1 700 t.

Further, the cartography of the annual overall production shows the economic advantages of the medium confinement zones (Zones II and III, Guelorget et al., 1982). Here, the malacological communities consist mainly of young individuals undergoing a phase of exponential weight increase, explained no doubt by the local concentration of larvae and the flow of nutriment originating in both the marine domain and the lagoon itself (Fig. 32).

Thus, the paralic domain shows capacities of production which are considerably superior to those of the marine milieu. Another fundamental characteristic is the small number of species constituting this production, which is moreover limited to a specific zone in the confinement area (moderate confinement zone where mixed species flourish).

3.8 QUALITATIVE AND QUANTITATIVE BIOLOGICAL ZONING OF THE PARALIC MACROBENTHOS ON HARD SUBSTRATE

The study of the soft-bottom benthic settlement of several Mediterranean lagunar milieux gave an insight into the biological organization of the paralic domain. Is the qualitative and quantitative biological zoning, established according to confinement and which applies to other biological compartments such as phytoplankton and ichthyofauna, also valid for benthic communities on hard substrate? The question is all the more important as hard substrates are rare or absent in the natural paralic milieux - especially in the lagunar milieux - and most of the lagoon-bred species are fixed species of considerable economic importance (oysters, mussels). Because of this lack of hard substrates in the paralic milieux, the authors followed the establishment and the qualitative and quantitative evolution of the benthic communities on bare artificial surfaces, immersed in the lagoons of Diana and Urbino, and the Etang du Prévost: a general organization system of the fixed communities appears both with regard to the specific composition and to the characteristic values of the macrobenthofauna.

3.8.1 Biological Zoning

This is an exact parallel with the zoning calculated for the benthic populations of the sediment.

Zone I includes most of the thalassic species of the infra-coastal communities of the Mediterranean rocky coastlines (Peres and Picard, 1964).

In zone II, the taxonomic diversity is considerably reduced and comprises only just over fifty thalassic and mixed species. Among these, the more commonly found are: Hydroides elegans, Serpula vermicularis, Bowerbankia imbricata, Bugula stolonifera, Anomia ephippium, Modiolus barbatus, Botryllus schlosseri (see Appendix 2). The plant

communities are dominated by Bangia fuscopurpurea, Ceramium rubrum, Padina pavona, Cystoseira (various species), Acetabularia mediterranea, Codium vermilara, Halimeda tuna. In the vicinity of the boundary between zones II and III reef structures with Neogoniolithon notarisii may appear (Denisot et al., 1981).

In zone III, mainly the mixed suspension-feeding species develop: Mytilus galloprovincialis, Ostrea edulis, Avicula hirudo (Bahiret el Bou Grara), Ciona intestinalis, Styella plicata, Phalusia mamillata, Bugula neritina, Membranipora membranacea, Balanus eburneus, represent most of the fixed fauna. In the flora, various species of Enteromorpha and Ulva lactuca appear.

Zones IV and V are characterized by a small number of paralic species, and the community is dominated by the Cirripedia Balanus amphitrite amphitrite (associated with B. eburneus), the pelecypod Brachydontes marioni and the Polychaeta Mercierella enigmatica (Vuillemin,1985), which is liable to produce reef-type structures, as for example in the northern lagoon of Tunis. The flora is almost exclusively composed of Chlorophyceae Enteromorpha gr intestinalis and Ulva lactuca locally associated with Gracilaria verrucosa.

All fixed macrofauna disappears in zone VI to give way to a cyanophytic covering (Lyngbya confervoides, Callotrix eruginea, C. scopulorum, Anabaena sp., Oscillatoria nigro-viridis).

3.8.2 Biological Gradients

These were analysed from data collected in the Etang du Prévost (Fig. 33) and were confirmed by studies on other Mediterranean lagoons, the Venice lagoon (Barbaro and Francescon, 1976; Francescon and Barbaro, 1976) and lagoons of eastern Corsica.

From the communicating link with the sea towards the confines of the lagoon, these can be observed:

− a significant decrease in number of species − a considerable increase in density − a decrease in biomass, despite the increase of density, with regard to the

small size of the individuals (lagunar dwarfism).

This pattern, identical to that of soft-bottom communities, remains the same whatever time the colonization of the immersed areas takes. However, it is obvious that both biomass and density increase with time at a given point.

As with the soft-bottom communities, the paucity of species, counter-balanced by a high density, generates an incomparably greater biomass than in the marine domain, especially in zone III where mixed species thrive. In this zone, after a year's immersion, at the Etang du Prévost can be collected a biomass of 11 000 g/m2 (in dry weight after decalcification), 96% of which consists of Mytilus galloprovincialis 1/.

This shows how well-founded the empirical methods are, based on an ancestral knowledge of the milieu. They allowed the development of conchyliculture in the paralic domain, whereas random trials, often carried out for political reasons without any previous knowledge of the host milieu, have never led to profitable production because they were situated in zones of inadequate confinement. The economic interest of the zoning of the paralic milieux can be understood in terms of confinement, since it allows the definition of optimal zones of conchyli-cultural - or even aquacultural - activity. It is even possible to envisage, through development work on the passes, the modification of a given basin with regard to the confinement scale.

3.9 CONCLUSIONS REGARDING THE CONFINEMENT PARAMETER

Because confinement appeared to be the fundamental parameter of the organization of the paralic domain, the authors thought it necessary to propose an explanation and define its main levels. On the basis of the qualitative biological zoning of the benthic macrofauna, they have established a scale of confinement on which are expressed the various qualitative and quantitative parameters which describe ths biological organization.

The arrangement proposed for the dynamics and the biological structure of the ecosystems of the Mediterranean paralic domain appears to apply to each of them, taking into consideration its morphological and hydrological particularities. Thus the different zones defined can vary as to their surface area and localization, but in all cases the zoning is respected.

The study of other biotic (meiofauna, microfauna) and abiotic (Appendix 3) parameters will permit to refine, and indeed perhaps enlarge towards the Far paralic, the zoning which has been established for the Near paralic Mediterranean domain.

However, the knowledge of extra-Mediterranean milieux such as the Caribbean mangrove swamps (Belle Plaine and Manche à Eau; Guelorget and Gaujous, 1983), the Red Sea lagoons (Guemsah; Ibrahim et al., 1982) and the Persian Gulf lagoons (Khour el Hadid lagoon; Perthuisot and Jauzein, 1978), and bibliographical data confirm the general nature of the proposed model (Appendix 6). 1/ "Rhopism" and "rhopic barriers" are sometimes mentioned; notions which involve the various major ionic

relations of the milieu. A definite value of a particular relation would be "limiting" for a particular species; moreover certainty of the rhopic values would determine the morphology and/or the layout of the tests (pelecypods, ostracodes). As far as ostracodes are concerned (Carbonel, 1982; Paypouquet, 1977)., it seems that the last-mentioned type of rhopic action could be suggested. However, present knowledge of the hydro-chemistry and biology of paralic milieux leads one to entertain doubts about the part it plays in the distribution of species.

1/ Thus, at Salin-de-Giraud, for instance, in the zones where the water flow is great (near the sluice valves, in the by-pass channels), the biological associations show a confinement lower than the totality of the condensers where they belong

1/ These are species of the paralic milieux that can be found in the sea (and conversely). The possibility of a genetic difference between the paralic populations and the thalassic populations cannot yet be discarded (work in progress with Madame Thiriot - Zoological Station, Villefranche-sur-Mer.

1/ Among the Peridinieae, it can be noted that Exuviella marina and Prorocentrum micans, "mixed" species of notable size, are to be found especially in zones II and III, whereas their paralic homologues, the smaller Exuviella compressa and Prorocentrum scultellum, colonize mainly zones IV and V (Appendix 3).

1/ It should be remembered that pelecypods (and gastropods) are often the only paleontological remains in the fossil paralic series. If a specific analysis of the tests allows for a determination of the position of the paleofauna in question on the confinement scale, insofar as such a scale is established for the period and the biogeographical province considered, the paleobiometrical measurements of these tests are likely to yield valuable information about the biological productivity of the immediate environment and, with reference to the organization of the present-day paralic domain, about the level of paleobiological production of the sedimentary basin concerned.

1/ The figures obtained at the Etang du Prévost (Fig. 33) can be compared with those collected by Relini et al. (1972)in the the Venice lagoon, where the fouling production varies between 3 370 and 18 510 g/m2 (dry weight) on an annual cycle.

4. HYPOTHESES CONCERNING THE INFLUENCE OF CONFINEMENT ON PARALIC COMMUNITIES

4.1 CONFINEMENT IN TIME, STABILIZATION OF THE GRADIENT, THE ROLE OF DEPTH

There are at present certain basins having poor communication with the sea but which do not have the special biological characteristics for high confinement: this is the

case of the Gulf of Amurakikos in Greece. In this respect, it can be noted that the volume of water if can tains (8 km3) is high with regard to its surface area (460 km2). Moreover, it is of recent formation, since the Amurakikos depression has been flooded by sea for a relatively short time, 8 000-10 000 years at the most. It can therefore be assumed that the initial stock of marine water has not yet had time to become sufficiently impoverished, bearing in mind its communication with the open sea. The rarefaction of certain species (according to local fishermen) is however perhaps the indication of the beginning of confinement.

Other examples are even more striking. The Caspian "Sea" (77 000 km3 and 436 000 km2) has been cut off from the marine domain since the end of the Tertiary, i.e., for several million years. However, it still contains 60% of thalassoid forms (Zenkevitch, 1957), and therefore has not yet shifted into the continental domain (apparently most of the basin is situated in zones III and IV as defined above), in spite of the great time-lapse since it was cut off from the sea. Moreover, the maps of biomass distribution established by Zenkevitch from 1935 onwards show that the upper layer of the deep southern basins is less confined than the shallow northern basin (Appendix 3).

The Aral "Sea", whose history is very similar to that of the Caspian Sea but which is much less deep, shows a distinctly higher degree of confinement, with communities far richer in freshwater species (zone VI and beyond).

Therefore, the depth parameter plays an important part in the speed of confinement of a basin in the process of being cut off from the sea; it may also be deduced that the deeper a paralic basin is, the slower its biological response to a diminishing exchange with the sea will be.

It was suggested earlier that confinement influences the communities through an impoverishment of the milieu in certain "vital" elements of marine origin - or, at least, coming essentially from the sea - consumed and immobilized by the living beings themselves, whose activity (with regard to the solar energy available) is at most proportional to the surface area of each basin, whereas the initial reserves of "vital" elements are proportional to the volume of the basin. Thus, in extremely shallow paralic basins, the confinement gradient very quickly reaches its state of equilibrium. In deep basins, on the contrary, the impoverishment of the milieu and the stabilization of the confinement gradient are much slower and, if ever they become completely cut off from the sea, a considerable lapse of time is needed for their total "continentalization" to occur.

4.2 INTRACONTINENTAL "PARALIC" BASINS

Mention has already been made of the Caspian which today is an intracontinental basin but which once was in communication with the sea.

This is not the case with some smaller basins - fossil or present-day - of North Africa and the Sahara, which contain or have contained in a recent past, typically paralic flora and fauna: for example, the chotts of pre-Saharan Tunisia (Coque, 1962; Levy, 1982) the Pleistocene lake of Shati, in Libya 1/ , the Fayoun in Egypt, which is the outlet of a branch of the Nile and where a typically paralic fauna thrives 300 km away from the sea (Cerastoderma glaucum, Bittium reticulatum, Brachydontes marioni).

Finally, there is the Sebkha Mellala near Ouargla, in Algeria, where Boye et al. (1978) described associations of Cardium, Melania, Hydrobia, Melanopsis, with Foraminifera (Ammonia, Nubeculariidae, Discorbidae) and Oogonia of Characea, in sediments dated between 8 000 and 10 000 years ago.

These were paralic communities which could imply:

− on the one hand, that the continental waters which fed the endoreic basin were re-creating a biochemistry close to that of the paralic domain

− on the other hand, that the milieu had been inseminated by paralic species, and therefore that there was a relationship with the sea, via the usual paralic domain

This second phenomenon is easily explained: perhaps air-borne transportation by paralic birds or by waterspouts, etc.

The first opens up strange horizons, and it is possible to imagine the fortuitous existence of endoreic basins whose catchment area is such as to allow the creation of biochemical conditions compatible with the development of thalassic species.

The authors propose that these endoreic basins with a paralic flora and fauna be termed "telehalic".

4.3 IMPORTANCE OF TRACE ELEMENTS IN THE BIOLOGICAL EXPRESSION OF CONFINEMENT

In the preceding sections, it has already been suggested that confinement should correspond to an impoverishment of the milieu in "vital elements" coming essentially from the sea, and consumed or trapped by the living beings themselves. Among these elements are vitamins and trace elements which include the heavy metals.

In the marine domain, numerous studies have shown that indeed a great many of these elements are "limiting" as far as their concentration and above all their "bio-availability" are concerned. Obviously, the critical values vary according to the species (Provasoli, 1963, Belser, 1963). Moreover, it is likely that these "vital elements" include bodies as yet unknown.

Considering only the heavy metals, many studies have shown the accumulation of certain of these in the tissues of benthic animals and, consequently, pro parte in the sediments (Table 6). In this respect, molluscs, especially Lamellibranchia and Gastropoda, and also Polychaeta, seem particularly efficient.

Thus, the zones of the paralic domain where molluscs predominate (zones II, III, IV) constitute a trap for heavy metals, and a screen for the populations of the more confined zones. It is possible to consider that in these zones, it is the species whose requirements in heavy metals are lower, or species which cannot tolerate them in very high concentrations which develop. For example, according to a study on the Nereis diversicolor of the Humber estuary (Jones et al., 1976) it seems that these Polychaeta lose their osmoregulatory capacity when the milieu is too rich in Cu. This seems to be true for other paralic species, particularly crustaceans.

