Geconcerteerde Akties Oceanografie

Technisch Rapport Benthos 81/06

This paper not to be cited without prior reference to the author

Harpacticoid Copepod community Structure ~n two North Sea Estuaries

in Relation to Pollution.

D. C. Heip, R. Herman & M. Vaeremans

Marine Biology Section, Zoology Institute,State University of Ghent, Belgium.

Harpacticoid Copepod COlIllJ1unityStructure in two North Sea

Estuaries in Relation to Pollution

D. Van DalIllJ1e,C. Heip, R. Herman & M. Vaeremans

Marine Biology Section, Zoology Institute,State University of Ghent, Belgium

KEYWORDS:

Copepoda; meiobenthos; community structure; estuaries; inter-

tidal; pollution; heavy metals; North Sea.

ABSTRACT

The harpacticoid copepod assemblages of two estuaries in

the Netherlands, the Westerschelde and the Eems-Do LLard , are

compared. Both estuaries have similar physical characteristics

but the Westerschelde is much more polluted than the Eems-

Dollard. Harpacticoid copepod assemblages in both estuaries

are similar in terms of species composition. However, there

is a striking difference in quantitative characteristics :

the highest annual averages of density, biomass and diversity

in the Westerschelde are below the lowest annual averages in

the Eems-Dollard estuary. Because both estuaries are exten-

sively monitored, data concerning many possible causes of this

difference could be compared. It is argued that it is the

high load of heavy metals in the Westerschelde which is re-

sponsable for the observed decline. Harpacticoid copepods

seem to be a very suitable tool in ecological monitoring of

estuarine systems and can provide a good indication of gene-

ral 'health' of rivers and coastal zones.

Introduction

The monitoring of pollution in the sea has its principal

aim in public health considerations but concern for the ad-

verse effects of pollution on living marine resources is also

an important motive; the possibility of looking for effects

in marine organisms and communities more directly - in the

field - is clearly attractive but has not received appropria-

te attention (McIntyre & Pearce, 1980). In routine monitor-

ing biological parameters are usually discarded, mostly becau-

se the inherent variability of biological material and exist-

ing environmental 'noise' inhibit a clear interpretation of

the response of biological parameters to changes in the en-

vironment. In research, the main effort is still on the che-

mical analysis of the water and biological methods are, in

many instances, used only additionally or not at all.

Moreover, much of the existing research effort in the bio-

logical monitoring of pollution is directed towards the indi-

vidual organism because non-specif:i,ceffects of pollution can

be diagnozed by physiological, pathological and biochemical

responses which enable quantitative assessment of animal health,

with the additional advantage that an early warning can be

signalled (Bayne, 1979).

In this paper we will argue the case for what thus appears

to be the most unpopular way to look at the impact of pollu-

tion, i.e. by looking for changes in biological communities

in the field. There are several drawbacks in this approach

which we recognize : it provides no early warning as it will

show changes only when they are already important; and, these

changes will be difficult to interprete and impossible to link

to some particular cause with absolute certainty. Moreover,

ecological monitoring will only be useful if done over time

periods which are long compared to the response time of the

system.

But, there is not a single method which will provide the

environmentalist with all the answers. The effect of many

kinds of pollutants acting together is far from simple and

predictions based on simple concentration measurements as ln

routine chemical analysis are so over-simplified that they are

extremely dangerous. The availability of pollutants to organ-

isms is often not a linear function of total concentration,

e.g. copper in sea water is complexed actively by dissolved

organic matter and this complexation is pH-sensitive whereas

cadmium is complexed by chloride ions and is not significant·-

ly affected by dissolved organic matter. Also, the physiolo-

gical effect of some pollutant on an organism is governed by

many extrensic and Lnt.rins i,ccauses of variation which necessi-

tate the simultaneous measurement of a lot of environmental

factors together with intrinsic causes such as reproductive

and nutritional status (Bayne et al., 1980).

Monitoring the canges in communities in the field has at

least this advantage that these changes are directly measured.

In many existing estuaries and coastal areas an early warning

is no longer particularly needed. What we need is some set of

measures that allow us to see if things go better or worse -

even if we cannot explain the change in terms of causal rela-'

tionships.In this study we look at assemblages of harpacticoid cope-

pods from two estuaries in the Netherlands. Harpacticoid

copepods are very small crustaceans (about 0.5 to 1 rom) which

live in or upon the sediments. Their possible utility in eco-

logical monitoring has been discussed by Heip (1980) who al-

ready stressed that they should prove to be excellent 'indi-

cator-organisms', as they live associated with the sediments,

which are the ultimate sink and thus the best indicators of

pollution in most instances, and as they are small and should

therefore be more sensitive.

The two estuaries which we will compare are the Western

ScheIdt, the most southern estuary in the Netherlands, and the

Eems-Dollard, which is its most northern estuary, on the bor-

der with Germany. Although situated in the Netherlands, the

Western ScheIdt carries a huge load of many different kinds

of pollutants originating from human activities ~n Belgium

(and even France) and it is an open sewer right to the Belgian-

Dutch border. The Eems-Dollard is only occasionally polluted

by organic matter from potato-flour mills, annually in early

autumn. For both estuaries a lot of data concerning water

quality have been collected over the year by the Rijkswater-

staat (Water Control. Public Works Department) and by several

Belgian groups connected with the Management Unit of the Math-

ematical Model of the North Sea and the ScheIdt Estuary (Min-

istry of Public Health, Belgium). Apart from this difference

in the degree of pollution both estuaries are comparable and

they harbour very similar harpacticoid species.

Material and Methods

The localisation of the stations is charted in Fig. 1. TheF1l -t .,,~

,.1 term 'stationgroup', is used to design all stations from a

particular transect. The Vlissingen-stationgroup includes the

I'

I!..?

7 stations (WS 51-57) from Breskens to Vlissingen. The Ter-

neuzen-stationgroup encloses the station WS 41-,45, the

Ossenisse and Valkenisse transects comprise resp. the stations

WS 31-34 and WS 21-25. Most of the stations are intertidal

and situated on shallow sand- or mudflats between + 1.0 m and

- 3.0 m MTL. Truly subtidal samples were taken in the chan-

nels at depths of -25 to -60 m on each transect during Sep-

tember 1980. During this same period macro-algae were col.Lec;-:

ted a.n intertidal pools on sandbanks in the mouth.

In the Westerschelde estuary 21 stations divided over four

transects were sampled from September 1978 till September 1979

on the following dates: 27-29.09.1978; 11-13.12.1978; 03-05.

04.1979; 24-26.06.1979; 03-05.09.1979. Meiofauna at the Doel

transect was sampled and studied a year earlier (Holvoe~ 1978)

from May 1977 till May 1978 over 6 sampling periods on the

following dates: 05.05.1977; 17.06.1977; 01.09.1977; 18.10.

1977; 26.01.1978; 09.05.1978.

The sampling technique differs according to tidal level.

At the deeper stations a modified Reineck boxcorer (Farris and

Crezee, 1976) was used, from which four subsamples were taken.

Two replicates for meiofauna were fixed with warm formalin

(lO°C) to a final concentration of 4 %, the two other cores

for chemical and sediment analysis were frozen immediately.

In shallow water (-3.5 m) samples could be collected with a

'meiosticker', a telescoping tube (max. length 5.5 m) equiped

with a head into which a plastic core can be screwed (Govaere

and Thielemans, 1979). At the other intertidal stations the

samples were handcollected. Plastic cores with a 10.2 cm2

inner surface were used for all direct sampling and subsampling.

In the Eems Dollard estuary benthic harpacticoids weresampled from September 1976 till July 1977.

At Uithuizerwad, 9 stations on a transect of 2400 m were

sampled in autumn (21.09.1976) and winter (01.02.1977); at

Eemshaven a 150 m transect (8 stations) and at Hoogwatum a

100 m transect (4 stations) were sampled in autumn (23.09.1976)

and spring (25.03.1977). At the Reidersplaat the 4 stations

along a 400 m transect were sampled in winter (28.12.1976) and

spring (29.03.1977). The Heringplaat-stations (interdistance:

150 m) were sampled only in sunnner 1977 (27.07 •.1977) and 5

stations at the Oost-Friesche Plaat (interdistance : 500 m)

were sampled in late summe r and autumn 1976 (15.09.1976; 16.11.

1976). The location of these stationgroups is also marked in

fig. 1. All stations are situated on intertidal sand~ or mud-

flats.

All samples were handcollected with a 3.4 cm inner diameter

core except at the Oost-Friesche Plaat where a 2.4 cm inner

diameter core was used.

In the laboratory the samples were passed over 250 l.lmand

38 l.lmsieves.

For sandy sediments, the meiobenthos was elutriated from

both fractions using the trough method (Barnett, 1968; Heip,

1976).

For muddy sediments the organisms from the smaller fraction

were extracted using a density gradient centrifugation techni-

que (Bowen et al., 1972; de Jonge and Bouwman, 1977). After

staining with rose Bengal all groups of the meiobenthos were

counted, and the harpacticoids were studied systematically

(Van Damme et al.., 1981).

For estimating biomass, a Mettler ME 22/BA 25 microbalance,

accurate to 0.1 ~g, was used. The individual adult dry weight

per species was measured after drying the specimen in an oven

at 110° C for 2 h. Copepodites were assumed to have one eight

of adult weight.

As measure of species diversity the Brillouin formula wasused: H = -N1log __ .E.~__ . (Pielou, 1975).

n n .... n1 2 S

Median grainsize values were obtained according to the

technique described in the IPB handbook (Buchanan & Kain, 1971).

Sediment analysis was done on a 25 g ovendried subsampLe . For

classification of the sand fraction the Wentworth scale was

used. Stations with a mud fraction (silt-clay particles

< 62 11m) lower than 2 % are considered here as 'pure sand sta-

tions', while stations with a mud content in excess of 2 % are

here termed 'muddy sand stations'.

The amount of organic matter was estimated by loss of

weight on ignition at 550°C (Wollast, 1976). For the deter-

mination of organic carbon we used the method of El Wakeel and

Riley (1956).

The mean annual salinity per station was used to classify

the different station groups into the salinity zones of the

Venice system (Remane and Schlieper, 1971).

Chemical analysis of the water column and determination of

trace metals in water and sediment of the Verdonken Land van

Saaftinge was done by Dr. F. Vaes at the Provinciaal Instituut

voor Milieuhygiene, Oost-Vlaanderen (for heavy metals X-ray

fluorescence and atomic absorption spectrofotometry was used,

for detergents in Saaftinge the methylene blue method).

Characteristics of the two e$tuaries---_.-The main hydrodynamical and physical characteristics of the

two estuaries are summarized in table 1. Differences in these

factors and topography (the Westerschelde is elongated and

narrow, the Eems-Dollard truncated and broad) are correlated

with the distance of the :respective estuaries to the bottle-

neck of the English channel (Van Veen, 1950). Tidal volumes

and tidal amplitudes are higher in the Westerschelde and hence

turbulence is higher, which is still more pronounced because

the Westerschelde is open and directed to the prevailing winds

and not protected by an island chain as the Eems-Dollard.

Lateral extensions, traces of fermer inundations, occur in

both estuaries. The Verdronken Land van Saaftinge (the Drowned

Land of Saaftinge) results mainly from an inundation for mili-

tary purposes in 1584 (Brand, 1975). It has evolved into a

high saltmarsh (27 km2) with a Spartrina and Pueeinel.l.ia type

of vegetation, crisscrossed by an intricate system of tidal

creeks connected with the Westerschelde. During low tide these

creeks drain completely. During flood the tidal currents are

strong s~nce the tidal amplitude is about 4.5 m in that area

of the Westerschelde (Wolff and Beeftinck, 1975).

In the Eems-Dollard estuary, the Dollard is a vast interti-

dal mudflat (100 km2) interlaced with creeks. Vegetation is

confined to the edges. It was formed by repeated inundations

between the 13th and the 16th century (De Smet & Wiggers, 1960).

In the Westerschelde, pure sands with a median grai,n size

around 200 l.lmand a mud content never in excess of 2 % are

found on the sandflats situated near the tidal channels, where

tidal currents and wave turbulence are strongest. The sediment

is well oxigenized up to the maximal depth sampled (.:!:. 20 em)

and is displaced during floodperiods in a 'sawteeth'-movement

around the banks (Peters and Sterling, 1976). The organic

matter content averages between 1.5·-2.5 % and the organic car-

bon content is around 0.05 % (table 2).

Pure sand stations occur also in the Eems-Dollard in areas

with similar hydrodynamical conditions, but the mean grain sl--

ze here is smaller (110 to 125 11m) and the organic carbon corr-

tent higher (0.11-0.26 %). Muddy sands occur over the whole

Westerschelde estuary, along the edges, where currents and wave

turbulence are minimal, allowing fine particles to settle. The

grain size is on average around 150 11m and the mud percentage

is well above 3.0 % throughout the year. Stations with very

soft sediments (black muds) occur at Doel (WS 14 : avo 53 %

mud) and at the Vlissingengroup (WS 51 : avo 21 % mud).

The organic matter content of muddy sands lies aroung 5.0

to 7.0 % and organic carbon averages between 0.30 and 1.60 %,

except at Doel where it is higher. At this station group, all

stations, even in turbulent waters, contain more than 3 % mud

during at least part of the year, due to their location in the

flocculation zone (see further) (table 2). The top sediment

(5 em) of the saltmarsh of Saaftinge consists also of black

muds and the sands below are anoxic.

The muddy sands of the Eems-Dollard have a mud content of

4 to 14 % and an organic carbon content of 0.80 to 2.70 %.

Ttans£ott and accumulation of pollutants

Westerschelde estuary and Saaftinge saltmarsh

About 1.52 x 106 ton/yr of suspended solids are discharged

into the estuary by the river Schelde, highly polluted by or-

ganic compounds (N = 2.0,600 ton/yr; P = 125,000 ton/yr; O.M. =250,000 ton/yr) and heavy metals (Zn = 2000 ton/yr; eu = 49.5

ton/yr; Pb = 450 ton/yr; Hg = 1.52 ton/yr).

In a zone with a salinity between 1 0/00 and 5 0/00, most

of the suspended matter flocculates and is sedimented. The

corresponding zone of highest mud accumulation and turbidity

is located between Antwerpen and Doel (where residual bottom

currents are zero, Peters and Sterling, 1976). The seaward

limit of the flocculation zone may shift as far as Hansweert

during periods of increased river discharge (250 mS/s) (Peters,

1975). About 1.2 x 106 ton/yr (60 %) of solids of continental

origin are trapped in this area together with 0.8 x 106 toni

yr (40 %) of solids, mainly coarser clastics, of marine origin.

Most of the heavy metal load is coprecipitated and fixed in

the sediment as insoluble sulphides (Wollast, 1976; Wollast

and Peters, 1978). The amounts of these pollutants in the se-

diments and in the suspended matter are therefore very high

(table 3).r-:5 .::"')

Degradation of the organic matter may release again com-

plexed or adsorbed heavy metals. Most of the organic matter

1S removed from the water by sedimentation in biodegradation

in the flocculation zone. Only 20 % reaches the seaward part

of the estuary (Wollast, 1976). As a result of the activity

of heterotrofic bacteria, the waters in the zone Antwerpen-

Bath are frequently anoxic, especially in summer and toxics

such as H2S and ammonia are released (Billen et aI., 1976).