4.4 CONCLUSIONS REGARDING THE INFLUENCE OF CONFINEMENT

It can be supposed that if confinement indeed corresponds to an impoverishment of the waters in "vital elements" its influence is two-fold:

− it sets limits to the extension of species for which there is a minimal threshold, for one or several elements

− it contributes to the development of species for which there is a maximal threshold beyond which one or several elements would act as poisons

In detail, the situation is likely to be more complex and, there may be an interaction (synergy) between several chemical variables of the milieu. However, schematically, the first type of action could apply to mixed species, and the second to strictly paralic species.

Thus, the biological zoning of the paralic milieux would be controlled by all the biohydrochemical gradients of the "vital elements", each playing a positive, neutral or negative part, according to the species.

In this discussion, the following comments extracted from a study by Amanieu, Ferraris and Guelorget (1978) should be considered:

"From a historical point of view, the "i" ecosystems are presumed to have preceded the "s" ecosystems, in order of evolution 1/, from a geographical point of view, the "i" strategies are presumed to have been preserved in those regions of the globe where the factors of the milieu show great fluctuations, whereas the "s" strategies are presumed to have developed in the sites with a stable environment."

"We were discussing ... the reputation of the lagunar ecosystems of being fragile, juvenile and highly productive. Here we have not broached the subject of productivity which is indeed very high; but then, we have stated why the term juvenile seemed to us inadequate. Lastly, as far as "fragility" is concerned, we consider that it cannot be disassociated from the stability which we have discussed. Indeed, the coastal lagoons are fragile and vulnerable, both to natural disturbances and to the inconsiderate behaviour of man. But, on what time scale and to what extent? Even since Strabon and Pliny has it not been common knowledge that the "malaigues" ruin our lagoons and that excessive fishing depletes them? Thus, for over two thousand years, our fragile lagoons have been in the process of dying. Yet how many "stable" peaks have for ever disappeared in the meantime? All through the cyclic vicissitudes from malaigues to "martegades" (winter freezing-over), through years good and bad, it may be that these everlasting swings of the pendulum, these steps forward hesitating between the worst which never happens and an ever-hoped-for improvement, are not accidents of the ecosystems, an ecosystem into which nearly 100 generations of local fishermen have been integrated, but its very nature. It is true that "Homo economicus" now has the means, with sea-front developments, motorways, stabilized shores and "lagooning", to stabilize and "valorize" once and for all these great coastal ponds which will soon survive only as a memory." 1/ This intracontinental paralic system has recently been the subject of a pluri-disciplinary study (Petit-Maire et al.,

1982). Taking into account the considerations developed in the present work, the authors express utmost reserve as to the paleo-environmental reconstitutions (incidentally, based on contradictory data), especially concerning the salinity of the milieu.

1/ The i-type cenotic strategies (Blandin et al., 1976) concern ecosystems which are poor in species but rich in individuals (typical case of paralic communities). The s-type strategies concern ecosystems with high diversity and rich in species (thalassic communities).

5. THE SCIENTIFIC IMPORTANCE OF THE PARALIC DOMAIN

If the paralic domain' is seen as an autonomous entity, in terms of the Earth and its history, it takes on a dimension other than that of a mere contact (or exchange)* zone between the land and the sea.

5.1 FROM A BIOLOGICAL AND PALEONTOLOGICAL POINT OF VIEW

The sediment associated with the first traces of life on Earth ("sensu lato" stromatoliths) along with various considerations (concentration of nutritive elements, indications of shallow or intermittent water levels) imply that it was truly in a paralic

milieu that life first appeared on our planet, about 3 000 million years ago (Cloud, 1968). In this respect, it is indeed the paralic rather than the marine domain which appears as the original one (see inset, Fig. 34).

The paralic domain is an obligatory transit channel for anadromous and catadromous species. Moreover, because of its high productivity (Fig. 34), it represents a place of growth for many species which breed at sea, and is probably indispensable for the completion of the reproductive cycles of many marine species.

A remarkable property of the strictly paralic stocks is their slow almost non-existent evolution. This is the case of the "lagunar" molluscs whose shape has hardly changed since the Tertiary, or certain Foraminifera, which have remained identical since the Cretaceous (Ammonia tepida). Lingula are an example of a paralic "panchronic" form (Appendix 3). By comparisjon, the rapid evolution of marine or continental stocks is clearly revealed by stratigraphy (cephalopods, echinoderms, branchiopods, mammals, etc.). There is an obvious reason for the stability of paralic stocks: the species are adapted to a certain physical and chemical variability of the milieu and, if confinement is the essential parameter of the distribution of the species, there is always a zone in the paralic domain where an adequate degree of confinement allows them to survive. Thus, the physical or chemical variations of the milieu exercise only a low or indeed non-existent "selective pressure" on the paralic stocks, and the genetic shift is extremely weak. Conversely, in the marine or continental domains, the stability of the abiotic parameters of the milieu is greater and their variations slower: there, the species are much more specialized and unable to adapt to sudden changes should they occur: these stocks are therefore condemned to evolve or disappear in case of "crisis" in the domain (variation of the salinity, temperature, currentology, etc.).

One may even wonder whether the paralic domain does not constitute some sort of potential reservoir for stocks capable of replacing the marine or continental stocks in case of a crisis leading to the disappearance of the latter.

The problem arises of the species that hitherto referred to as "mixed" i.e., species which colonize both the marine and the paralic domain, in spite of the obvious differences in the biological dynamics of these domains. It should be noted that many mixed "species" have an ambiguous taxonomic status, with duplications of specific nomenclature or an adventitious nomenclature (a variety of the form referred to, or "endemic").

Moreover, recent research in population genetics (electro-phoretic spectral analyses of the inner milieu) detects notable differences within a same mixed species, between the marine and the paralic populations (Worms and Pasteur, 1982; Buroker et al., 1979; Kerambrun, personal communication).

This would tend to show that in any case, at the borderline between the paralic domain and the marine domain, the notion of species is to say the least ambiguous - as regards mixed species, naturally.

This could well be taken as an indication of the colonization of marine species by stocks from the paralic domain. Could there be a symmetrical phenomenon as regards continental species?

Thus, it seems that far from being a marginal or secondary domain its permanence and its biological stability cause the paralic domain to be a fundamental element in the history of the biosphere.

5.2 FROM A GEOLOGICAL POINT OF VIEW

The various types of sedimentary deposits in the paralic domain have been considered, and also the importance of the biogenic carbonates in the Near paralic, that of the evaporites (Appendix 3) and of the "deltaic" structures in the Far paralic. The main characteristics of these deposits is their high speed of sedimentation (the highest known in present-day natural conditions) by comparison with the oceanic domain where the rate of sedimentation is minimal, and with the continent where erosion predominates. The paralic milieux make a leading contribution to the accumulation of sediments on the continental shelves.

The importance of the sedimentation of organic matter should also be remembered. The high intrinsic productivity 1/ (Fig. 35) of the paralic domain is combined with its ability to conserve organic matter, owing to the prevalence of reducing zones, hyperhaline or otherwise. The coincidence or the frequent proximity of mother rocks, detrital or carbonated deposits (possibly dolomitized under the influence of magnesian brines) liable to constitute reservoirs, and muddy or evaporitic sediments able to produce excellent covering, explain the presence of a great number of hydrocarbon deposits in the paralic series. Coal beds, for a large part, also belong to the paralic domain (Barrabe and Feys, 1965). However, whereas with petroleum and natural gas the organic matter is essentially autochthonous, the continent makes a large contribution in the case of coal.

The observation of present-day nature offers only an incomplete idea of the geological importance of the paralic domain (Perthuisot, 1980)1/. The study of the sedimentary series of the great plateaux of the past shows that at certain periods in time, particularly outside the "orogenic phases", the paralic mi lieu could spread considerably, over a region several thousands of kilometres wide (Busson, 1972) 2/.

These few considerations show the fundamental role, too often ill-known, played by the paralic domain in the geological and biological history of our planet. 1/ Here the shift in the curves of the benthic biomass and the phytoplanktonic biomass with regard to an

increasing confinement (Fig. 29) should be remembered. It implies that, in high confinement zones (IV and V), the primary production shows a large surplus with regard to consumption: this surplus or organic matter will be deposited and form in fine a mother rock, if the conditions of conservation of that organic matter during the diagenesis are favourable.

1/ The Baltic is one of the largest paralic basins in the world today, with a surface area of 420 000 km , and a length of 1 500 km (Appendix 5).

2/ Busson was one of the first geologists to state the existence of an original domain between the sea and the continent, which he calls the "intermediary domain", but he considered it more as a "mixing zone" (particularly for fauna) than as an autonomous entity.

6. THE ECONOMIC IMPORTANCE OF THE PARALIC DOMAIN

6.1 MINERAL PRODUCTION

The paralic domain supplies fossil energy resources, coal and oil and the fossil or present-day evaporitic areas abundantly supply minerals and elements indispensable for human activities:

− halite (NaCl) comes almost exclusively from the paralic domain; world production was 143 million t in 1971

− gypsum (CaSO4 , 2H20) is mainly used for making plaster but is also a constituent of cements

− potassium salts, sylvite (KC1) and carnallite (KMgCl3, 6H20), essential constituents of fertilizers (world production in 1971: 17 million t

− elements extracted from connate brines or from the mother-waters of salt marshes (Mg, Br, K, etc.)

There seem to be genetic connections (which are incidentally variable) between the confined milieux and certain metal-bearing deposits, particularly of lead, zinc, copper, magnesium (Routhier, 1980; Laginy, 1980; Quemen'eur, 1974). As seen before, the processes of biological concentration of some of these elements are probably closely related'" to the notion of confinement.

6.2 FOOD PRODUCTION

The rich biological resources and the high productivity of paralic milieux explain their having been exploited since antiquity and probably even since long-distance prehistoric times.

6.2.1 Gathering of "Sea Food"

This is, of course, the first form of exploitation, and can be intensive, for - unlike the very fragile marine ecosystems which can be seriously perturbed by over-harvesting (even to the extent of a possible disappearance of certain species) and furthermore highly sensitive to "pollution" - the paralic domain, because of its biological stability and high productivity, appears to be practically inexhaustible; for example, the conehylicultural yield of the lagoons of Diana, Thau and Urbino is around 15 t/ha/year of marketable products.

6.2.2 Halieutic (Fish) Production

Amanieu and Lasserre (1981) demonstrated the production levels of the Mediterranean coastal lagoons and their contribution to the enrichment of demersal fishing. The lagoon fishing-grounds represent an 8-10% contribution to the entire Mediterranean catch (the Black Sea excluded )1/; but it must also be emphasized that these lagoons represent feeding nurseries for young fish during their first years of life; later these same fish will be found in the marine fishing-grounds.

(a) Levels of halieutic production in the Mediterranean lagunar sites

After analysis of data collected from 46 peri-Mediterranean lagunar sites, Amanieu and Lasserre (1981) demonstrate "the extreme disparity of the outputs which vary from 6 kg/ha/year (Nador lagoon, Morocco), to 149 kg/ha/year (Venice lagoon, Italy) or even 172 kg/ha/year (the Ebro estuary,Spain); therefore, an average figure would be meaningless. On the contrary, it would be useful to have a better knowledge of the Mediterranean lagoons' production potentialities. A preliminary estimate would be different from the above, the variations noted at present being largely due to the conditions of exploitation. If the true potentiality is equal to the highest yield of the Spanish or Italian lagoons, i.e., around 150 kg/ha/year and by extrapolating to the 883 720 ha of available lagoons around the Mediterranean, a potential annual production is

obtained of 125 000 t, more than twice the present yield. However, such an estimate remains speculative and artificial.

(b) Contribution of the lagunar sites for the enrichment of inshore fishing

According to Amanieu and Lasserre (1981): "The lagunar sites are very irregularly distributed along the coast of the Mediterranean. The level of inshore captures also varies considerably and, so it seems, in relation to the proximity of lagoons likely to contribute to trophic supplies, or indeed recruitment. Thus, the lagunar milieux of the Adriatic and the Gulf of Lions, which cover around 280 000 ha, i.e., nearly 34% of the totality of the Mediterranean lagoons, are themselves among the most prolific sites (between 80 and 150 kg/ha/year). They furthermore run parallel with the coastline where the production of demersal fishing grounds is particularly high, about 35-50 t/km of coast. On the contrary, such regions as the French Riviera, the south of the Peloponnese in Greece, or the Libyan coast, are characterized both by a low rate of productivity (between 0 and 5 t/km of coast) and by the absence of coastal lagoons. Such an estimate can only remain rough insofar as it does not take sufficiently into account the surface areas of the fishing grounds, which depend on the local width of the continental shelf".

Thus, compared with the sea coast, the lagoons contribute largely to the increase of the halieutic demersal potential: the zones of high demersal production correspond to the zones where the lagoons are large and productive. The paralic domain appears here again as a highly productive ecosystem which exports a good deal of its excess energy towards the sea (Fig. 36).

6.2.3 Aquaculture

As confinement contributes to the reduction of species and simultaneously the increase in their density, it is clear that the paralic milieux are propitious for mono- or pauci-specific culture, provided that the species chosen are adequate: it is from this point of view that aquaculture has developed, first empirically, and now more rationally. Today, most aquaculture takes place in paralic milieux.

6.3 FUTURE PROSPECTS

6.3.1 Biomass

The considerable productivity of the paralic domain can no doubt be used for the creation of energy if, for instance, the production of organic matter by phytoplankton or by the algal fields of Cyanophyceae, or again the amazing rate of growth of certain algae such as the Ulva and the Enteromorpha (Appendix 10) are considered.

A reasonably long-term exploitation of such potential resources can therefore be envisaged.

6.3.2 A Strategy for the Development of the Paralic Domain

Given that the hypotheses concerning the paralic domain and its dynamics continue to be proved correct through observed facts, an overall strategy for the development of these milieux is proposed.

First, the place of each basin on the confinement scale and the shape of the various confinement zones it includes determine its biological potentialities (species likely to develop, productivity rate, location of the beds, etc.). The ecological characterization in terms of confinement of these stretches of water (Guelorget et al.,

1983) is then an essential phase of their development, without modifying the initial natural environment (Appendix 6).