Nitrogen and phosphorus are also precipitated through the ac-

tivity of auto- and heterotrofic organisms but relatively larg-

ge amounts still persist in the seaward part of the estuary

(Hansweert-Vlissingen) and even at the mouth ammonia is still

present. The longitudinal profiles of oxygen, ammonia, total

phosphates and organic carbon are listed in table 4.

About 2.2 x 105 ton/yr of solids of continental origin are

deposited in the seaward part of the estuary. The concentra-

tions of heavy metals, especially copper and to a lesser de-

gree zinc, in the suspended matter decrease as they dissolve

or become desorbed, e.g. dissolved eu increased from about 10

~g/l at Doel to 20 ~g/l at Vlissingen; dissolved Pb increased

from about 7 ~g/l at Antwerp to 40 ~g/l at Doe L, then decrea-:

sed again seawards to 20 ~g/l (Wollast, 1976). Dissolved Hg

remained constant around 0,1 ~g/l (Anonymous, 1978-1979).

Iron and magnesium on the other hand precipitate in this

zone as hydrous oxides (Wollast and Peters, 1978). Since in

this form they are known scavengers of heavy metals in sol-

ution (Duinker and Nolthing, 1976, 1977) this could partially

explain the still very high amounts of heavy metals in the

sediments of the zone Hansweert-Vlissingen. Hoenig (1978)

also discovered the existence of a relatively small but im-

portant accumulation area in front of the harbour of Terneuzen

and noted periodical high peaks of dissolved and particulate

heavy metals in this zone. The suspended matter, deposited in

the Terneuzen area (2 x 104 ton/yr (Wollast, 1976))originates

from the Gent-Terneuzen channel, which runs through an indus-

trial complex (a.o. chemical and petrochemical, woodpulp and

sugarbeet industry, metallurgy) and which is regularly flushed

by highly polluted Schelde waters in order to halt the land-

inward progress of the salinity front (Bakker and Heerebaut,

1971). Although the input of solids from the channel is minor,

the relative concentration of heavy metals in the particulate

matter is extremely high (table 3).

Apart from the ones already mentioned, the concentration

of the following toxic compounds have been measured in the

surface waters of the Westerschelde during the period of our

survey (Anonymous, 1978-1979).

Pesticides and PCB's occur over the whole Westerschelde ~n

extremely low concentrations; only lindane is sporadically

found in the watercolumn (0.01 jlg/l). The concentrations of

aldrin, telod:dn, dieldrin, endin, DDE, hexachlorobenzene and

PCB's are similar or even lower in mussels and shrimps from the

mouth (Vlissingen) of the Westerschelde estuary when compared

to populations of other localities along the Dutch coast and

specifically the Eems-Dollard estuary (see further). Fluoride

is found ~n a concentration of about 1.5 mg F/l over the whole

estuary, this value is 2-3 times higher than in all other Dutch

waters. Phenol does not reach the seaward part in high con-

centrations; at the mouth the concentration is sporadically in

excess of 1.0 ug /L. In this region concentrations of oil are

usually around 0.1 jlg/grwith a maximum of 1.2 jlg oil/gr.

Polynuclear aromatic hydrocarbons were present at Doel in con-

centrations of 0.3-0.7 jlg/l. Synthetic anionactive detergents

are practically continually present in a concentration of

0.04-0.05 mg maxonol oT/l over the whole estuary (at Doel,

Nihoul and Boelen (1978) found values between 0.07-1.39, ex-

pressed in mg/l). During a Belgian survey (1971-1975) cyanide

was not detectable in most samples collected near Doel; in on-

lyone sample a concentration of 11 ~g/l was recorded (Nihoul

and Boelen, 1978).

Sediments, a.o. the toxic muds before the harbour of Ter-

neuzen, are continually dredged in the Westerschelde and (in

the Dutch part) the dredge spoil is again dumped 1n the estu-

ary in floodchannels, a.o. at Everingen (fig. 1) so that

coulds of muds are transported streamupward with the flood

currents (this is done on purpose in order to counter or halt

relief changes in tidal channels which could be hazardous for

the naval traffic). The amount of dredged sediment in 1972 was

9 x 106 m3/yr (Theuns, 1975).

In september 1980 a water and sediment sample was taken at

the mouth of the main creek in the Saaftinge saltmarsh. Nu-

trients in the water column are low (table 4) and comparable

to the concentrations found at the mouth of the Westerschelde.

Concentrations of heavy metals, especially copper, are very

low in the sediments in comparison to the Westerschelde (table

3). Ammonia however is relatively high (1.6 mg N/l), fluoride

(0.69 mg F/l) about half of the concentration found in the

estuary, while the amount of detergents was higher (3.2 mg/l).

The main cause for the lower concentrations of most toxic

compounds in the saltmarsh is probably the very small degree

of exchange between the tidal watermass that moves in and out

of the creeks and the Westerschelde waters (De Pauw, 1975).

However, there is probably also a mechanism of selective trans--

port of suspended matter at work.

The Eems-Dollard estuary

The amount of suspended solids carried by the river Eems

is estimated at 65,000 ton/yr (Hinrich, 1974). The turbidity

maximum is limited to a narrow area between Ditsum and Emden

(Postma, 1960). All the particulate matter originating from

the river is transported by tidal currents to the eastern

(German) part of the Dollard where is settles. Freshwater is

also carried into the Dollard from the Westerwoldse Aa river.

This river is organically polluted by potatoflour mills. The

input of organic matter varies from 1200 ton in summer to

25,000 ton in late autumn, this is about 0.5-10 x 103 ton C/yr

(Schroder et al. 1976). Concentrations of nutrients and or-

ganic matter in this estuary therefore reach maxima in the

south-eastern part of the Dollard (Oost-Friesche plaat), where

annual averages of oxygen, nutrients and organic matter are

similar to the values occurring near the seaward limit of the

flocculation zone in the Westerschelde (table 4). During the

peak activity of the potatoflour mills, maximal concentrations

of these parameters are even above the ones occurring at Doel.

Nutrients N, P and Si are never limiting for the primary pro-

ducers and during autumn the water becomes anoxic and high

amounts of ammonia and H2S are observed (Bouwman & Kop, 1979).

The concentrations of oxygen and nutrients in the rest of the

Eems-Dollard are similar to the ones occurring in the seaward

part of the Westerschelde.

The Dollard sediments are predominantly (as is the rest of

the estuary) of marine origin (~ 90 %), and the apport of

solids from the North Sea is estimated at I x 106 ton/yr

(De Smet & Wiggers, 1960).

The concentrations of heavy metals are very low in com-

parison with the Westerschelde, both in sediments and in sus-

pended matter (table 3). However, comparison is rendered dif-

ficult because Salomons and Mook (I977) and Essink (1980) ex-

press the concentrations at 50 % sedimentparticles smaller

than 16 um ('because of the linear' correlations between heavy

metal concentrations and the amounts of particles < 16 ]lm l.n

sediments, making it possible to characterize the content of a

specific metal of a whole group of co-genetic sediments by a

single value' (De Groot, 1973».

The Westerschelde values are expressed as concentrations in

mg/kg dry weight of the total sediment, which rarely contains

more than 20 % mud. If the Westerschelde values would be cal-

culated at 50 % particles < 16 ]lm (but this would be a danger-

ous extrapollation) the differences in heavy metal concentra-

tions between both estuaries would be even more pronounced.

From 1960 till 1975 the Eems-Dollard estuary received sev-'

eral tons of mercury per year, mainly from a chloro-alkaline

plant and a pesticide factory. The concentration of dissolved

Hg varied from 0.31 ]lg/l in 1975 to 0.10 ]lg/l in 1978, while

the mercury content of the particulate matter averaged between

1.76-1.77 ]lg/g (Essink, 1980). The average concentration in

the seaward part of the Westerschelde is in comparison 506.6

]lg/g (min. : 324 ]lg/g) according to Hoenig (1978), but Ano-

nymous (1978-1979) cites similar values for the Westerschelde

as those occurring in the Eems-Dollard, so Hoenig's values are

probably erroneous.

Salomons and Mook (1977) found no evidence of mobilisation

of trace metals from the sediment in the Eems-Dollard estuary.

Koeman (1971) found pesticide concentrations at Delfzijlof 0.009 ppm wet weight of aldrin, 0.002 ppm telodrin, 0.065

ppm dieldrin, 0.020 ppm endrin and 0.015 ppm DDE in muscle

tissue of MytitU8 eduZis. Concentrations at the mouth of the

Westerschelde were at least 2-3 times lower except for endrin

which was equal (0.017 ppm). Hexachlorobenzene was present

usually below measurable concentrations « 0.01 jlg/l) in the

surface waters (Fonds and Greve, 1973) and with a concentra-

tion of 0.05-0.99 ppm wet weight in shrimps (Crangon crangon),

Le. about 2-3 times higher than found in these organisms at

the mouth of the Westerschelde (Hagel and Tuinstra, 1974).

Concentrations of PCB's on the other hand are similar in

Eems-Dollard shrimps (0.05-0.14 ppm wet weight) and at the

mouth of the Westerschelde (0.07-0.18 ppm wet weight) (Ten

Berge and Hillebrand, 1974; Essink, 1976).

We possess no figures on concentrations of phenols, fluori'-

des and oil in the surface waters of the Eems-Dollard. Accord-

ing to De Groot (1973) the main mobilizing compounds of heavy

metals in the Eems are phenoles originating from peat layers.

The maximal amount of anionactive detergents measured in the

Eems river was 0.3 mg/I.

The Harpacticoid fauna

Density and biomass

In the Westerschelde the annual average density over all

pure sand stations is highest at the Vlissingen station group

at the mouth (26.4 ind./10 cm2), peaks down at the Terneuzen

stations (0.64 ind./10 cm2), increases again at Ossenisse

(12.0 ind./10 cm2) and decreases to very low levels at Valke-

!'/,I

h (I",~"

nisse and zero at Doel. Biomass shows a similar trend with

6.6 ~g dwt/IO cm2 as the highest average. Maximum density

noted was 128 ind./IO cm2 (Vlissingen station group), maximum

biomass 33 ~g dwt/IO cm2 (Ossenisse station group) (table 5).

In muddy sands a similar trend 1n density and biomass is

observed with respective averages of 10.36 ind./IO cm2 at the

Vlissingen stations, 2 •.30at Terneuzen, 9.60 at Ossenisse,

0.77 at Valkenisse and 0.10 ind./IO cm2 at Doel (table 5, fig.

2). Maximum density recorded was 68 ind./IO cm2, maximum bio-

mass 146.5 ~g dwt/IO cm2• In 66 % of all samples (pure sands

and muddy sands) no harpacticoids were found. Per station

group this percentage is resp. 22 % at Vlissingen, 50 % at

Terneuzen, 60 % at Ossenisse, 82 % at Valkenisse and 94 % at

Doel.

In the Saaftinge saltmarsh the annual average density and

biomass were respectively 69.7 ind./IO cm2 and 236 ~g dwt/IO

cm2 (table 5, fig. 2).

A survey in September 1980 of the subtidal sediments (muddy

sands and pure sands) along the Westerschelde yielded maxima

of 13 ind./IO cm2 at the mouth. No harpacticoids were found

in the subtidal region upstream from Terneuzen. Investigation

of harpacticoids of macroalgae in tidal pools at the mouth

during the same period yielded very low densities, approxi.-'

matively 0.10 ind./IO cm2, although other meiofauna groups such

as nematodes, turbellarians and gastrotrichs were numerous.

The average annual density in the Eems-Dollard is highest

at the station group at the mouth with no significant differ-

ence between pure sandy sediments (87.7 ind./IO cm2 at Eems-

haven) and muddy sands (89.9 ind./IO cm2 at Uithuizerwad).Upstream in the Eems estuary, densi~y is halved (46.1 ind./

10 cm2 at Hoogwatum). Similar densities were found in the

outer part of the Dollard with higher values in the muddy

sand stations (44.5 ind./IO cm2 at Reidersplaat) than in the

pure sand stations (36.0 ind./IO cm2 at Heringsplaat) (arti-

fact due to low number of samples ?). At the most polluted

stationgroup (Oost-Friesche Plaat), the lowest average of

17.3 ind./IO cm2 was recorded (table 5, fig. 2). Biomass

shows a similar trend, declining from 147.9 l1g dwt/IO cm2 at

the mouth (Eemshaven) to 29.2 l1gdwt/IO cm2 at the Oost-Frie-

sche Plaat. Maximum density and biomass noted in this estuary

were respectively 478 ind./IO cm2 and 602.911g dwt/IO cm2 (Uit-

huizerwad (table 5).

Density is zero in 22 % of the samples at the Oost-Friesche

Plaat. At all other station groups Harpacticoids occur in all

samples.

Community structure and diversity

A total of 23 species was found on the intertidal flats of

the Westerschelde estuary (table 6). They can be divided in

two distinct communities : a mesobenthic community, consisting

of small interstitially living grazers and an epibenthic com-

munity consisting of large burrowing and epibenthic detritus-

feeders. There is no overlap or intermediate community in the

Westerschelde, because the two habitats are incompatible for

the communities involved : in the pure coarser sands the ab-

sence of detritus or epibenthic diatoms and the high turbulen-

ce exclude the presence of epibenthic copepods. In the low

energy zone the interstitial habitat is unavailable because

of the fine texture of the sands « 200 ym) and a too high

mud content in excess of the limit of 7 % (according to Ward,

1975) or 2-3 % (according to Van Damme and Heip, 1977 and

Govaere et al., 1980).

The occurrence of a relatively well structured interstitial

community is restricted to 2 very turbulent stations at the

mouth (Vlissingen station group). Here a total of '7species

was found with as dominant forms Paral-eptiaetacue eepinul.atue

(33.8 %), KUopsy Tlue eonetx-ictue (29.2 %) and Parameeochra

simiUs (26.5 %). In the eu-polyhaline zone at Terneuzen only

1 species was recorded: Euaneul.apygmaea (3 specimens) al-

though the sediment is suitable for interstitial life at 3

stations (15 samples). In the poLy-: mesohaline zone, 3 spe-

cies were found with P. espinuZatus extremely dominant (93 %).In the mesohaline zone (Valkenisse) only a few specimens of

P. espinuZatus were found, while at Doel the pure sand habitat

does not exist (table 6).

Mean annual diversity is highest at the mouth (Vlissingen)

with H = 1.08, falls to zero at Terneuzen and rises again to

H = 0.11 at the Ossenisse station group and is zero at Valke-

nisse (table 5). Maximum diversity (H = 1.67) was noted at

the Vlissingen station group during autumn. Diversity was ze-

ro in 80 % of the pure sand samples.

The best structured epibenthic community again occurs at

Vlissingen with a total of 13 species, decreasing to 8 at Ter-

neuzen, 7 at Ossenisse and 3 at Valkenisse and Doel.

Tachidius discipes is dominant in the eu-polYhaline (35 %)and poly-mesohaline zone (66 %) and StenheUa paZustris in

OJ.0'1,

the meso-(81 %) and meso-oligohaline zones (57 %) where T.

diseipes did not occur (table 6). T. discipes., S. palustris

and Asellopsis intermedia represent 80-90 % of the whole epi-

benthic fauna in all salinity zones.

Mean annual diversity of this community declines from H =0.37 at Vlissingen to H = 0.24 at Terneuzen, H = 0.16 at

Ossenisse, H = 0.04 at Valkenisse and H = 0.01 at Doel (table

5, fig. 2). Maximal diversity of H 1.24 was recorded during

winter at the Vlissingen transect. Diversity was zero in 84 %

of the muddy sand samples.