However, if, in the marine or continental domain, man has only limited means of action over the fundamental parameters of the milieu, the situation is different in the paralic domain, since it is possible, often quite cheaply, to influence the confinement. This is exactly what salt-producers do in the salt marshes, when they adjust the inflow of seawater and the displacement speed of the brines, according to meteorological variations and, with a specific aim, the production of salt.

It is therefore possible to envisage, for each paralic basin, the regulation of the confinement, and possibly its gradient values, to the quantitative and qualitative requirements of a coherent economic policy. Thus , the present effort to develop the coast must necessarily be accompanied by the study of the paralic milieux. 1/ The overall production of Mediterranean fishing is relatively well known, around 1 200 000 t/year (Statistical

Bulletin, GFCM, No. 3, 1980). According to Levi and Troadec's figures (1974), the exploitation of the totality of the lagoons represents a third of the demersal fishing in the Mediterranean.

7. CONCLUSIONS

The existence of a fully fledged ecological domain has been confirmed; this term being understood in the widest and most multidisciplinay sense which has its own dynamics: the paralic domain. Its principal originality lies in its obvious geographical dispersion at the present time and in the spatial and temporal variability of its abiotic parameters. Paradoxically but logically, these two fundamental characteristics lead to both a deep biological unity and biological stability.

The geographical dispersion of the paralic domain is only apparent, since every coast, even that most open to the sea, has its more or less extensive paralic fringe, if only because there is always an intertidal fringe (Appendix 3). Moreover, there are many transfer agents between the different basins, the sea itself, the living beings it contains, sea birds, etc.

The variability of the abiotic parameters results in incomparably greater hydrochemical gradients than those found in the other domains of the hydrobiosphere, even if one considers endoreic basins, which are often highly homogeneous at a given moment of the seasonal cycle.

The biological unity of the paralic domain appears in a qualitative and quantitative zoning common to the entire sub-domain close to the marine domain (the Near paralic). This zonal organization, which affects the species (mainly thalassoid) depends on the confinement (relative to the sea), an inconspicuous but obligatory parameter in any paralic system. Beyond this, in the Far paralic, the communities very rapidly become richer in freshwater species, or in species characteristic of the evaporitic milieux: confinement becomes too great and appears therefore to lose its leading biological role. However, perhaps later studies may prove the contrary.

In order to be able to compare the spatial and temporal variations of the different biotic and abiotic parameters which, in each basin of the Near paralic or within the sub-domain as or whole, describe the confinement field (and gradient), is a scale proposed based on the communities of benthic invertebra which best assimilate the minor fluctuation of the milieu. The other links of the trophic chain, along with the quantitative variations of the different biological parameters (number of species, biomass, density, production, productivity), can satisfactorily and logically be incorporated in this scale.

The exact way in which confinement affects the distribution of the paralic living beings still remains unclear, but various reasons allow the supposition that confinement results in an impoverishment of the milieu in "vital" elements of essentially marine origin, trace elements, small organic components (vitamins, alkaloids, etc.), around which the communities are organized.

These different considerations lead to a qualification of the paralic domain as a biological cross-roads between the marine and continental domains (evaporitic and freshwater), to abandon the marginal status it was hitherto conceded, and on the contrary to grant it a key position in the history of the biosphere. The paralic domain is moreover a preferential place for the accumulation of biogenic sediments (deltaic series, evaporites) and organic matter on the surface of the Earth, and, in this respect, plays a fundamental part in the history of the lithosphere.

To conclude, the various original characteristic features of the paralic domain - both biological and geological - open up considerable economic possibilities, incomparably superior to those of the Oceans, both in the present and in the fossil realms. In this respect, the study of confinement and its various mineral and biological expressions appears as an effective, reliable, and inexpensive instrument for the development and the survival of the paralic domain.

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Table 1

Taxonomic resources of the phytoplankton in various Mediterranean paralic basins.

Basin Taxonomic resources Prevailing confinement zones

Lagoon of Diana 41 II (III) Mar Chica 29 II Lagoon of Mauguio 13 IV (V) Lake Melah 10 IV - V Bermuda Triangle 8 IV - V (VI) Lagoon of Biguglia 27 V - VI

(freshwater influences)

From FRISONI's data, personal communication

Table 2

Number of species and diversity of the ichthyological populations of the lagoons of the eastern plain of Corsica. From FRISONI et al., 1983.

BIGUGLIA DIANA URBINO

Mouth Centre of lagoon South North North South

Sedentary 78 % 82.0 % 94 % Migrant 21 % 10.4 % 4 % Occasionals (% of the captures*) 1 % 7.6 % 2 % In numbers M/S * Ratio (numbers) (1981) 0.31 0.07 0.09 0.15 0.03 0.02Number of species* (Migrant + sedentary) (numbers) 16 11 23 20 20 18ESH (numbers) Maximum 0.659 (Spring) 0.716 (Summer) 0.558 (Summer)

Minimum 0.463 (Winter) 0.304 (Winter) 0.342 (Winter)

* All periods taken together

Table 3

"Sedentary" species : percentages of captures (Over one year)

BIGUGLIA DIANA URBINO Mouth Centre North South Mouth North South

Year 81 81 81 (79) 81(79) 81 81 81 Atherina 26 61 83(87.8) 68(71.3) 38 65 72 Aphanius 36 20 13(2) 17(13) 59 29 25 Syngnathus 1 18 2(3.6) 9(2.3) 1 4 1.5 Mottled Goby 38 1 2(6.6) 6(13.2) 2 2 1.5

weight Atherina 58 81.5 89 87 51.5 75 75 Aphanius 24 15.0 9 12 47.5 20 23 Syngnathus 0.5 2.5 1 0.5 0.5 2.5 1.5 Mottled Goby 18 1 1 1 0.5 2.5 0.5

Table 4

Quantity of Molluscs in the Etang du Prévost

N = Density in number of individuals per m2

B = Biomass in grammes of decalcified dry weight per m2

Fine sands Muddy sands Lagunar muds. North emis West. bas. East. bas.

St.X St. 15 et 16 St. 7

N B N B N B N B N B Molluscs 9052 401,3 3083 33,8 1083 15,7 4031 8,4 919 8,3

Total fauna 9716 415,4 4010 44,9 6282 22,6 7331 16,9 2851 14,9

Table 5

Average values of density and biomass, annual production of the main Pelecypod Molluscs at different stations in the Etang du Prévost

Station X Station 16 Station 15 Station 7 N B P P/B N B P P/B N B P P/B N B P P/B

Venervpis decussate 641 56,7 221,7 3,9 427 8,6 30,4 3,5 139 0,7 2,2 3,1 Venerupts aurea 2705 54,6 190,9 3,5 1546 39,8 65,4 1,6 1301 13,5 45.3 3,3 Scrobicuhria plane 4901 122,9 451,9 3,7 1952 6,3 34,3 5,4 1162 5,1 16,2 3,2 322 9,6 29,1 3.0Cerastoderma glaucum 189 7,4 24,2 3,3 68 1,3 0.1 0,1 234 2,6 8,4 3,2 428 9,8 43,9 4,5Abra ova ta 312 0,5 1,0 1,9 561 1.6 4,3 2,7 506 1,4 5,3 3.8

TOTAL 8436 241,6 888,7 4305 56,5 131,2 3397 23,5 76,4 1256 20,8 78,3

Station 12 Station 11 Station 4 Station 3 N B P P/B N B P P/B N B P P/B N B P P/B

Venervpis decussata 11 0,2 0,6 3,0 Veverupis aurea 30 0,5 1,4 2,8 Scrobicuhria plana 22 1,1 4,4 4,0 Cerastoderma giaucum 2401 11,0 26,8 2,4 6071 8,0 38,2 4,8 613 9,4 22,3 2,4 260 4,4 10,9 2,5Abra ovata 503 1,2 6,4 5,3 1220 0,8 2,5 3,0 492 2,6 6,1 2,3 199 0,7 2,8 4,1

TOTAL 2904 12,2 33,2 7291 8,8 40,7 1168 13,8 34,8 459 5,1 13,7

N : Number of individuals per m2 B : Weight of dry organic matter in g/m2

P : Production of dry organic matter in g/ m2/year.

Table 6

Some heavy metal concentrations measured in the tissues (whole animal) of marine and/or paralic species (in ppm of the dry weight)

Ag Cd Co Cu Zn Pb Monodonta crassa (1) 1,1 33 30 Littorina littorea (1) 1,6 45 50 Thais lapillus (1) 28,5 198 535 Murex brandaris (1) 32,2 270 285 Anadara granosa (2) 56 Meretrix casta (2) 44 Cassostrea madrasensis (2) 99 Ostrea edulis (3) 1,8 90 710 0,1 Mytilus galloprovincialis (3) 0,5 2 36 0,2 Scrobicularia plano (4) 122 32,4 55 372 Nereis diversicolor (4) 8 3 25 947 Sea water (ppm) (5) 0,0003 0,0001 0,0005 0,003 0,01 0,00003

1. From BOUQUEGNEAU and MARTOJA (1982) 2. From KUMARAGURU and RAMAMOORTHI (1979) 3. From BURELLI et al., (1979) 4. From LUOMA and BRYAN (1982) 5. From GOLDBERG (1963)

Table 7

Comparison of the number of species in the Caspian fauna and the Mediterranean fauna

Mediterranean Sea Caspian Sea Group Number of

species % of total

fauna Number of

species % of total

fauna Echinodermata 101 1,7 0 0 Protozoa 138 2,3 3 0,7 Polychaeta 433 7,2 5 1,2 Pelecypoda 366 6,1 16 4,0 Gastropoda 937 15,6 26 6,5 Superior Crustaceans 620 10,3 90 22,5 Fish 529 8,7 71 19,0

Table 8

Monthly variations of the total lagunar algal (macrophyte) biomass (expressed in tonnes of drained wet weight). Etang du Prévost. From RIOUALL, 1976.

WEST BASIN CENTRAL BASIN,, EAST BASIN OVERALL LAGOON 25 April 1975 900 600 3700 5200 20 June 1975 1700 1800 3700 7200 23 July 1975 600 1800 1600 4000 25 August 1975 - - - - 10 September 1975 - - - - 28 October 1975 300 300 6300 6900 26 November 1975 400 200 2600 3200

7 January 1976 700 300 2900 3900 16 February 1976 600 200 1700 2500 18 March 1976 1200 800 3700 5700

Table 9 Monthly variations of the lagunar algal biomass (tonnes) for the four principal types. Etang du Prévost. From RIOUALL, 1976.

ULVA ENTEROMORPHA GRACILARIA STICTYOSIPHON 25 April 1975 3900 - 100 1000 20 June 1975 6800 - 200 - 23 July 1975 3800 - - - 25 August 1975 - - - - 10 September 1975 - - - - 28 October 1975 200 6250 150 - 26 November 1975 350 2700 100 - 7 January 1976 1000 2200 150 100

16 February 1976 1000 1200 150 50 18 March 1976 4600 800 50 100

Table 10

Present-day stratigraphic classification of the Paleogene formations in the Paris Basin (POMEROL, 1973), and equivalences with the classification of FURON and SOYER (1947).

Furon et Soyer (1947)Layers Sub-layers Layers

AQUITANIAN CHATTIAN STANPIAM s.s. STAMPIAN

STAMPIAN s.l. SANNOISIAN SANNOISIAN LUDIAN LUDIAN

MARINESTAN BAHTONIAN AUVERSIAN

BARTONIAN

LUTETIAN LUTETIAN CUISIAN CUISIAN

YPRESIAN SPARNACIAN SPARNACIAN THANETIAN THANETIAN

DANO-MONTIAN MONTIAN

Table 11 Composition of total thalassoid fauna for each of the Paleogene layers of the Paris Basin. From FURON and SOYER's data, 1947.

The continental forms (Limnaeidae, Helicidae, Pupidae, etc ...) were not counted

MONT. THAN. SPAR. CUIS. LUTE. BART. LUDL SANN. STAM. CHAT.Foraminifera 4 1 3 8 10 1 Coelenterata 10 2 30 23 Echinodermata 10 72 2 2 1 Bryozoa 1 23 130 12 9 Brachiopoda 2 2 18 2 Pelecypoda 21 137 14 340 550 380 44 2 118 Gasteropoda () Hydrobiidae

45 229 (7)

56(11)

555(10)

1136(19)

779(21)

59(12)

18 (9)

185 (7)

11(8)

Pteropoda 3 Cephalopoda 2 3 5 12 5 Anthropoda 6 5 5 2 Amphinora 3 7 1 Scaphopoda 7 21 10 1 Dominance of zones O-I IV-V V-(VI) (II)-III 0-I 0-I (II) IV-V V-Vl (II)-III VI

Fig. 1 - Map of isohalines in the Etang d'Urbino in February 1978 by

northerly wind. The encircled figures correspond to the salinity averages (in ‰) calculated over 1978 and 1979.

Fig. 2 - Salinity levels (in ‰) in the Etang du Prévost at

ebb-tide and by northerly wind and at flow tide by southerly wind. From GUELORGET and MICHEL (1976).

Fig. 3 - Total concentrations (in g/l) in the surface waters of the Bahiret el Biban, in August 1976. From MEDHIOUB (1979), modified. Appendix 1 contains a conversion table for the different parameters which can express the degree of dilution or concentration (by evaporation) of sea-water.

Fig. 4 - Salinity levels (‰) in the Etang de Biguglia, from May 1978 to March 1979.

Fig. 5 - Salinity levels (in ‰) of the Dybsø Fjord in May 1955

(Figures not in circles) and Aug. 1955 (Figures in circles). From MUUS (1967).

Fig. 6 - Macrophytobenthic associations in the Dybsø Fjord. From MUUS (1967).

Fig. 7 - Salinity levels (in ‰) in the Kysing Fjord. From

MUUS (1967).