In the subtidal area of the eu-polyhaline zone a totally

different epibenthic community occurs, consisting of Pseudo-

nuchocamptue proorimue, MicroarthY'idion littorale., AJrrphiascoi-

des debi.l.ie and Robertiquxmeua ep ; Only some specimens of T.

discipes were found on the macroalgae.

At the saltmarsh of Saaftinge, a total of 5 species was

recorded with as dominant one Nannopus palustris (63.7 %).The Saaftinge epibenthic community is totally different from

the Westerschelde estuary in species composition and structure,

since, except for Stenhelia paZustris, the other species do

not occur or are very rare in estuarine samples. Mean annual

diversity was H = 1.14 (max H = 1.39 in summer; min H = 0.81

in winter) (table 5, fig. 2).

In the Eems-Dollard estuary a total of 24 species was re-

corded (tabie 7). They all belong to the epibenthic community

and are found on the pure as well as on the muddy sands. Their

presence on the first is probably made possible by the large

amounts of epibenthic diatoms which cover the surface as a

film during low tide. An interstitial harpacticoid community

is absent in pure sands due to the small grain size « 200 ~m).

In the eu-polyhaline zone (Uithuizerwad, Eemshaven) a total

of 21 species was recorded with no outspoken dominance. The

3 most abundant species are Haieotrinoeoma ourtrioorne (19.0 %),

Harpatrioue fZexus (16.2 %) and Tachidiue dieeipee (15.1 %).

In the poly-mesohaline zone (Hoogwatum) the total was 17

species with as dominant forms Microarthridion l.ititorale

(57.4 %), T. di.eeipee (11.8 %) and Hs ourbiooime (10.4 %). In

the mesohaline zone (Reiderplaat, Heringplaat) at the mouth of

the Dollard, the number of species decreased to 11 with domi-

nance of H.curticorne (42.6 %),Paronychocamptus nanus (16.9 %)and M. UttoraZe (10.7 %). In the meso-oligohaline zone (Oost-

Friesche Plaat), l.nthe organically polluted south-eastern cor-

ner of the Dollard, the total number of species was reduced to

6 and only 2 species, StenheUa paZustris (49.1 %) and Nanno-

pus paluetx-ie (39.9 %), were still relatively abundant. Both

are typical for intertidal black muds (Lang 1948; Noodt, 1957)

and Nannopus paZustris can survive prolonged periods (96 h) in

anoxic conditions (Coull et al ,, 1979). However, during the

period of maximal sewage output of the potatoflour mills, only

StenheUa paZustris occurred.

Highest annual average diversities occur at the mouth of the

estuary with H = 2.20 in pure sands (Eemshaven) and H = 1.47 in

muddy sands (Uithuizerwad). Diversity does not decrease sig-

nificantly upstream since values of H = 1.42 (Hoogwatum) and

H = 1.44 (Reiderplaat) were recorded. The relatively low

value of H = 0.93 at Heringplaat, the other station group at

the mouth of the Dollard, maybe an artifact due to the small

number of samples. The lowest annual average was noted at the

Oos t.-Eri.e sche Plaat : H = 0.41 (table 5, fig. 2). Maximum

diversity of H = 2.68 was recorded in March at Eemshaven.

Diversity is zero in 1 % of all Eems-Dollard samples, which

correspond to the 22 % of the Oost-FI'iesche Plaat samples and

one of the Uithuizerwad samples (with a low density).

Discussion

In both estuaries, mean annual density and biomass decline

from the station group closest to the mouth to the one most

inland (fig. 2). While in the Eems-Dollard this decrease is

gradual, this is not the case in the Westerschelde where an

inverse peak occurs at the Terneuzen station group. Whereas

there is some resemblance in pattern between the two estuaries,

absolute values of all community parameters are completely

different. The highest annual averages of density and biomass

in the Westerschelde are lower than the lowest annual averages

occurring even at the Oost-Friesche Plaat group in the Eems-

Dollard (fig. 2). On the other hand, density recorded in the

Saaftinge saltmarsh approaches the highest ones found in the

Eems-Dollard estuary, while the biomass recorded in this salt-

marsh exceeds all values recorded from this latter estuary.

Sunnner averages recorded in the adjoining estuaries Ooster-

schelde and Grevelingen are about 4-7 times higher than the

peak summer densities (23 ind./IO cm2) noted in the Wester-

scheLde • Surkyn (1977) found averages of 119 ind./10 cm2 on

intertidal mudflats of the Oosterschelde (n = 7) and 200 ind./

10 cm2 on permanently flooded mudflats in Lake Grevelingen

(n = 7).

Values recorded in literature are also substantially high-

er, both for organically polluted as for unpolluted environ-

ments. Arlt (1975) records densities of 88 ind./IO cm2 in

front of a sewage outlet of a small town, 165 ind./IO cm2 .30

meters furthe r off and 96 indo /10 cm2 at an unpolluted station

on muddy sand and sandy sediments of the oligohaline Greifs

Walder Bodden. Warwick et al. (1979) record an average of

279 ind./IO cm2 on an intertidal mudflat of the Lynher estu-

ary and Coull et al. (1979) and Fleeger (1980) give a range

of average monthly densities of resp. 73-262 ind./IO cm2 and

75-620 ind./IO cm2 for S.-Carolina salt marshes. In a long

term study (9 years; 205 samples) Heip (1980) found a har-

pacticoid density of on average 211 ind./IO cm2 in a shallow

brackish water pond.

Although Westerschelde and Eems-Dollard are closely rela-

ted with regard to species composition, since 65 % of the

epibenthic and burrowing species of the Westerschelde occur

also in the Eems-Dollard, the qualitative parameters reflect

a impoverishment of the fauna in the first estuary. In the

first place there is the overwhelming dominance of the same 3

species AselZopsisintermedia., Taehidius diseipes and Stenhe-

lia palustris along the whole salinity gradient in the Wes-

terschelde, whereas dominance shifts to different combinations

of species in the Eems-Dollard at each station group, and the

above mentioned species remain unimportant except at the Oost-

Friesche Plaat. Also interesting to note is the presence of

Mie:'f'oarthridion littorale in all intertidal station groups of

the Eems-Dollard and its confinement to the eu-polyhaline sub-

littoral in the Westerschelde, although the species must be

extremely plastic physiologically (Coull et aT., 1979). M.

littorale is dominant and often the only species occurring in

the polluted subtidal muds before the Belgian coast (Van Damme

and Heip, 1977; Govaere et al.; , 1980).

In summer samples of an intertidal mudflat of the Ooster-

schelde an average diversity of H = 1.8 was recorded; dominant

were Aeel.lopei.e intermedia (37.4 %), HarpaetricuefZexus (36.1%)

and Axenoeetel.La ep; (15 %). In a subtidal mudflat in the

G:revelingen the average diversity was H :::;1.9 and the dominant

species we.re Canuel-la epp: (C. fureige.ra-C. perpl.eea : 56.3 %),

Ameira pamnila (20.8 %) and Harpactricue flexus (7.1 %) (Surkyn,

1977) •

Fleeger (I980) records monthly mean averages of H :::;1.44'-

1.69 for two subtidal stations and H:::;1.50 for an inte:rtidal

site at North Inlet, South Carolina. The highest annual aver-

age diversity for the epibenthic community recorded at the

Westerschelde (H :::;0.59) is beneath the lowest average found

at the Dollard and is about 3-4 times lower when compared to

the saltmarsh of Saaftinge and adjacent estuaries or with the

monthly averages recorded by Fleeger (1980). Heip (1980) re-

corded in his long term study a mean annual diversity of H :::;

I .06.

The very low average values of all pa:rameters in the Wester-

schelde are in the first place due to the complete absence of

Harpacticoids in most samples (65 %). In the remaining ones

density remains also low as does dive:rsity, since the average

number of species per sample is usually also very low (H :::;0

in 80 % of all samples). This impoverishment is apparently

not confined to the intertidal flats since epiphytic and sub-

tidal epibenthic populations also a:re present in very small

numbers only and confined to the seaward part of the estuary.