Fig. 8 - Mangrove around the Grand Cul de Sac Marin (Guadeloupe).

Fig. 9 - Two paralic basins in the Guadeloupe Mangrove and their surface

water concentration levels. From data by GAUJOUS (1981).

Fig. 10 - Map of Cl concentration in the waters of the Khour el Aadid

(Emirate of Qatar, Persian Gulf), in January 1977. From PERTHUISOT and JAUZEIN (1978).

Fig. 11 - The Bocana de Virrila (Peru) and its total concentration levels (in

g/l). From MORRIS and DICKEY (1957).

Fig. 12 - The salt marsh of Salin-de-Giraud (Camargue). Passage

of virgin waters and density levels. Saturation in gypsum is obtained for a concentration of 150 g/l. Saturation in halite for a concentration of 350 g/l (See Appendix 1). From PERTHUISOT (1983).

Fig. 13 - The Baltic Sea and its salinity levels (‰) in May (average over

several years).From BOCK (1971).

Fig. 14 - Plant associations in the salt marsh of Salin-de-Giraud

(Camargue) 1. Communities of Ruppia spiralis and Enteromorpha gr.

Intestinalis 2. Algar fields of Lyngbya and Microcoleus 3. Algar fields of Microcoleus, Oscillatoria, Phormidium and

Spirulina 4. Phytoplanktonic communities of Dunaliella salina and

microphyto-benthic communities of Oscillatoria, Phormidium and Spirulina.

From THOMAS (1983) and PERTHUISOT (1983).

Fig. 15 - Quantitative variations of benthic macrofauna in the Etang du Prévost (France) and the Bahiret el Biban (Tunisia). From GUELORGET and PERTHUISOT (1982).

N.B. Local deposits of organic matter (contact between marine waters and lagunar waters, hydraulic umbilicus, zones of intense biosedimentation, inflows of polluting effluents) influence the benthic communities by transforming their qualitative and quantitative features: increase of detritivorous species to the detriment of suspension-feeders, appearance of species indicative of "organic pollution".

Fig. 16 - (a) Decrease in number of molluseun species in the

North Sea-Baltic transition area, related to mean salinities.(From Sorgenfrel, 1958: by courlesy of the author.)

(b) The coast of southern Texas showing, in circles, thedifference in relative diversity of the fossilizable faunas atthree localities. (Founded on data In Kornicker and Odum,1958.)

(Reproduction of two nearly consecutive figures, In AGER, D.V., Principles of Paleoecology, McGraw Hill Ed. (1963)

Fig. 17 - Diagram of the geochemical and biological organization of the paralic domain, with regard to confinement and hydrous deficit, taken as the sole parameters (see definitions, convention and comments in the text). From PERTHUISOT and GUELORGET (1982).

− In the above, the isohaline values of the totality of the paralic domain are diagrammatically represented as the branches of hyperbolae. Indeed, determining a saline concentration involves determining a ∆h, T couple (hydrous deficit, confinement) so that ∆h, T = constant.

For example, the isohaline corresponding to the normal concentration

(36 g/l) is represented on the diagram by the two axes, one for a zero hydrous deficit the other for a zero confinement.

One understands that the higher in absolute value the hydrous deficit is, the less necessary it is that confinement should be great in order to reach a specific concentration, conversely the smaller the hydrous deficit is (in absolute value), the greater the confinement has to be in order to reach any given salinity.

− In the simplest cases (salt marshes) where the hydrous deficit over the whole of the basin can be considered as uniform, the former is represented by a horizontal segment (or a thin rectangle), along which the geochemical and biological zoning of the basin can be read. The figure becomes more complex as the tributaries appear. They are generally situated around the continental or distal ends of paralic basins, when the contact with the sea is only occasional, when the seasonal climatic variations and the local hydrography entail considerable variations of the hydrous situation (e.g., Tunisian coastal sebkhas), or when the water stratification - to which depth contributes- removes a confined body of water from atmospheric influence. This is why it is impossible to maintain a rectangular diagrammatical representation : Therefore, the drawing is covered with branching lines in an attempt to picture the particular conditions of the distal zones.

− The various basins have been arranged on the diagram in the best way possible according to our work and that of other authors.

Fig. 18 - Diagram of the dominant sedimentological features of the paralic

domain with regard to confinement and hydrous deficit. From PERTHUISOT and GUELORGET, 1982, 1983. 1. Terrigenous detritals. 2. Biodetrital limestone. 3. Biochemical limestone. 4. Evaporites. 5. Autochthonous organic matter (oil ?). 6. Detrital organic matter (coal ?).

Fig. 19 - A diagrammatic representation of the biological zoning (Roman figures) in the pattern, of the Mediterranean paralic ecosystem. See definitions in the text. From GUELORGET et al., 1983.

Fig.20 - Diagrammatic map of confinement zones in the Bahiret el Biban.

From GUELORGET et al., 1982. N.B. The shading scale used here is the same as in figures 19 and 21-27

Fig. 21 - Diagrammatic map of confinement

zones in the Nador lagoon (Mar Chica or Sebkha bou Areg). From GUELORGET et al., 1983.

Fig. 22 - Biological zoning in the lagoons of the Louros Delta (Amvrakikos, Greece). Iso-halines in ‰. From GUELORGET et al., 1983.

Fig. 23 - Biological zoning in the lagoons of the eastern plain of southern Corsica. From GUELORGET et al., 1983

Fig. 24 - Biological zoning in the Etang de Biguglia in the northern part of the eastern Corsican plain.

From GUELORGET et al., 1983.

Fig. 25 - Biological zoning in "Lake" Melah (Algeria).

From GUELORGET et al., 1983.

Fig. 26 - Biological zoning in the Palavas lagoons (France)

From GUELORGET et al., 1983

Fig. 27 - Biological zoning in the Bermuda Triangle (Santa Pola, Spain) From GUELORGET et al., 1983, PERTHUISOT et al., 1983 and C. MONTY (in preparation). Isohalines en g/l. Unbroken line ; June 1981. Dotted line : October 1981.

Fig. 28 - Relative quantities of "migrant" and "sedentary" species in the ichthyofauna of the lagoons of Diana and Urbino From XIMENES, 1980.

Fig. 29 - A. A diagrammatic representation of the biological zoning (Roman

figures) defining the scale of confinement (in Arabic figures) in the model of the Mediterranean paralic ecosystem .

B. Variations showing the phytoplanktonic and benthic biomass in relation to the confinement scale. The single points of the diagram (maximum, curve crossings) remain fixed in relation to the biological zoning. The corresponding values of the biomass can vary according to the overall productivity of each basin.

Fig. 30 - Annual malacological production of the Etang du Prévost.

From GUELORGET et al., 1982.

Fig. 31 - Seasonal evolution of the malacological production of the Etang du Prévost. From GUELORGET et al., 1982.

Fig. 32 - Situation of the stations and levels of total malacological production (in kg/ha/year) in the different parts of the Etang du Prévost. From GUELORGET et al., 1982.

Fig. 33 - The biological gradients (specific richness, density, biomass) of the hard substrate macrobenthofauna in the Etang du Prévost. The stations concerned (X, 7, 11) are shown in Fig. 32. The biomass measured in January'76 corresponds to an annual production.

Fig. 34 - Comparative production of some terrestrial and aquatic systems, (Adapted from BASSON et al., 1977, and ALLEN et al., 1979)

Fig. 35 - The potentialities of the Near paralic in the genesis of hydrocarbons in

relation to confinement. In parallel, with an idealized section of a paralic basin, a diagrammatic representation of the various values which control the accumulation of potential mother rocks. It is noticed that, with regard to an increasing confinement, there are relay points between the various links in the chain of the primary production. Moreover, the organic matter, a consequence of the biological activity of all the consumers (dead bodies, various secretions, faeces), contributes to the organic sedimentation : it has been shown that the recent development of intensive aquacultural activities speeds up the enrichment of paralic sediments in organic matter.

Fig. 36 - A comparison between demersal halieutic production and lagunar production along the Mediterranean coastline. From the FAO Atlas (1972) and AMANIEU and LASSERRE (1981), modified. The coasts with a high demersal production always border on a large paralic domain.

Fig. 37 - Map of aragonite content (in %) of the superficial sediments in the lagoon of Logarou (Amvrakikos, Greece).

Dotted line : Presence of highly magnesian calcite (note that this is found in zones which are not greatly confined in relation with the sea and in the marginal backwaters where Pirenella conica are found in profusion on a cyanobacterian substrate). The aragonite maximum seems to be in relation with the contact between the confined waters which have travelled along the northern fringe of the basin, and "fresh" waters coming from the Bay of Amvrakikos. From FRISONI et al., 1982.

Fig. 38 - Situation of observation stations in the lagoons of Belle Plaine and Manche à Eau.

Fig. 39 - Abiotic parameters. Correlations between variables and principal axes.

Fig. 40 - Analysis of abiotic data into principal components Positioning of the stations on the main plane.

Fig. 41 - Multidimensional positioning of the stations according to similarities of benthic populations. S : threshold of similarity.

Fig. 42 - The Caspian Sea, bathymetric map. From ZENKEVITCH (1951) and TASKIN (1954) in ZENKEVITCH (1957).

Fig. 43 - Diagrammatic map of surface currents in the Caspian Sea. From ZENKEVITCH (1947) in ZENKEVITCH (1957).

Fig. 44 - Salinity levels in the Caspian Sea. From DZENS-LITOVSKIJ, 1962, (modified).

Fig. 45 - Benthic biomass distribution in the Caspian Sea, in 1935. From 2ENKEVITCH

(1951). in ZENKEVITCH(1957).

Fig. 46 - Distribution of Nereis succinea biomass in the northern basin of the Caspian

Sea, in 1948. From ZENKEVITCH (1951) in ZENKEVITCH (1957).

Fig. 47 - Diagrammatic map of confinement zones in the Caspian Sea from data by ZENKEVITCH (1957). This map concerns only the upper water level (0 - 200 m).

Fig. 48 - Diagrammatic section of the Caspian Sea and zoning linked to "bathymetric confinement". From data: by ZENKEVITCH (1957). Profile A - B (Fig. 42).

Fig. 49 - Map of total algal biomass distribution (drained wet weight) in the Etang du Prévost. From RIOUALL, 1976. 1. 17 April 1975 - 2. 25 April 1975.

Fig. 50 - Monthly variations of total and specific (Ulva, Enteromorpha, Stictyosiphon) algal biomass in the central zone of the Etang du Prévost (drained wet weight in g/m2). From RIOUALL, 1976.

Fig. 51 - Biological zoning of the intertidal zone on rocky surface at Port Aransas (Texas). From HEDGPETH (1953) in DOTY (1957).

Note the increase in the intertidal zone on surfaces exposed to the waves, and the distance of the supratidal zone from the mean tide level. On the contrary, on sheltered surfaces, zonation is much tighter. On exposed surfaces the confinement gradient is reduced by wave lapping, and spray.

Fig. 52 - Extent of the different layers of the Paleogene in the Paris Basin (for the Stampian, see figures 53, 56 and 57). From FURON and SOYER, 1947.

Fig. 53 - Evolution of confinement in the Paris Basin during the Paleogene, reconstituted according to thalassoid paleofauna.

Fig. 54 - Situation of principal Stampian fossil deposits in the Paris Basin. From ALIMEN (1936).

Fig. 55 - An attempt to reconstitute the confinement zones of the Lower Stampian in the Paris Basin. The figures represent the diversity of species in each deposit according to ALIMEN (1936). The underlined figures denote mixed or indeterminate fauna.

Fig. 56 - An attempt to reconstitute the confinement zones of the Upper Stampian in the Paris Basin. The figures represent the diversity of species in each deposit according to ALIMEN (1936). The underlined figures denote mixed or indeterminate fauna.

Fig. 57 - Diagrammatic paleogeographical map of the Stampian in N.W.France.

From POMEROL, 1973. Dotted areas : Marine facies. Lined areas : Lagunar facies of the Isle of Wight.

Fig. 58 - An attempt at paleogeographical reconstitution of the Stampian

in N.W. France according to paleofauna. Dotted areas : Marine domains. Lined areas : Paralic domain (near paralic).

Fig. 59 - The Baltic Sea and its principal basins.

Fig. 60 - Density of benthic macrofaxina in the Baltic in June and July 1967. From ANDERSIN et al., 1976.

Black dots indicate stations devoid of macrofauna ; grey areas indicate regions where the dissolved oxygen level is below 2 ppm ; black areas indicate regions where H2S is present.

Fig. 61 - Biomass (dry weight) of benthic macrofauna in the

Baltic in June and July 1967.

From ANDERSIN et al., 1976.

Same symbols as for fig. 60.

Fig. 62 - Diversity of benthic macrofauna species

(Shannon rating in PIELOU, 1966), in the Baltic in

June and July 1967.

From ANDERSIN et al., 1976.

Same symbols as for fig. 60. Triangles represent stations where only one species was found.

Fig. 63 - Increase in percentage of the total number of individuals with the

increase in the number of taxa, in different parts of the Baltic. From ANDERSIN et al., 1976.

Fig. 64 - Distribution of Ophiura albida and Asterias rubens in the Kattegat and at the entrance of the Baltic Sea. From BRATTSTROM in SEGERSTRÅLE (1957).

Note the "Channel effect" opposite the Øresund and the other Danish straits where the water flow ensures a sufficient stream of "vital elements" to allow good development of Echinodermata. This "Channel effect" was suggested by A. JAUZEIN ; it corroborates the authors1 hypothesis as the ways in which confinement acts.

Fig. 65 - Map of confinement zones in the Baltic Sea.

Fig. 66 - Currents and principal tributaries of the Nador lagoon.

Fig. 67 - Bathymetry and granulometry of the sediments in the Nador lagoon.

Fig. 68 - Salinities and localization of the zone of maximal phytoplanktonic biomass (Summer 1982).

Fig. 69 - Confinement zones in the Nador

lagoon.