As to the planktonic copepods, their longitudinal profile

of annual average standing stock shows similar characteristics

as observed for all parameters studied here (De Pauw, 1975).

According to this author, the quantitatively most important

16

species Eurytemora affinis occurs in the highest concentrations

in the area before the Saaftinge saltmarsh. Here, the per-

centage of egg carrying females and nauplii was also highest,

while the relative number of males increased with increasing

distance from this area. De Pauw (1975) considers the area

just in front of and in the saltmarsh as the distribution and

reproduction centre of this planktonic copepod.

Compared to the related epibenthic isoconnnunities occurring

in other estuarine systems and in the Eems-Dollard estuary,

the poverty of the Westerschelde fauna is striking. Yet a

large numbe r of abiotic factors similar in both estuaries.

There are 3 basic differences : (i) The Westerschelde estuary

is more turbulent, (ii) The amount of solids of continental

origin that is sedimented is much higher and (iU) Water and

sediments are more polluted. These differences may explain

the impoverishment of the copepod fauna of the Westerschelde.

1. The possible influence of turbulence, dredge spoil dumping

and increasing load of fine grained solids :

The Eems-Dollard and the saltmarsh of Saaftinge are protect-

ed from extreme physical conditions, while the Westerschelde

is not. Giere (1968) and De Pauw (1975) consider the degree

of exposure to high turbulence and current velocity more im-

portant for the production of the planktonic fauna than the

nutrient supply. However, the high turbulence in the Wester-

schelde is beneficial for diversification of meiobenthic fauna,

since areas of coarse grained sands, where interstitial life

can develop, are maintained on the sandflats between the tidal

channels. Moreover, at the mouth were turbulence and stream

velocities are highest, sometimes removing the upper 20 cm of

sand during storms, a well developed interstitial fauna exists.

Thus, higher turbu lence alone can not be the limiting factor

at the mouth.

However, upstream the areas of coar-se grained sands are

confined and can be considered as islands. The continuous

dumping of dredge spoil and the increasing amounts of solids

brought in by the Schelde could affect the interstitial fauna

drastically When the interstitial pores would be even brief-

ly clogged. As interstitial copepods have no free swimming

larvae, recolonization of the 'islands' would be difficult.

This could theoretically explain the near-absence of inter-

stitial fauna in such area's as the sandflats at the Terneuzen

station group. However, occasional clogging was not observed

during our campaigns and the sand remained very pure. And, of

course, clogging can not explain the poverty of the epibenthic

communities which live on intertidal mudflats in low energy

zones where continuous sedimentation of finer clastics is a

normal process and where occasional physical disturbances ap-

pear to have no or little effect on the meiofauna (Sherman and

Coull, 1980).

2. The possible influence of high amounts of organic matter and

nutrients :

The drastic decrease of the copepod fauna at the Oost-Frie-

sche Plaat (Eems-Dollard) during late autumn clearly coincides

with the maximum output of organic material from the potato

flour mills. Organic pollution can hence be considered detri-

mental at the concentrations recorded.

Since at the Doel transect extreme conditions, as occurring

at the Oost-Friesche Plaat during a limited period, persist

almost throughout the year, this type of pollution can explain

the poverty of the benthic copepod fauna in such situations.

At the remaining station groups of the Westerschelde, the

nutrient load and oxygen depletion are much lower than at the

Oost-Friesche Plaat and the poverty of the epibenthic popula-

tions on the mudflats, especially in the seaward part of the

Westerschelde estuary, can therefore not be explained by this

type of pollution.

3. The possible influence of heavy metal pollution:

Concentrations of heavy metals in sediments, particulate

matter and water of the Westerschelde are higher than in the

Eems-Dollard and ~n the salt marsh of Saaftinge. Very high

concentrations occur in the flocculation zone and, in the sea-

ward part, in front of the harbour of Terneuzen, where an ~n-

verse peak occurs in all parameters studied. The absence of

interstitial life ~n pure sands at the Terneuzenstation group

may be due to a periodical sedimentation mixing of even small

amounts (not enough to fill up the interstiti.al pores) of par-

ticulate matter from the Gent-Terneuzen channe.l,with its ex-

treme high load of heavy metals. By breakdown of organic

matter in the sediment organo-metallic complexes are formed

which increase significantly the amounts of heavy metals in

the interstitial waters of the oxigenized layer (Bryan, 1976).

Little is known about the effect of heavy metal pollution

on copepods. They may take up trace metals, already concen-

trated in detritus, diatoms and bacteria, via the digestive

tract or in solution via the body surface (absorption) (Peil:es,

1976). Adsorption of heavy metals to the exoskeleton of co-

pepods (a.o. Euterpina acutifrons) has also been demonstrated

(Martin, 1970).

Bioassays on the effects of heavy metals on copepods are

scarce and limited to epiphytic and planktonic forms. Corner

and Sparrow (1956) determined the lethal concentration of a

number of mercury and copper compounds, - generally considered

as being the most toxic e,lements together with silver (Bryan,

1971) - for Acartia cZausi and Hoppenheit and Sperling (1977)

determined the lethal concentration of Cadmium for Trisbe

hoZothuriae. Reeve et ale (1976) cite 24 h L.C.50-values for

mercury of 3 ppb for nauplii of Acartia tonsa. All these le-

thal concent rat i.ons are on order of magnitude higher than con-

centrations found in the Westerschelde. However, drastical

sublethal effects, especially on feeding and egg production,

have been observed at concentrations lower or similar to those

occurring in the Westerschelde. Reeve et ale (1976) found a

clear downward trend in these activities for Acartia tonsa at

concentrations of 10 to 20 ppb Cu and almost no egg production

at 50 ppb Cu (24 h L.C.50 for A. tonsa was 104-311 ppb Cu).

For CaZanus pZumchrus Reeve et ale (1977) found a 6-fold re-

duction of egg production at a concentration of 5 ug Cull and

no production at all at 10 ug Cull (24 h LoC .50 for C. plumchrue

was 2778 ppb Cu).

The mean annual concentration of dissolved copper in the

Westerschelde lies around 10 ppb at Doel increasing to 20 ppb

towards the seaward part (Wollast, 1976). Concentrations of

dissolved mercury are similar in the Westerschelde (Anonymous,

1978-1979) and in the Eems-Dollard (Essink, 1980) (0.50-0.01

11g Hg/l), but as Corner and Sparrow (1956) found that copper

l.ncreases the permeability of Acartia cZausi to mercury

poisons, the toxicity of mercury compounds may be different

in the two estuaries.

4. Possible influence of chronic oil pollution

According to Mironov (1969) a concentration of 0.001 ml/l

of crude oil is sufficient to shorten_life span of Acartia

cZausi while Ustach (1979) found that the water soluble frac-

tion of 200 111Louisiana crude oil per liter seawater and 1/2

and 3/4 dilutions thereoff halved egg production in Nitocra

affinis. Ott et al. (1978) found a significant reduction of

broodsize, life span and number of naupli for Eurytemora affi.-

nis after chronic exposure to naphtalene and methylated de-

rivates at concentrations of around 10 ppb. At the seaward

part of the Westerschelde the concentration of unsoluble oil

(which is however not toxic) in the surface waters was around

1 11g/g in fall and winter and below the detectable level in

spring and sunnner during our survey (Anonymous 1978-1979).

The concentrations of polycyclic aromatic hydrocarbons at Doel

are already at least one order of magnitude lower than the

value cited by Ott et al. (1978). In the seaward part of the

estuary the concentration is still much lower. No major oil

spill occurred in the Westerschelde in the years previous to

our investigation.