Fig. 70 - "Positive" socio-economic aspects in the immediate surroundings of the Nador lagoon.

Fig. 71 - Water and energy sources, means of access, around the Nador lagoon.

Fig. 73 - Proposition for short-term

aquacultural development for the Nador lagoon.

Fig. 72 - "Negative" socio-economic aspects in the immediate surroundings of the Nador lagoon.

Fig. 74 - Longer-term development of aquacultural activities in the Nador lagoon, after modification of communication with the sea.

Appendix 1

CONVERSION TABLE of different parameters which may be used to express dilution or concentration by evaporation of sea-water in the Mediterranean (at 20° C). Figures kindly provided by the Compagnie des Salins du Midi and the Salines de l'Est.

Volune of solution

d° Baumé Density Concentrationg/l

Salinity ‰

(CI) g/l

(Cl) mole/l

∞ 0 1 0 0 0 0 100000 1,0000 0,4 0,4 0,2 0,006 10000 0,4 1,0026 3,7 3,7 2,1 0,06 5000 0,7 1,0051 7,4 7,4 4,1 0,11 2000 1,8 1,0129 18,6 18,4 10,2 0,29 1212 3,0 1,0212 30,6 30 16,9 0,48

1000 3,6 1,0257 37,1 36 20,5 0,58 Normal sea water 973 3,7 1,0264 38,1 37 21,1 0,60

943 3,8 1,0271 39,0 38 21,7 0,61 924 3,9 1,0278 40,0 39 22,2 0,63 902 4 1,0285 41,1 40 22,8 0,64 713 5 1,0360 51,8 50 28,8 0,81 591 6 1,0435 62,6 60 34,8 0,98 504 7 1.0510 73,3 70 40,8 1,15 436 8 1,0590 84,8 80 47,2 1,33 387 9 1,0665 95,7 90 53,2 1,50 345 10 1,0745 107 100 59,6 1.68 310 11 1,083 119 110 66,4 1,87 282 12 1,091 131 120 72,8 2,05 260 13 1,099 142 130 79.2 2,23

240 14 1,107 154 140 85,6 2,41 Saturation in gypsum 219 15 1.116 167 150 93,8 2,65

201 16 1.125 182 162 102 2,88 186 17 1,134 195 172 110 3,10 173 18 1,143 210 184 119 3,36 162 19 1.152 223 194 127 3,58 152 20 1,161 237 204 135 3,81 143 21 1,170 251 215 143 4,03 134 22 1,180 266 225 152 4,29 127 23 1.190 282 237 162 4,57 120 24 1,200 298 248 171 4,82 114 25 1,210 314 260 180 5,08

110 25,6 1,216 323 266 186 5,25 89,2 26 1,220 327 268 186 5,25

Saturation in halite (rock salt)

60,4 27 1.230 335 272 186 5,25 44,6 28 1.241 344 277 186 5,25 35,3 29 1,252 354 283 186 5,25 29,3 30 1,263 364 288 186 5,25

Appendix 2

LIST OF THE PRINCIPAL COMMON MACROBENTHIC SPECIES IN THE MEDITERRANEAN PARALIC DOMAIN

(P) indicates strictly paralic species

A. FLORA

Chlorophyceae Bryopsis plumose Caulerpa prolifera Chaetomorpha linum (P) Cladophora albida C. laetevirens C. vagabunda (P) Codium fragile C vermilara Enteromorpha clathrata E. flexuosa E. intestinalis (P) E. linza E. prolifera Monostroma oxyspermum Percusaria percusa Ulothrix flacca U. pseudoflacca Ulva lactuca (P) U.rigida Ulvopsis grevillei Urospora mirabilis

Pheophyceae Acinetospora sp. Cutleria multifida Ectocarpus confervoides E. siliculosus Giffordia granulose G. recurvata Haloglossum compression Petalortia fasciata

Punctaria latifolia Scytosiphon lomcntaria Stictyosiphon adriaticus

Rhodophyceae Antithamnion sp. Bangia fuscopurpurea Callithamnion granulatum Ceramium diaphanum C ruhrum C. tenuissimum Chondria tenuissima Erythrotrichia sp. Goniotrichum alsidii Gracilaria verrucosa (P) Lophosiphonia subadunca Neogomolithon notarisii (P?) Polysiphonia brodiaei P. denudata P. mottei Porphyra leucosticta

Characeae

Phanerogamia Althenia filiformis (P) Cymodocea nodosa Potamogeton pectinatus (P) Ruppia spiralis (P) Zostera marina Z. noltii

B. FAUNA

1. Soft bottom

Nemertea

Cereoratulus marginatus (P)

Annelida-Polychaeta Armandia cirrosa Audouinia tentaculata Capitella capitata

Eteone picta Glycera alba Glycera convoluta Heteromastus filiformis Leiochone clypeata Lumbriconereis impatiens Magelona papillicorris

Mercierella enigmatica (P) Microspio meckzrnikovianus Nephtys hombergii Nereis diversicolor (P) Ophiodromus flexuosus Owenia fusiformis Pectinaria koreni Phyllodoce mucosa Pista cretacea Platynercis massiliensis Polydora antennata Polydora ciliata Scolelepis fuliginosa Stylarioides monolifer

Gastropod Molluscs Akera bullata (P) Aplysia depilans Bittium reticulatum (P) Cerithium vulgatum Gibbula adamsoni Hydrobia acuta (P) Hydrobia ventrosa (P) Murex trunculus Nassarius (Hinia) mamillata Nassarius reticulates Natica (Neverita) josephina (P) Philine quadripartite Pirenella conica (P) Rissoa grossa Urosalpinx (Ocinebra) edwardsii

Pelecypod Molluscs Abra ovata (P) Acanthocardia echinata Acanthocardia paucicostatum Barnea Candida Brachydontes marioni (P) Cerastoderma glaucum (P) Cerastoderma exiguum (P) Corbula (Varicorbula) gibba Corbula (Lentidium) mediterranea Donax semistriatus Donax trunculus Dosinia exoleta Gastrana fragilis Loripes lacteus Mactra corallina M. corallina var. stultorum Mactra solida Microchlamys varia

Modiolus barbatus Musculus marmoratus Mytilus edulis Mytilus galloprovincialis Ostrea edulis Pholas dactylus Scrobicularia plana Solecurtus chamaesolen Solen marginatus Striarca lacteal Tellina tenuis Thracia papyracea Venerupis aurea Venerupis decussata Venus gallina V. gallina var. radiata

Crustaceans Artemia salina Athanas nitescens Carcinus mediterraneus (P) Corophium insidiosum (P) Crangon crangon Cyathura carinata Cymodoce truncata (P?) Erichthonius brasiliensis Gammarus aequicauda (P) Gammarus insensibilis (P) Idothea balthica Idothea balthica basteri Idothea viridis Jassa falcate Leander serratus Lilljeborgia brevicornis Microdeutopus gryllotalpa (P) Palaemonetes varians Penaeus kerraturus Portumnus latipes Portunus holsatus Sphaeroma hookeri (P) Sphaeroma rugicauda Upogebia littoralis

Echinoderms Asterina gibbosa Holothuria polii Ophiuridae Paracentrotus lividus

Dipterous insects (larvae) Chironomidae (P)

2. Hard bottom

Hydroida Obelia plana

Annelida Polychaeta Armandia cirrosa Capitella capitata Hydroides elegans Mercierella enigmatica (P) Polydora ciliata Pomatoceros lamarckii Scolelepis guliginosa Serpula vermicularis

Bryozoa Bowerbankia imbricata Bugula neritina Bugula stolonifera Conopeum seurati (P) Cryptosula pallasiana Membranipora membranacea

Pelecypod Molluscs Anomia ephippium Cerastoderma glaucum (P) Microchlamys varia Modiolus barbatus Musculus marmoratus

Mytilus edulis Mytilus galloprivincialis Ostrea edulis

Crustaceans Balanus amphitrite amphitrite (P) Balanus eburneus Balanus perforatus Corophium insidiosum (P) Gammarus aequicauda (P) Gammarus insensibilis (P) Jassa falcate Lilljeborgia brevicornis Melita palmate Microdeutopus gryllotalpa (P) Cymodoce truncata Idothea balthica (P?) Idothea viridis Sphaeroma hookeri (P) Pachygrapsus marmoratus

Tunicata Botryllus schlosseri Ciona intestinalis Phalusia mamillata Styella plicata

Appendix 3

COMMENTS AND DIGRESSIONS

1. SALINITY TOLERANCE, EURYHALINITY, STENOHALINITY

In ecology, "tolerance" of species with regard to salinity is often mentioned, and species are defined as "stenohaline", supposedly tolerating only very minor fluctuations in salinity, as opposed to species defined as "euryhaline"; capable of adapting to much more variable saline concentrations.

As far as thalassoid species are concerned, these distinctions must be totally re-examined since, for example, sea urchins (Paracentrotus lividus) which are generally considered to be particularly stenohaline, are to be found in "abnormally" low salinity conditions (Vonitza Bay). Moreover, even for euryhaline species, the question remains as to whether a salinity limit really exists. There has been a recent study on brine pools situated in a sebkha of the Gulf of Suez fed by subterranean marine inflows: these pools contain an association of Cyanophyceae grazed upon by Pirenella conica and small fish (Aphanius?) which, when disturbed, hurry to concealment beneath salt rafts. The salinity concentration is near saturation (350 g/l). Thus the ranges of salinity indicated by the authors for any thalassoid species result above all from their personal experience.

Perhaps the terms "stenohaline" and "euryhaline" are applicable to continental species, but the observation of endoreic basins passing from almost freshwaters to evaporitic brines casts a certain doubt on this.

In the authors' opinion, these terms are erroneously precise, and should be abandoned.

2. CONFINED MILIEUX VS REDUCING MILIEUX

According to the definition of confinement as used here, a confined environment is not synonymous with a reducing environment in spite of the anoxia (lack of oxygen) implied by the term confinement - from a very anthropomorphic point of view.

However, in the paralic domain, confinement often leads to the reducing character of the milieu, notably in the region of the bed, for different reasons:

− Lateral variations of salinity and therefore of density are liable to induce the stratification of successive layers of water (one above the other), which contributes to the anoxia of the lower layer. This phenomenon occurs particularly in hyperhaline milieux and/or when a salt "wedge" (coin sale) develops where freshwater and seawater are in contact (estuaries, delta channels, mangroves).

− The generally slow hydrodynamic quality of paralic basins increases this phenomenon, for it tends to stabilize bodies of water; this is obviusly not the case in estuaries where salt zones move with the tide and the flow of the river. Estuaries are rarely anoxic milieux.

− The temperature and salinity of the waters limit oxygen solubility, thus lessening the effects of photosynthesis. Paralic milieux in hot and/or arid climates are more affected by anoxia than elsewhere.

− Intense biological production, which may be implemented by continental inflows, results in considerable sedimentation of organic matter. The saprophytic microorganisms which consume it contribute to the anoxia of the mi lieu by their respiration and possbly by the production of reduced compounds (H2S, CH4, NH4).

In general, reducing milieux are more frequent in the Far paralic and in environments with evaporitic tendencies.

3. LAGUNAR DWARFISM AND GIGANTISM

Paralic species and the forms of mixed species near their limit of confinement are generally small compared either with neighbouring thalassic species, or with the forms of mixed species in slightly confined zones. "Lagunar dwarfism" is often referred to.

A possible reason for this property in species undergoing severe confinement and often "abnomal" salinity levels, might appear to be the energy expended in "contending" with this situation. This anthropomorphic explanation is not satisfactory, for thalassic species living in conditions of "abnormal" salinity are not particularly small in size. It seems rather that the effort to reproduce in paralic populations (cenotic strategy type; according to Brandin et al., 1976) takes place to the detriment of growth and thus of individual size: this could be one of the biological manifestations of deficiency connected with confinement. Moreover, and especially for mixed species near their limit of confinement, infantile mortality may contribute to the rarity of well developed adult forms (lagunar infantilism).

Finally, lagunar gigantism is occasionally referred to: this concerns only the mixed species in relatively little-confined zones where there is no problem of deficiency and where the abundance of food contributes to faster growth than at sea. Examples are oysters, and especially mussels, often much larger in lagoons than their equivalent in a marine environment, or again sea-bream (Sparus aurata) which for example in a lagoon north of Tunis reach a weight of over 1 kg in two years, while the sea bream remaining in the Bay of Tunis weigh only a little over 200 g at the same age (Chauvet, 1979).

4. MIXED SPECIES

"Mixed" species here refers to those which may be found both in the marine and paralic domains, without going into detail as regards classification.

These species enter the paralic milieu in two different ways:

− Passive establishment of all species of phytoplankton (holoplanktonic species) and of certain benthic species whose meroplanktonic larvae are brought in, along with the former, by the action of currents and tides.

− Active establishment of ichthyofauna (migrant) species which penetrate the paralic environment for feeding or reproduction purposes.

Thus the word "mixed" applied to thalassoid species found in paralic milieux does not have exactly the same meaning when applied to different biological aspects, in particular depending on the mobility of the different species concerned.

5. ABIOTIC PARAMETERS

Abiotic parameters are all the non-biological "parameters which participate in the functioning of the ecosystem. As far as paralic milieux are concerned, and taking into

account their morphological characteristics (bathymetry, surface area, direct contacts with the numerous adjacent ecosystems) the most significant parameters appear to be:

− Hydrological parameters (ionic concentrations, temperature, pH, levels of dissolved oxygen, turbidity, content of nutritive salts).

− Sedimentological parameters (granulometry, biodetrital phases, pelite content, mineralogy of the carbonated matrix, content of organic matter).

5.1 HYDROLOGICAL PARAMETERS

(a) As far as ionic concentrations of the major elements are concerned (salinity values of different ion ratios), it has been amply proved that they have no influence whatever on the organization and biological structure of paralic ecosystems. Nevertheless, if local climatic and hydrographic data are taken into consideration, their study often permits an approach to the hydrological system.