5. Possible influence of other known to toxicants :

Of the remaining toxicants, only anionactive synthetic

detergents and fluoride are continually present in the Wester-

schelde. The anionactive detergents, the only ones still in

use in Belgium and the Netherlands, are the least toxic, but

according to Bellan (1976), all tensioactive substances are

dangerous, even in very low concentrations. Arnoux and Bellan-

Santini (1972) found alterations in composition of a medi ter-

ranean Cystoseira etx-icta community starting at concentrations

of 20 to 50 ~g maxonol OT/I.

While the concentration of detergents in the Westerschelde

could have an effect on qualitative aspects of the communities,

it is extremely unlikely that it would affect the quantitative

parameters, since all concentrations for which short term ef-

fects are cited are of the order of 1-100 ppm, while for chro-

nic exposure effects a concentration of 0.1 ppm seems necess-

ary (Duursma and Marchand, 1974. Using 2 and 4 mg/l of dom-

estic detergent and 0.8 mg/l LAS, Fava and Crotti (1979) found

that the mean number of nauplii of the copepod Tisbe hoZothu-

riae either decreased or increased according to the number of

adults present. No increase of mortality in the adults was

noted; however, an earlier study (Fava and Dalla Venezia, 1976)

recorded 30 % increase in cumulative mortality after 6 days in

the 4 mg/l concentration. It must also be pointed out that

detergents are present in the saltmarsh of Saaftinge in similar

concentrations as in the Westerschelde and that Arlt (1975)

found high densities of Harpacticoids in front of an urban

sewage outlet.

Fluoride is considered a pollutant because marine organisms

can store it in large quantities (Peres, 1976; Perkins, 1976)

and it is hazardous to man. There exists no literature on

toxic concentrations for marine organisms.

Other pollutants are only intermittently present in the

Westerschelde. Pesticides, the most dangerous and persistent,

are either absent or occur in very low concentrations (lin-

dane: 0.01 ]lg/l) conform to the permissive level suggested

by Perkins (1976).

Sporadically concentrations of 1.0 to 5.0 ]lg/l of phenoles

occur at the mouth of the Westerschelde. Welch (1980) re-

commends an allowable ma.ximum concentration of 0.1 mg/l for

freshwater and aberrant behaviour in marine organisms (molluscs)

is only noted above 10 ppm levels (Perkins, 1976).

6. Possible influence of other biota

Small diatoms, probably an important foodsource for epi-

benthic copepods, were counted from three Westerschelde and

two Saaftinge samples on one occasion. They were present in

similar quantities and food in the form of diatoms or organic

matter may be excluded as a limiting factor. Meiobenthic pre-

dators such as Protohydra, do not occur in the Westerschelde,

but are abundant in the saltmarsh. There exist however im-

portant populations of the shrimp Crangon erangon and preda-

tory polychaetes in the estuary (Vermeulen, 1980), which can

increase the stress put upon the epibenthic copepod population.

CONCLUSIONS

Benthic harpacticoid copepods are a successfull group in un-

stable, estuarine environments and are as a rule quantitative-

ly, the best represented meiobenthic taxon after the nematodes.

Their extreme impoverishment in the Westerschelde estuary is

clearly abnormal and indicates severe stress. Through com-

parison of the chemical and physical characteristics of the

Westerschelde with other estuaries and an evaluation of the

scarce data from bioassays, it appears that chronic pollution

by heavy metals especially copper, which are present through-

out the year in sufficient quantities to produce important

sublethal effects, is the most probable cause for the decline.

In the Westerschelde, ecological monitoring clearly de-

monstrates the reduced quality of the environment in a way

that could be achieved only with difficulty by other monitor-

ing methods. For instance, neither oxygen nor ammonium, or

the amount of nutrients and organic matter are abnormal in the

seaward parts of the estuary and the sandy sediments themselves

appear pure to the eye. Neither does the remaining studied

fauna indicate a diminished quality. Nematodes are well re-

presented in the estuary, with mean annual densities in excess

of 1000 per 10 cm2 except at Doel and in some of the stations

1n front of Terneuzen (Van Damme et al., 1981). Wolff (1973)

found that diversity of macrofauna in the Westerschelde was

similar to that of other estuaries in the Delta region. Re-

sults from a more recent survey failed to demonstrate a gene-

ral decline in number or diversity of the macrofauna, except

locally (again at Doel and at some Terneuzen stations) (Ver-

meulen, 1980). For planktonic species in the Westerschelde,

the influence of pollution is difficult to assess (De Pauw,1975; Bakker & De Pauw, 1975), especially in the seaward part

where stocks are regularly renewed with the flood currents.

In the coastal zone of the North Sea, partial impoverish-

ment of the macrofauna could only be determined in the area of

severest pollution, where almost no harpacticoids were found,

while the region with a distinctly impoverished harpacticoid

fauna is much larger (Govaere et al., 1980).

It thus appears that harpacticoids are more sensitive to

pollution than many other benthic taxa. This has ecological

consequences for littoral and estuarine fish species which are

known predators of harpacticoids, especially the younger sta-

ges. Moreover, copepods could be used as an early warning in-'

dicator on an ecological level. They are easily recognized as

a group, even by technicians, and detailed determination to the

species level is not necessary. Especially for benthic species,

the number of samples needed to evaluate annual averages of

community parameters such as density and biomass is small. It

thus appears that a routine procedure involving sampling of

harpacticoids may be a simple and cheap monitoring technique.

Further studies on other estuaries and additional information

in the form of bioassays on sublethal toxicity effects are

necessary to corroborate these findings.

Acknowledgments

This research was made possible through grants from the Belgian

Ministry of Science Policy, Services of the Prime Minister and the

help from Rijkswaterstaat (Water control, Public Works Department,

The Netherlands). We acknowledge therefore II'.C. Bakker and

Ir. J. Gosse from Rijkswaterstaat advisor department at Flushing

for the logistic support to make this study possible. We thank

especially the crews of the R.V. WELSINGE and the R.V. WIJTVLIET

for their enthusiastic field assistance in the ~vesterschelde. For

technical assistance we could acknowledge A. Braeckman, M. De Keere,

W. Gijselinck, A. Van Bost and D. Van Gansbeke. We also thank

Dr. G. Billen, D. Claeys, Dr. N. De Pauw, II'.J.A.W. de Wit,

Dr. K. Essink, M. Holvoet, II'.H. Koopmans, Dr. F. Vaes, K. Wil-

lems and Prof. R. Wollast for discussion and information and

C. Lostrie for typing several manuscripts.

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Fig. 1. Localisation of sampling stations.

Fig. 2. Longitudinal profile of average annual density N

(ind./IO cm2); average annual biomass B (1Jgdwt/IO cm2

) and

average annual diversity H (bits/ind) of the epibenthic har-

pacticoids in the Westerschelde and Eems-Dollard estuaries.

THE ~... 52N

NETHERLANDS'~f~ER"'A]'"1. .••••.• / .:

l, \ 'ANTWERPEN -, , f..

BELGIUM .: I')6'SRUSSEL ( .. :..., __

4,Ot 60e

N· 86 WESTERSCHELDE ESTUARY "-w 236,2• 67,92020

10 10

6\

\\\\\\\

\\\\\\

\." "....,_.. _-_..-- -0- -- ." .~'::

I ...,.--------r-==-r.:---I Ossenisse Valkenisse \ Doel

Terneuzen SaltmarshSaaftingen

o

Q

-,------Vtissingen

N. 86100 150

EEMS - DOLLARD ESTUARY

4 60

20 30

<:> "'---7~I Heringplaat I

Reiderplaat OostfrieschePlaat

iUithuizerwad

Eemshaven

iHoogwatum

(_~ ~.60E

NORTH SEA

52N

uo1.5

10

·05

H'o

25

20

·15

10

05

,.".'c;

Table 1 : Main hydrodynamical and physical characteristics of

the Westerschelde and Eems-Dollard estuaries.