(b) Temperature is obviously an important ecological factor, especially in these generally very shallow environments. It influences particularly the biological cycles of the majority of the species involved: nutritional activity (filtration rate, for example), sexual maturity (period and numbers attracted) 1/.

(c) The turbidity of paralic milieux is dependent upon phytoplanktonic activity (green waters, and red waters in salt production condensers) and upon the concentration in the water of mineral particles in suspension as a result of the disturbance of the milieu. Whatever its origin, high turbidity is a habitual characteristic of paralic milieux. There are, however, basins where it remains low (lagoons in arid climates): the biological structure of the ecosystem is not modified because of this.

(d) The pH and the level of dissolved oxygen are measurements which are very complex to determine, in which salinity, temperature, water disturbance and various biological considerations (chlorophyll synthesis, respiration, bacterial reduction, etc.) intervene. Except in extreme examples – in particular evaporitic zones - or in environments subject to dystrophic crises in summer, the values of these parameters remain within a range such that they have little or no influence on the biological organization of the ecosystem.

(e) The importance of nutritive salts (phosphate and nitrogen compounds) is emphasized, particularly in basins receiving continental inflows. Even if there is no doubt that these salts affect the activity of chlorophyll organisms, they have only a minor influence on the specific qualitative composition of the plant populations, since these are identical both in paralic basins receiving continental inflows and in those which do not.

In the natural paralic domain, one can assume that the notion of "limiting factor" is illusory, otherwise fundamental qualitative and quantitative differences in plant populations would be observed in the different basins: the nutritive elements necessary for the proliferation of plant species are always present in sufficient quantities in natural paralic basins. On the other hand, it is likely that many hydrochemical parameters (levels of trace elements, of discrete organic compounds, etc.) intervene in the organization of paralic ecosystems. Knowledge of these parameters still remains imperfect both in the laboratory and in the field; furthermore all the chemical substances which intervene in confinement and their respective roles are still unknown.

5.2 SEDIMENTOLOGICAL PARAMETERS

(a) In the majority of cases, the detritic elements of the paralic sediments are fine to very fine (sands, pelites, clays). Added to this is the extent of the carbonated biochemical and organic phase which is always extremely fine. In general, mean granulometry decreases significantly from zones under marine influence towards the continental shores, moreover the winnowing of the edges tends to concentrate the finer particles in the lowest zones. In the paralic milieu, it is however possible to find granulometric anomalies sometimes linked with coarse continental inflows, but most often due to the biodetrital phase (Foraminifera sands, accumulation of test debris, Lithothamnion fades, Hydrobiidae levels, Cardium muds, etc.). Thus the majority of lateral variation in the granulometry of paralic sediments is in relation with the field of confinement, notably via the biodetrital phase.

(b) Organic enrichment is a fundamental characteristic of paralic basins except when the system of currents prevents this, particularly in the case of estuaries. It is the result of trapping organic matter of marine and/or continental origin, and the intrinsic production of the milieu (it is possible to measure up to 60% of dry organic matter in paralic sediments).

The amount of organic matter in the superficial sediment is of ten a good indication of confinement. Moreover, local accumulation of organic matter for diverse reasons (hydraulic bubble, intensive aquaculture) results in a notable transformation of the benthic populations and, in particular, in the development of detritivorous species at the expense of suspension feeders. The interest of measuring this parameter is thus undeniable.

(c) The mineralogical composition (qualitative and quantitative) of the carbonated matrix depends on various factors among which rank temperature, salinity, the photosynthetic activity of the milieu, the activity of the bacterian flora, the mineralogical composition of. the tests on. species living in the milieu, etc. Moreover, and this complicates the situation, certain types of minerals, such as aragonite and highly magnesian calcite, are metastable under natural conditions and are transformed at speeds varying with the chemical characteristics of the environment: carbonates are extremely elusive.

In the present-day, Near paralic milieux, associations with calcite, highly magnesian calcite and aragonite are very generally found. The variations of relative proportions of these minerals, along with the Mg content in the calcites are likely to provide indications as to the milieu. Thus, for example, high aragonite content can indicate zones of contact between bodies of water of different confinement levels (Perthuisot et al., 1983; Frisoni et al., 1983) but the spatial variations of the relative proportions of the different types of minerals in these milieux are difficult to interpret in detail (Fig. 37) given the present level of knowledge.

In the Far paralic (beyond zone VI) in the evaporitic or pre-evaporitic milieux, highly magnesian carbonates, dolomite, huntite, magnesite appear (Perthuisot , 1971, 1975, 1980) while in freshwater milieux, the carbonated phase is generally calcic (calcite, aragonite).

6. THE ROLE AND EXPRESSION OF CONFINEMENT IN THE MANGROVE LAGOONS: BELLE PLAINE AND MANCHE A EAU (GUADELOUPE) 1/ From Gaujous (1981) and Gaujous and Guelorget (1983)

The sedimentological and hydrological parameters measured at the different stations (temperature, salinity, dissolved oxygen, pH, bathymetry and vegetable debris content of sediments) were analysed together into principal components. From this, it is clear that the various stations fall into two distinct groups corresponding to each of the lagoons (Figs. 38, 39 and 40). The lagoon of Belle Plaine has on the whole warmer and more saline waters and higher pH, than those of Manche à Eau: the increase in the value of these parameters appears on the chart (Figs. 39 and 40) as a shift to the northeast (towards the right and upwards). This can be explained by the greater influence of marine waters in the lagoon at Belle Plaine, while Manche à Eau communicates with the sea only by a complex system of canals. However, within each group a southeast/northwest gradient can be observed going from the stations close to the sea (2, 9, 10), which are low in oxygen, shallow, and rich in vegetable debris: this hydrological and sedimentological gradient is one of the expressions of the confinement gradient.

A similarity study (Legendre and Legendre, 1979) carried out at the same stations comparing the densities of each zoological group (echinoderms, crustaceans, gastropods, pelecypods, annelids) gives a totally different picture from the preceding one (Fig. 41). The stations form three groups subdividing the previous ones :

− A first group includes the stations (1, 6, 8, 7, 11) where all the zoological groups are represented. In particular, this is the only group where echinoderms are present.

− A second group of stations (2, 4, 9, 10) where only annelids and pelecypods are present.

− Lastly, stations 3 and 5 where very few thalassoid species and individuals are to be found.

The first group corresponds to stations under pronounced marine influence (zone II), the second group to typical paralic stations (zones III and IV), and stations 3 and 5 to marginal backwaters (zone V) where the fauna is considerably impoverished.

Thus, even the mathematical treatment of the abiotic data on the one hand and the biological data on the other, shows that the biological organization of paralic milieux is independent of the physiochemical characteristics of the milieu, and follows exactly the confinement gradient. This is expressed, in each of the mangrove basins studied, by apparently concording salinity gradients, levels of dissolved oxygen, and vegetable debris contents, but the comparison of the two basins shows that there is not an unequivocal relation between the value of one or other of these parameters and the confinement level.

If, from among these parameters, the best indicator of confinement has to be singled out, the vegetable debris content in the sediment would emerge: this extrinsic factor measures the continental influence at a given point, owing to the omnipresence of an abundant continental biomass. Nevertheless, it is emphasized that if the quantity of vegetable debris gives a clear picture of the confinement in this type of milieu, the other abiotic parameters (dissolved oxygen, pH) are closely subordinated to it.

7. THE CASPIAN SEA

The Caspian Sea is a unique example of a basin of marine origin separated from the sea a long time ago geologically, and which still shows all the qualitative and

quantitative abiotic and biological characteristics of the paralic domain. Zenkevitch and other Soviet authors (Zenkevitch, 1957) have made interesting studies.

The Caspian Sea is 1 200 km Long, between 200 and 500 km wide, with a surface of 436 000 km2 and a volume of 77 000 km3 It comprises three regions (Fig. 42): the northern basin, which is very shallow and receives the majority of the continental water inflow notably from the Volga and the Ural rivers; the central basin reaches over 700 m in depth and flows partially into the Kara Bogaz Gol Bay, where evaporites are deposited (Dzens-Litovskij, 1956, 1962); the southern basin which also belongs partly to Iran, reaches over 900 m in depth.

From the hydrological point of view, the same divisions as above are found and each basin has its own hydrodynamic system, as each large mass of water rotates upon itself (Fig. 43). Hydrous exchanges between the different basins seem to be above all due to movements of perilittoral drift.

The content of salts dissolved in the surface waters follows a north-south gradient with freshwaters at the mouths of the large rivers coming from the north, and obviously in the peripheral evaporitic annexes (Fig. 44). The two deep basins have very homogeneous waters from the surface down to the bottom, with concentrations of between 12 and 13 g/l. The chemical composition of the waters of the Caspian Sea does not (or no longer correspond (s) to a dilute seawater, notably by its richness in sulphate ion.

The flora and funa comprise a small number (around 400) of species of macrofauna 1/, among which three groups can be distinguished.

− Common freshwater species are principally localized in the extreme north of the basin (Abramis brama, Salmo trutta, Cyprinidae, Lucioperca lucioperca, Eriocheir sinensis, Astacus);

− Common paralic species:

Phytoplankton: Exuviella, Rhizosolenia, Skeletonema Phytobenthos: Potamogeton pectinatus, Ruppia spiralis, Zostera

noltii Zoobenthos: Cerastoderma glaucum, Corophium, Nereis succinea,

Pontogammarus, Mytilaster Fish: Atherina, Syngnathus, Pomatoschistus, Gobius

− Endemic species - 60% of the fauna is endemic, including for instance certain Cardiidae (Adacna, Monodacna, Didacna), the Micromelaniidae, a great number of crustaceans, and among the fish, Clupeidae (Caspialosa, Clupeonella) and Acipenser stellatus. These endemic species appear to derive from common paralic species, such as the Micromelaniidae which some consider as Hydrobiidae, or mixed (migrant) species, such as Leander, the Acipenseridae, Clupeidae, Pleuronectidae, Petronyzonidae, Hugilidae.

From these data, so general that they tend to conceal the real diversity of the basin, it is reasonable to try to establish coherent zoning in terms of confinement. The absence of echinoderms (Table 7) and a great number of other thalassic groups (Radiolaria, corneous, and siliceous sponges, Siphonophora, Anthozoa, Ctenophora, Nemertea, cephalopods, Tunicata) shows that the basin as a whole is situated beyond zone II on the scale of confinement of the Mediterranean paralic domain.

The hydrochemical gradients, the presence of freshwater populations in the northernmost part of the basin and the presence of strictly paralic species (Cerastoderma glaucum, Nereis succinea) lead to the conclusion that the Caspian Sea occupies confinement zones III, IV and V and VI, with a fringe situated in the Far paralic: the Volga Delta and similar regions (freshwater pole) Kara Bogaz Gol (evaporitic pole). Furthermore, given the elongated shape of the basin, the disposition of the continental inflows and the bathymetric profile, the possibility of a confinement gradient increasing from the waters of the central and southern basins towards the northern limits can fairly safely be assumed.

The quantitative biological data given in Zenkevitch's publication permit this zoning to be specified:

The map of benthic biomass distribution (Fig. 45) shows a maximum of biomass in a curved zone between the northern and central basins, as we 11 as along the northwest and northeast shores of the latter. This region corresponds clearly to zone III, a zone where, in the paralic domain, the benthic biomass generally - if not always - reaches its maximum.

The map of the distribution of Nereis succinea biomass (Fig. 46) traces the outlines of zones IV and V very exactly: this species, which is strictly paralic, characterizes these two zones. It is probably in this region that Zenkevitch notes the phytoplanktonic blooms which are practically exclusive to Exuviella (a strictly paralic species, zone V) sometimes reaching 4.5 to 6.5 g/m3 : these blooms clearly indicate the region of maximal phytoplanktonic biomass. Here, in a basin separated from the marine domain, is evident the organizational system of the paralic domain, and, in particular, the remarkable feature of the crossing of the benthic biomass curve and that of the phytoplanktonic biomass in zone IV (Fig. 47).

Thus, in spite of the many centuries since it was in contact with the marine domain, the Caspian Sea is far from being totally "continentalized": the qualitative and quantitative organization of the paralic domain is found there. It seems that this phenomenon is linked to the great depth of the basin which has still not had time to become highly confined since its separation from the marine domain, except in the deepest parts of the basin which are affected by bathymetric confinement and anoxia (Fig. 48) as shown by' the very low levels of biomass encountered.

Thus, similarly to normal paralic basins where the sea represents a vitalizing source, the upper water level in the two deep basins of the Caspian Sea acts as a vitalizing reserve for the more confined zones, and the organization of the confinement fields depends on it.

8. LIVING FOSSILS AND PANCHRONIC FORMS

The stability of paralic settlements and species have been mentioned. It seems that a certain number of coincidences in present-day nature tend to indicate that the paralic domain could represent a "sanctuary" or a "depository" of panchronic forms and living fossils. This idea is based on several considerations:

− the similarity and morphological invariability of paralic molluscs and crustaceans since the beginning of the Tertiary era and for certain among them, since the beginning of the Secondary era;

− the presence, particularly in the Far paralic, of archetypal forms such as the phyllopod Artemia salina, the mystococarid Derocheilocaris, the archaic

Chlorophyceae Dunaliella salina the agnathe Petromyzon, the white whales in Saint Lawrence Bay;

− the obligatory biological relationship, for trophic or gene tic reasons, between the paralic domain and species descending from very ancient and not highly evolutive origins. Examples are "green" turtles, the Galapagos iguana, sturgeon, eels, sea lice (Polyphemus) (whose larval form is said to be trilobitic);

− the obligatory paleobiological relationship, in what was the newly-born paralic domain, of species nowadays separated by the effects of global tectonics.

Obviously there is the problem of the "panchronic forms" and the "living fossils" which in present-day nature are not clearly geographically linked to the paralic domain. The coelacanth (Crossopterygian), can be mentioned, certain species which inhabit medio-oceanic rifts (Pognophora), the Monoplacophore Neopilina galathea, the Nautilus, etc.

It is noticeable that in most cases, in contrast with species developed for deep-sea living (bathyal and abyssal forms), these species do not present morphological or anatomical characteristics corresponding to their natural environment (Latimeria's large eyes and "legs" which seem incongruous in its habitat and which would tend to indicate a coastal ethology). Moreover, these species are found in limited, often highly restricted geographical areas, and what is more, present anomalous hydrological characteristics. There are paralic basins isolated in the interior of continents (the Caspian Sea, Fayoum). Could they not have counterparts here and there in the midst of the marine domain, relict paralic bubbles, a heritage from the geological past of the Earth?

Vacelet (1983) proposes that submarine caverns conserve living fossils.

9. PALEONTOLOGY, CONFINEMENT, AND EVAPORITIC DEPOSITS

The fact that there is no unequivocal relationship between salinity and confinement leads to doubt about the relationship between biological activity and evaporitic sedimentation, that is, concerning the chronological relationship between saline deposits and biogenic facies.

First, an examination of the paleontological content of paralic formation (associated or otherwise with evaporites) does not permit the reconstitution of the salinity of their place of deposit. Thus the classification of a large number of paralic formations from the French Paleogene as having a brackish water facies, i.e., hypohaline, needs to be totally re-examined, particularly in cases which are associated with evaporites. Other arguments, notably geochemical and isotopic, might no doubt be better used, although the latter should be subject to great caution (Jauzein, 1982).

Even the attempt at reconstitution of paleosalinities from fossil ichthyofauna, which are often remarkably conserved in paralic formations (Gaudant, 1982) runs into two problems:

− incomplete knowledge of present-day natural phenomena where much still remains to be discovered, and where many surprises are possible, in particular concerning "salinity tolerances" of a given species;

− the geographical and/or ethological drift, always possible, in any case always conceivable, of descendence in the course of geological time.

Even obviously thalassic fossil settlements are not indicators of a "normal" salinity of the environment, since in present-day natural conditions, typically thalassic flora and fauna are to be found in milieux deviating appreciably from the marine "norm". Moreover, in the case of large "deep basins where communication with the sea is becoming limited, climatic and hydrographical conditions may be such that the concentration of the waters by evaporation takes place more rapidly than the progression of the confinement of the basin: depth is an important factor here, since in zones of great depth, biological activity impoverishes the milieu at only a very slow rate. It becomes possible to imagine highly hyperhaline basins where communities characteristic of low confinement still thrive, even thalassic communities. These considerations perhaps explain the presence of Coccolithophorida planktonic nanoflora in the Mediterranean Messinian (Rouchy, 1981). In fact, the possibility of a growth of reef-type structures in basins depositing evaporites cannot be completely excluded: if this is hardly conceivable for coral reefs, the majority of Coelenterata being highly sensitive to confinement, it is however mo re so for Stromatoporidae or Cyanobacteria. The latter, for example, form reef-type structures contained within the Miocene gypseous series of the Gulf of Suez.

10. ALGAL PRODUCTION IN THE ETANG DU PREVOST

Riouall (1976) establishes monthly maps of the macrophytic biomass distribution in the Etang du Prévost (Fig. 49) and thus calculates the total lagunar algal biomass (Table 8) and the specific lagunar biomass for the four principal types (Ulva, Enteromorpha, Stictyosiphon, and Gracilaria) (Table 9). He estimates lagunar productivity at around 6 900 t (drained wet weight) during October (Fig. 50) giving a yield of 25 t/ha. This yield may reach 50 t/ha in certain zones. He calculates the lagunar production during the rest of the year (20 t/ha) at 5 400 t, giving a total for the whole year of 12 300 t (47 t/ha). Converted into weight of dry matter, the total lagunar plant productivity reaches 2 000 t, representing a yield of 7 t/ha.

For comparison, Mercier (1973) evaluates the productivity of the entire lagunar complex of Bages-Sigean at 1 t of dry matter per hectare (1.4 to 2 t/h for the surfaces alone covered by vegetation, 4 t/h of dry matter for the richest vegetations). There too, the dominant types of algae are Ulva and Enteromorpha.

11. THE INTERTIDAL ZONE OF ROCKY COASTLINES

Many studies have been made of the biological zoning of rocky coasts; to give an example, a description follows of the eastern coasts of the USA (Doty, 1957):

The so-called sublittoral fringe, situated in the vicinity of the lowest tideline, is characterized by a flora of Laminaria associated with Zostera and Phillospadix. The highly diversified fauna includes clearly thalassic groups, Nudibranchia, Tectibranchia, echinoderms, etc.

The coastal zone possesses a different flora and fauna, with fewer species represented (Anthopleura elegantissima, Pachygrapsus crassipes, Pagurus granosimanus, Thais emerginata, etc.). The lower part of this zone is characterized by Mytilus californianus, Iridophycus coriaceum, Mitella polymerus, Tegula brunnea. The upper part still maintains a flora (Fucus and Pelvetia) and a fauna which although not highly diversified is rich in individual specimens (Balanus glandula, Littorina scutulata, Patella).

The supra-littoral fringe, situated in the vicinity of the highest tideline, is often referred to as the "black zone" because of its colour, due to the presence of

Cyanophyceae such as Calothrix. The fauna consists only of Littorina, a grazing gastropod, and a very small number of detritivorous species such as Lygia.

This zoning (Fig. 51), which proceeds from a typically thalassic settlement to an association comprising only Cyanophyceae, a gastropod, and a small number of isopods, is to be found naturally with certain specific variations, on all rocky coasts, even those with the lowest tides (Pérès and Picard, 1964). The analogy between this coastal zoning and that in the Mediterranean lagunar milieux is striking, in spite of notable differences in the specific composition of the settlements, in the one case adapted to a soft bottom and a permanent depth of water, and in the other to a hard bottom and intermittent changes in water levels.

It may be presumed that the essential factor influencing the zoning of intertidal species is undoubtedly resistance to emergence.

Depending on current meteorological conditions, as far as the film of water covering the rocks is concerned, emersion will mean either an increase in salinity and possibly total dessication, or else dilution and transition to freshwater: here are found the two hydrochemical extremes of the paralic domain.

A similar zoning may be observed in the intertidal zones on a soft bottom (beaches).

Thus it would seem that the "pattern" of the intertidal zone is a typically paralic pattern, in which confinement may be measured by the mean length of time of emergence.

1/ The maintenance at adequate temperatures by artificial means of species which in their natural habitat are exposed to marked seasonal temperature variations, permits an early gonadial maturation and the production in hatcheries of several successive layings in one year.

1/ Section 1 of the main text discusses these two paralic basins

1/ The determinations of these species are for the most part outdated, and systematic classification has greatly evolved in the last few decades. Because of this, the specific denominations used may, in certain cases, be unreliable. In the interpretation of Zenkevitch's data, the generic classification has been maintained.

Appendix 4

USE OF THE NOTION OF CONFINEMENT IN AN ATTEMPT TO RECONSTITUTE PALEOMILIEUX IN THE PALEOGENE OF THE PARIS BASIN

Here, an attempt is made to apply the biological zoning of the paralic domain to a fossil basin using a method which is particularly efficient for present-day conditions, to see how far it is possible to retrace the paleozonings - or the paleo-biogeography - of ancient paralic basins, including their quantitative aspect.

The transposition of an outline concerning present-day natural conditions to a sedimentary basin of the geological past encounters certain difficulties:

− First, the fossil fauna are different from present-day fauna, and this difference increases with the age of the basin studied. This explains the choice of the Paleogene in which the species differ only moderately from present-day forms, particularly concerning the malacofauna.

− In practice, it is very difficult, even impossible, to define with precision a time-surface that can be located on a sufficient number of sections or logs, especially in the neritic or paralic basins where changes in facies prevail. Even in the Paris Basin, which has been subjected to innumerable geological studies, it is rash to descend below a time-interval corresponding to the layer stage. One is thus led to integrate the greater part of each stratigraphic cycle and the outline becomes much less precise.

First, an attempt was made to reconstitute the evolution of the Paris Basin, in the Paleogene within the paralic domain, working from global lists of paleofauna; then, a more precise reconstitution of one layer only, the Stampian.

1. EVOLUTION OF THE PARIS BASIN IN THE ZONING OF THE PARALIC DOMAIN DURING THE PALEOGENE

The main reference document consulted was the "Catalogue des fossiles tertiaires du Bassin de Paris" by Furon and Soyer (1947), without which much bibliographic research would have been necessary and totally out of proportion with the subject. It is possible that this list may be incomplete for a number of recently discovered species. Moreover, the stratigraphic divisions adopted by the authors (Table 10) have been considerably modified over the last few decades, but this is of secondary importance to the discussion here.

It is very clear that this universal and synthetic list can only offer a very general view of the picture. Nevertheless, given the large numbers of thalassoid fauna species recovered over the entire basin for each whole layer, the presence or absence of certain zoological groups, particularly echinoderms and cephalopods, and the relative proportions of the different groups (Table 11), the dominant zone of confinement and the position of the basin in the diagram of the paralic domain, can be defined for each period (Figs. 52, 53).

The Montian comprises an essentially thalassic fauna, with very few paralic or continental species. It is therefore situated entirely in the marine domain (zones 0 and possibly I).

The Thanetian contains few zoo logical groups. The fauna is almost exclusively composed of molluscs including numerous examples of pelecypods and gastropods. The majority of species listed are paralic and with freshwater tendencies. The major part

of the Paris Basin in the Thanetian is therefore situated in the zones of high confinement (zones IV and V dominant).

In the Sparnacian, the fauna is also almost exclusively limited to molluscs, but the number of species present is considerably reduced; there is also a very high proportion of gastropods. Further, paralic species (Hydrobiidae, for example) are well represented, and even freshwater pelecypods are to be found. This situation is characteristic of a very highly confined basin, where zones V and VI dominate, in a position which approaches nearer to the freshwater pole. This point of view is confirmed by the existence of lignitic layers in the Sparnacian formations of the Paris Basin.

By comparison with the preceding layers, the Cuisian includes a great number of different species and numerous zoological groups are represented. However, the absence of echinoderms is a notable fact to be emphasized. Here too, the molluscs largely dominate the fossil settlements. The basin is in closer communication with the sea although without going beyond zone III. Zone III dominates largely, and the importance of biological production during this period can be affirmed.

In the Lutetian and Bartonian the maximum representation of the various zoological groups is reached with a very high total number of different species. The species listed are practically all obviously thalassic: these two layers clearly represent the maximum of marine influence on the Paris Basin in the Paleogene. Nevertheless, the Bartonian shows a slight tendency towards confinement, notably by the reduction in echinoderms. Obviously, the edges of the basin cover more highly confined zones, particularly at the end of the Lutetian in the direction of the evaporitic pole (Toulemont, 1980).

In the Ludian, a considerable reduction in the number of zoological groups can be observed and of the number of species within each group. The basin becomes distinctly more confined and zones IV and V are the most widely represented. However, the contact with the sea remains evident, at least temporarily, during this period, for echinoderms are still present. This high level of confinement on several occasions draws the basin towards the evaporitic pole.

The Sannoisian fauna is almost exclusively represented by a small number of typically paralic gastropods (9 Hydrobiidae out of 18 gastropods). The other members of the fauna indicate the importance of continental inflows. This layer is dominated by zones V and VI.

The Stampian is characterized by a fresh increase in numbers of species with the reappearance of thalassic species (echinoderms). Moreover, the abundance of molluscs, and notably of pelecypods, indicates the dominance of the highly productive zone III.

In the Chattian, the settlement is limited to a small number of paralic gastropods. Freshwater and terrestrial forms are very abundant: the basin is situated partly in the extreme paralic (zone VI and Far paralic) and partly in the continental lacustrian domain. Communication with the sea has no doubt disappeared at this time.

The outline proposed (Fig. 53) comes to no original conclusions as to the paleogeographical reconstitution nowadays accepted for the Paleogene in the Paris Basin. It may however be that there is too great a tendency to consider thalassoid fauna as being marine: thus only the Lutetian, Bartonian, and, to a lesser extent, the Stampian layers - except for the highly localized Montian - correspond to typical marine transgressions. The other periods correspond to greater or lesser extensions of the

paralic domain over the Paris Basin, which must have remained in connection with a relatively far-off marine domain.

For greater precision it is necessary to establish a zoning pattern from fauna collected in different quarries and within as short a period of time as possible.

2. TEST-ZONING OF THE STAMPIAN IN THE PARIS BASIN

An attempt to transpose the zoning pattern revealed by the present-day paralic domain onto a bygone period required access to a study dealing with a particular layer and to the most complete description possible of the paleofauna collected in numerous different deposits. The Stampian was chosen because the thesis by Alimen (1936) corresponds to all these requirements (Fig. 54). In an attempt to be even more precise, two periods were distinguished: the lower Stampian, excluding the Sannoisian, which at that time was considered to be independent of the Stampian (Table 10); it includes the Etrechy limestone site deposits, the faluns at Jeurs and Morigny, the giant oyster marls, the clays at Corbules de Frépillon, the sands and sandstones at Cormeilles, etc. Second, the upper Stampian (Shelly deposits at Pierrefitte and Ormoy and lateral equivalents). Finally, account was taken of the fact that certain deposits present mixed or indeterminate fauna (the Shelly deposit at Vauroux, for example). A total of 38 fossil deposits was counted.

The parameters taken into consideration in this study were the number of different species of benthic fauna present, the number of different zoological groups, and their relative proportions. It is however possible to pursue the matter further, in view of the large numbers of species present on the one hand and the density, when it is possible to have an idea of this, on the other, and to try to define which species are strictly paralic and characteristic of a high degree of confinement. In this category are species such as Cyrena convexa, Melania decussata, M. semidecussata, the Hydrobiidae, and a certain number of cerithiids, Cerithium conjunctum, C. plicatum, C. trochleare, and of course Cerithium (Potamides) lamarcki. There is some doubt regarding the oysters, Ostrea longirostris and (O. cyathula which by comparison with present-day conditions, should rather be considered as mixed species. However, there is at Evreux an Ostrea cyathula var. minor which could very well be paralic or indicate a highly confined milieu (dwarfism). Similarly,Natica crassatina seems to characterize the most confined zones and could well be a paralic species in spite of its large size.