Table 2 : Average values of median grainsize (Md llm),mud

percentage (fraction < 63 ].lm)and percentage organic matter

(O.M.) and organic carbon (D.C.) at each stationgroup, divided

in 'pure sand' stations (% mud < 2 % over the whole sampling

period) and 'muddy sand' stations (% mud> 2 % during the who-

le sampling period).

Table 3 : Trace metal concentrations in sediment and suspended

matter (1) Hoenig, 1976, (heavy metal concentration in sedi-

ment expressed in ].lg/gdwt of total sample); (2) Vaes, pers.

connn., (idem); (3) Salomons and Mook, 19'77, (heavy metals in

sediment expressed in ~lg/g of a representative sediment con-

taining 50 % particles < 16 um) ; (4) Data communicated by Lr,

J.A.W. de Wit, Rijkswaterstaat RIZA, Lelystad, The Netherlands.

Table 4 : Annual averages (from fortnightly samples) of sali-'

nity, oxygen, annnonia, total fosfates and total organic carbon

in surface waters of the Westerschelde estuary (Anonymous,1978-

1979) and Eems-Dollard estuary (Anonymous, 1976-1977). Total

organic carbon averages for the second estuary were calculated

from date from Laane (1980). The Saaftinge saltmarsh samples

were collected in July 1980.

Table 5 : Mean annual density N (ind./l0 cm2), biomass B (].lg

dwt/l0 cm2) and diversity H (bits/ind) of benthic harpacticoids

of the Westerschelde estuary, Saaftinge saltmarsh and Eems-

Dollard estuary, per stationgroup and according to sediment

composition (pure sands = p.s.; muddy sands = m.s.) at each

stationgroup. (st = number of stations; n = number of samples).

Table 6 : Species list of benthic harpacticoids of the Wester-

schelde estuary and Saaftinge saltmarsh, according to the sa-

linity zones (E.P. = Eu-polyhaline; P.M. poly-mesohaline;

M = mesohaline, M.O. = meso-oligohaline). Individual biomass

B. (in Vg dwt) per species, mean density N (ind./IO cm2);1

dominance in % and absolute frequency (st = number of stations

n = number of samples).

Table 7 : Species list of benthic harpacticoids of the Eems-

Dollard estuary, subdivided according to the salinity zones.

Type of estuary

Average flood volumeAverage freshwater volumeRange of normal tidal

current velocityVertical tidal amplitudeTidal waveLength of sea armWidth at landlocked mouth

Channel depth at low tide

WESTERSCHELDE EEMS-DOLLARD

coastal plain estuary withslight partial stratificationand vertical mixing (1)

1300xl06 m3

89 m3/sec(2)

(3)

(4)

idem(5)

410xl06 m3 (5)83 m3/sec (5)

1 .0-1 .5 m/ s (5)

2-3 m (5)idem (5)

33 km(+) (5)

9 km (6)

av:4-8 mmax. 16 m

"----------------'----------- ._--.-_._------''-------_._-

0.7-1.5m/s

4-5 m (3)semilunar M2, 12h25 min (3)

80 km (3)5 km (2)

av: 10-18 m max. 63 m

(l) De Pauw and Peters, 1973 ; (2) Theuns, 1975 ; (3) Peters and Ster-ling, 1976 ; (4) Valcke et al., 1966 ; (5) Dorrestein, 1960 ; (6) VanStraaten, 1960.(+) this is the length of the landlocked sea arm part of the estuary.

Md um % Mud % OM % OC---------

Westerschelde estuaryDoel ms (8) 190 9. 1 5.2Valkenisse ps (2) 183 8.5 1.2 0.04

ms (3) 147 8.8 7.7 1.62Ossenisse ps (3) 215 0.4 1.4 0.06

ms (1) 155 2.9 4.4 0.29Terneuzen ps (3) 210 0.5 2.4 0.05

ms (2) 143 5.2 5.4 0.34Vlissingen ps (2) 230 0.7 2.7 0.05

ms (5) 164 12.9 6.3 0.37Saaftinge ms (4) 104 16.2 1.3 0.18

Eems-Dollard estuaryOost-Friesche Plaat ms (5) 132 12.7 2.74Heringsplaat ps (5 ) 122 1.7 0.26Reiderplaat ms (4) 86 14.4 1.03Hoogwatum InS (4) 93 6.2 0.87Eemshaven ps (8) 114 1.1 O. 11Uithuizerwad ms (9) 112 4.7

-_ ...-

Westerschelde (1)

Flocculation zone(Antwerpen-Hansweert)

Sediment

Suspension

Znppm

Cuppm

Pb

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Sediment

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Suspension

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Sediment

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type st n N B H'-- -- - ------

Westerschelde-estuaryDoel ms 8 64 0.10± 0.05 0.23± 0.10 0.008±0.008

Valkenisse ps 2 10 0.90± 0.71 0.17± 0.17 0ms 3 13 0.77± 0.54 2.34± 2.21 0.04 ±0.04x 0.83± 0.42 1.40± 1.25 0.02 ±0.02

Ossenisse ps 3 15 12.0 ± 8.54 3.17± 2.21 O. 11 ±0.07ms 5 9.6 ± 7.31 15.34±15.10 0.16 ±0.16x 10.85± 6.58 6.21± 3.97 0.12 ±0.07

Terneuzen ps 3 15 O.64± 0.29 1.66± 0.80 0ms 2 10 2.30± 1.10 3.87± 1.81 0.24±0.13x 1.33± 0.52 2.58± 0.89 0.10 ±0.06

Vlissingen ps 2 10 26.40±10.84 6.67± 2.84 1.08 ±0.21ms 5 25 10.36± 3.25 20.17± 6.81 0.37 ±0.09x 14.94± 3.97 16.66± 5.13 0.57 ±O. 10

Saltmarsh 4 '7 69.71±24. 68 236.16±83.51 1.14 ±0.09Saaftinge ms

Eems-Dollard estuary~

Oost-Friesche Plaat ms 5 22 17.38± 4.15 29.23± 7.09 0.41 ±0.09Heringplaat ps 4 4 36.07±21.57 33.56±21.75 0.93 ±0.20Reiderplaat ms 4 17 44.57± 6.37 55.99± 9.41 1.44 ±0.11Hoogwatum ms 3 7 46.10±20.03 72.30±32. 84 1.42±0.13Eemshaven ps 5 17 87.79± 7.55 147.95± 13.12 2.20 ±0.07Uithuizerwad L ms 9 14 89.98±34.52 127.60±45.85 1.47 ±0.20

---'---

Westerschelde estuary

CanuelZa peppZexaHa'lectinosoma sarsiPseudobradya beduinaPseudobradya quoddiensisArenoseteZZa germanicaHaetriqerel la sp •Euterpina.acutifronsTachidius discipesHappacticus fZexusHarpactsicue ZittoraZisStenheZiapaZust.risRobertgurneya sp.NitocratypicaParamesochra simiZisKZiopsyZZus constrictusEvansuZa pygmaeaLeptastacus ZaticaudatusParaZeptastacus espinuZatusArenocaris bi:fidaHuntemannia sp.Paronychocamptus curticaudatusAseZZopsis intermediaPZathycheZipus ZittoraZisNumber of individuals/IO cm2

Total number of species

B.i,

3.90 0.068.40 0.051.500.101.50 0.060.63 0.010.63 -1.800.06I. 90 I. 741.80 0.011.800.033.19 0.880.80 0.250.20 -0.20 1.150.20 1.280.25 0.400.23 0.010.25 1.500.23 0.062.80 0.012.60 0.401.00 1.223.56 0.01

9.2921

Salt marsh of Saaftingest=4 n=7

AZteutha depressa 8.00Stenhe l.ia pal.uetirie 3. 19Nannopus paZustris 3.40Paronychocamptus nanus 0.60PZathycheZipus ZittoraZis 3.56

Number of individuals/IO cm2

Total number of species

EPst=12 n=69 s

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