Considered as a whole, the biological characteristics of the different fauna of the Stampian deposits permit an outline of the biological zoning of the two levels in this layer:

− Lower Stampian (Fig. 55)

Echinoderms and nummulites are present only in the southern-most part of the basin, where the paleofaunistic associations also show the maximum numbers of different species: only this southern zone is under direct marine influence (zone II in present-day paralic terms).

On either side of the corridor formed by the zone and towards the north of the basin, a significant decrease is found in diversity of species which is accompanied locally by a proliferation of certain species. The fauna here consists only of molluscs, with gastropods becoming largely dominant. This denotes a much higher degree of confinement (zones III and IV).

− Upper Stampian (Fig. 56)

The outline is schematically the same, but, on the whole, the confinement is more pronounced: except in the region of Pierrefitte and Ormoy, the diversity of species is considerably reduced, in particular in the entire northwest of the basin where the fauna is reduced to a few pelecypods (Ostrea cyathyla, Cyrena convexa) together with Potamides lamarcki and Hydrobiidae. Moreover, the local presence (Evreux) of layers rich in organic matter indicates the high degree of confinement of this whole region (zones IV and V) and its typically paralic character.

The pattern of the biological organization of the Paris Basin in the Stampian and the confinement gradients which can be inferred from it show clearly that the influence of the sea during this period was limited to the south of the basin: it is therefore in this direction that communication with the marine domain must be sought. This conclusion, in contradiction with those of Alimen, seems to be corroborated by recent discoveries (Pomerol, 1973) in the south of the Paris Basin. Communication towards the Atlantic approximately following the Seine Valley, suggested by Alimen, should be discounted, particularly in the Upper Stampian (Figs. 57, 58).

3. CONCLUSION

The application of the biological organization pattern of the present-day paralic domain to fossil paralic basins shows the usefulness of the classification categories which defined the confinement scale, in particular the benthic organisms. It is among these zoological groups that the most fossilizable species are to be found, leaving exploitable traces in sedimentary rocks. What is more, a study of the structure and organization of benthic communities often gives with a certain precision a good picture of their natural environment, assimilating as they do the fluctuations in the conditions of the milieu, in . the more or less long term. Further, present-day species of benthic macrofauna, especially the paralic forms., in general have their counterparts in geological formations, particularly those which are not too remote in time. This permits a relatively viable reconstitution, without too much systematic distortion (specific or generic) of the structure, organization, and even, to a certain extent, the production level of fossil paralic basins: the organizational pattern of the present-day domain appears to be transposable in its broad outline on the Paleogene of the Paris Basin.

Without minimizing the difficulties linked with the evolution of thalassoid stock of the possibility of movements and modifications of fossil fauna, the study of paralic paleofauna allows the ecological dynamics of the formations under consideration to be retraced and may be, in the light of these propositions, an efficient tool for paleogeographical reconstitution.

Appendix 5

THE BALTIC SEA

The Baltic Sea has been the subject of substantial biological studies (Segerstråle, 1957), but the recent works of Andersin et al. (1976) on the community structure of soft-bottom benthic macrofauna in different parts of the Baltic illustrate well the biological organization pattern of the paralic domain which proposed the present work. The results obtained by these authors show that the pattern is applicable to a biogeographical region very different from that of the Mediterranean. The large number of sampling stations, together with the existence of a pronounced hydrochemical gradient and a diversified bathymetry, reveals the majority of the aspects of confinement and their consequences which are shown clearly but gradually.

However, the studies of Andersin et al. cover neither the Kattegat nor the Øresund, the strait separating the towns of Helsingør in Denmark and Halsingborg in Sweden. This region is of great interest, as it corresponds to the transit zone where waters of marine origin enter the Baltic; various other works have been consulted to complement those of Andersin et al., including those of Bråttstrom (in Sergerst-råle, 1957).

The Baltic Sea (420 000 km2 , 1 500 km long from Copenhagen to the Bay of Bothnia, maximum depth 495 m) separates continental Scandinavia from the rest of Europe. It forms a Y shape whose vertical line is the main part of the Baltic with Denmark as its base; the two oblique arms represent respectively the Gulf of Bothnia and the Gulf of Finland (Fig. 59). This "sea" is the remnant of the basin which constituted the bed of the North European ice sheet during the last ice age. More than 250 rivers drain the Scandinavian shield and flow into the Baltic: this, along with the low rate of evaporation, explains the generalized low salinity level of. the basin, with a very pronounced negative gradient from the Kattegat towards the Bay of Bothnia where salinity is lower than 2 ‰.

Andersin et al. calculated for each station the density (in numbers of individual samples per m2 ) the biomass (in g of dry weight per m2 ) and the diversity of species.

The principal conclusions which can be drawn from the analysis of these three parameters are as follows:

− The density increases notably (Fig. 60) from the Arkona Basin in the south, where it is around 2 000 individuals/m2 , to the Gulf of Bothnia in the north, where it exceeds 3 000 individuals/m2 and may reach totals of over 5 000 individuals/m2. The density decreases slightly in the Bay of Bothnia. It is noted immediately that the deep zones of the Central Basin are devoid of benthic macrofauna resulting from the lack of oxygen and the presence of H2S. The significance and origin of this phenomenon are examined later.

− The biomass decreases significantly from the Arkona Basin which contains the highest levels of biomass, in general more than 200 g/m2 , towards the Gulf of Bothnia where the biomass totals vary between 10 and 20 g/m2 (Fig. 61 ). Here also, the deep zones appear to be anomalous: when the level of dissolved oxygen falls below 2 ppm, the biomasses fall below 2.5 g/m2 and in places decrease to 1 g/m2. Andersin et al. emphasize (and this is the case for other basins) that the decrease in biomass accompanying the increase in density towards the more remote zones is due to the dwarfism which affects paralic species: they notice particularly that the size and weight

of the crustacean Pontoporeia affinis which dominates the communities of the northern regions of the Baltic are larger in the Gulf of Bothnia than in the Bay of Bothnia further north. They ascribe this reduction in size and weight to the salinity, the temperature, the trophic conditions and the combined effect of other factors. (The authors of this document ascribe it to the confinement.)

− The diversity of species decreases drastically from the Arkona Basin towards the outer limits of the Baltic. In the Arkona Basin, Andersin et al., record 41 species, 20 of which are found only in this area. The Central Basin contains only 20 species, the Gulf of Bothnia no more than 8. A marked diversity of species gradient is demonstrated very clearly in Figure 62, which shows also that the majority of stations situated in the northern part of the Gulf of Bothnia are colonized by only one species, the detritivorous crustacean, Pontoporeia affinis. This very pronounced diversity of species gradient may also be observed in Figure 63 which shows the decrease in the number of taxa constituting the total community in the different basins from Denmark towards the outer limits of the Baltic.

The study by Andersin et al. of the faunal composition of the benthie communities allows the biological organization pat tern of the paralic domain to the Baltic Sea to be applied.

In the southern part of the basin, molluscs largely dominate since they represent between 70 and 90% of the total biomass; nearly all these molluscs are pelecypods including Macoma baltica. The high biomass totals recorded in this region and the absence of echinoderms - confined to the Kattegat and the Straits and disappearing immediately to the east of the Danish coast (Fig. 64) - situate it in zone III (Fig. 65).

The communities of the central basin (particularly in its northern half) and the Gu,lf of Finland are no longer dominated by pelecypods but by crustaceans, particularly Pontoporeia affinis, which may alone represent 90% of the community. These two regions can thus be placed in zone IV.

In the Gulf of Bothnia, the communities are always dominated by crustaceans and Pontoporeia affinis alone represents nearly the whole of the total benthic macrofauna. Macoma baltica is still occasionally found. The greater part of the Gulf of Bothnia (southern and central parts) can again be situated in zone IV.

In the extreme northern part of this Gulf (Bay of Bothnia) nothing but Pontoporeia affinis is found at the great majority of the stations studied: this region is situated in zone V.

Andersin et al. emphasize the anomaly represented by the zones where the oxygen level is low. These zones are sites particularly subject to the accumulation of organic matter which, as demonstrated, brings about a transformation of the faunal composition of the communities. This is shown, in these anomalous zones of the Baltic, by the clear predominance of the polychaete Capitella capitata, a species which indicates "organic pollution".

Finally, in the deepest zones where water movement is nil (bathymetric confinement) large quantities of organic matter accumulate, originating not only from the biological activity of the superficial water layers, but also from the considerable inflow from the tributaries draining regions of high production and good conservation of vegetable debris. These deep areas are obviously situated in zone VI.

Thus, the Baltic presents an interesting confinement gradient but it also demonstrates various aspects of the conditions of this confinement, notably in relation to bathymetry: here, as seen in other examples, it is characterized by an increase in organic matter and a tendency to anoxia.

Andersin et al. attribute the variations in the composition of soft-bottom benthic communities to the parameters of salinity and dissolved oxygen alone. Neither of these parameters is a determining ecological factor, but both are tracers expressing confinement: one longitudinally, salinity; the other, vertically, the level of dissolved oxygen. These authors also emphasize in their conclusion that the "stability" of the communities in the poorly oxygenated deep zones is greater in the Gulf of Bothnia than in the more southerly zones: this confirms the stability of paralic communities.

Appendix 6

USE OF THE NOTION OF CONFINEMENT FOR AN AQUACULTURAL DEVELOPMENT SCHEME IN A PARALIC MILIEU

EXAMPLE OF THE NADOR LAGOON (MOROCCO)

Taking into account the qualitative and quantitative biological organization of paralic ecosystems, it can first be described in terms of confinement which permits a rapid description - simply and precisely - of the natural resources of each basin and of their distribution.

The second step could be the optimalization of the potential resources through the implantation in an adequate confinement zone of an intensive conchyliculture with species that live naturally fixed onto a hard substratum (oysters and mussels) in soft bottom only biotopes. In that case, zone III should be chosen for it corresponds to the maximum productivity of the malacofauna. This optimalization, which of course is at least partially dependent on local socio-economic conditions, entails no artificial development liable to modify the natural site, i.e., the various abiotic and biological gradients of the ecosystem.

A later step would be to envisage, by means of installations - mainly hydraulic - the adaptation of the ecosystem, through an adequate translation within the confinement scale, to an aqua-cultural activity defined by socio-economic imperatives.

This gradual approach is akin to what is known as "ecological planning" as defined by Falque et al. (1975), and employed again by Jouveniaux and Palanchon (1978). This approach has been applied to the aquacultural development possibilities of the Nador lagoon (Frisoni et al., 1982).

The definition of the natural resources of this lagoon can be based on the study of currents and hydrology (Fig. 66), bathymetry and granulometry (Fig. 67), the hydrochemical gradients and the phytoplanktonic biomass (Fig. 68) and above all the biological zoning calculated according to the benthic fauna, which makes possible a mapping of the basin in terms of confinement (Fig. 69).

The Nador lagoon is located mostly in zone II; and this situation in the lower part of the confinement scale is rather a disadvantage, for it means that most of the water is in a zone of average productivity, with a diversified fauna little adapted to an intensively aquacultural orientation.

Taking into consideration the "positive" and "negative" aspects (Guelorget et al., 1983) resulting from the human environment of this lagoon (Figs. 70, 71 and 72), it is possible to envisage without too "massive" a development, two types of aquacultural activity:

− an extensive tapiliculture (breeding of Veneridae), in the zones close to the sand bar where the granulometry of the sediments and the bathymetry are propitious;

− intensive suspended mussel and oyster breeding in the more central zones which are deeper and slightly more confined, near to the zone of maximum phytoplanktonic production (Fig. 73).

It is clear that an adequate arrangement reducing the communication between the lagoon and the sea, would in time increase the average confinement of the basin

and would shift it towards zone III, optimal for malacological production. This displacement would also extend zones IV and V, principal phytop lankton producers, which would result in an increase of the basin's richness with regard to the suspension-feeding species cultivated there. This being so, it is possible to envisage the further improvement of aquacultural activities by the establishment of hatcheries, the transformation of the outlying salt marshes into breeding basins, the setting-up of fish cages, etc. (Fig. 74).

This brief explanation shows all the interest of the zoning of the paralic domain which can be perceived on the spot quickly, reliably and at little expense applied to the development of the coastal regions.

Perhaps this technique could be applied to the oceans?

Appendix 7

ONE LAST WORD ON GEOLOGY

One of the most remarkable features of confinement is that it can be applied to all the spatial scales. Compare, for example, the tiny Bermuda Triangle passing from zone III into zone IV in the space of a few hundred metres; with the Great Baltic Sea, where zones III to V stretch over more than 1 500 km. This is because, in every paralic basin, confinement depends on the effectiveness of the communication with the sea and of the currents within the basin with regard to its size and volume. Thus, the large paralic basins are similar - in the geometrical sense of the word - to the smaller ones, as far as their biolo-gical zoning is concerned. In this respect, it should be remembered that the intertidal zoning on rocky surfaces is simply the transposition over several metres - or indeed centimetres -of subvertical wall, of the horizontal zoning of the lagunar milieux which can stretch over several kilometres.

Apart from the biological zoning, confinement controls the hydrochemical zoning (for a given freshwater balance), and one notes the similarity of the large paralic basins of the past and the smaller basins of today: for example, the Vosgian sandstones and the Rhone Delta, the Silurian formation of Salina, Michigan and the Sebkha el Melah, the evaporitic Trias of the Saharan shelf and the salt marsh of Salin-de-Giraud.

All this, in spite of the space/time scale differences separating the knowledge of the Earth's past which man is trying to acquire from the present-day knowledge of nature, justifies the study of the present to better understand the past.