Nutrients and phytoplankton in the Gulf of Lions, northwestern Mediterranean

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<ul><li><p>Continental Shelf Research. Vol. 10, Nos 9-11. pp. 931-942. 1990. f1278-4343X/9tl $3.00 + 0.00 Printed in Great Britain. 19911 Pergamon Press plc </p><p>Nutrients and phytoplankton in the Gulf of Lions, northwestern Mediterranean </p><p>ANTONIO CRUZADO* and ZOILA R. VELASQUEZ* </p><p>(Received 24 April 1989; in revised form and accepted 9 November 1989) </p><p>Abstract--The mostly oligotrophic character of the Mediterranean Sea is altered drastically in areas receiving the outflow from large rivers. The Gulf of Lions, receiving discharges from the Rh6ne River, has nutrient and phytoplankton concentration much higher than the adjacent open northwestern Mediterranean Sea. </p><p>A surface layer of freshwater, with thickness that varies with the meteorological conditions between 2 and 40 m, overlies the deeper open seawater; this is advected onto the shelf and influences an area that covers the eastern half of the Gulf of Lions. </p><p>Most of the waters affected by the river discharges show property relationships indicating conservative behaviour, with very little or no loss of nutrients through phytoplankton uptake, particularly in winter. Phytoplankton populations in winter are sparse, with maximum densities just above and below the boundary between the fresh- and seawater. Diatoms are the main group of organisms, although dinoflagellates, coccolithophorids and cyanobacteria are abundant. Small heterotrophs (cilliates, tintinnids, etc.) are also abundant and are positively correlated with the diatoms. </p><p>A water balance model, linking the river discharge to the advective fluxes of water and nutrients, is proposed. The primary productivity supported by such fluxes is estimated. </p><p>INTRODUCTION </p><p>THE Med i ter ranean Sea is known to have a negative water balance. To compensate the excess evaporat ion over precipi tat ion and river runoff, surface At lant ic water flows into the Med i te r ranean basin whilst deeper Medi ter ranean water flows out at G ibra l tar to balance the salt (LACOMBE, 1988). Since less nutr ients are conta ined in the inflowing surface At lant ic water than in the deeper outf lowing Med i ter ranean water, the Medi terra- nean basin is relat ively impover ished in nutr ients with respect to the open ocean. Nutr ient loads or iginating in the r iver discharges within the Med i ter ranean basin are not enough to balance the losses unless the organic fraction of the nutr ients contained in the At lant ic surface water (COSTE et al., 1988) and the nutr ients or ig inated in the atmosphere and dePOsited with rainfal l (MIGON et al., 1989) or through dry deposi t ion (ALARCON and CRUZADO, 1989) are taken into account. As a consequence, the Med i te r ranean Sea is an ol igotrophic system; this more character ist ic of the eastern rather than in the western basin, part icular ly in summer (JACQUES and TREGUER, 1986). </p><p>*Centre d'Estudis Avan~ats de Balnes, P.O. Box 150, 17300 Balnes. Spain. </p><p>931 </p></li><li><p>932 A. CRUZADO and Z. R. VELASQUEZ </p><p>Exceptions to this rule are shelf areas of the North Adriatic and the Gulf of Lions, in the vicinity of the rivers Po and Rhrne, respectively, which are the largest ones in the basin, delivering an important nutrient load. The areas, particularly the Gulf of Lions, have been the site of extensive studies during past years (HEusSNER et al., 1988); these are related to the interrelationship between the pelagic and the benthic systems, much closer than in open-sea or coastal areas without the influence of continental drainage. The hydrology and ecology of such areas are of particular importance, for the understanding of the processes taking place in the continental margins and the exchanges with the adjacent open-sea regions. </p><p>The northwestern Mediterranean Sea exhibits the following features described below. - -The upper layers of the central western Mediterranean are occupied by Modified Atlantic Surface Water of salinity below 38%o; this constitutes a clear example of an oligotrophic system. - - In the northwestern parts of the basin, off the Gulf of Lions, an area of deep-water formation extends off the shelf to the southwest with surface water of salinity above 38%o (SALAT and CRtJZADO, 1981; FONT et al., 1988). This water, with a very small vertical salinity gradient, is formed during winter by cooling and evaporation of a well-mixed water column; it remains all year round, with high salinities close to those typical of intermediate and deep waters. - -A strong thermohaline front between the above water and the Modified Atlantic Surface in the central parts of the basin supports some upwelling which contributes to the fertilization of the Gulf of Lions (CRUzADO, in preparation). - -The Liguro-ProvenGal Current persistently flows over the slope, from the inner Ligurian Sea and along the coast of southeastern France, off the Gulf of Lions shelf and along the east coast of Spain, at least as far south as the strait between Ibiza and Spain's mainland (MILLOT, 1987, 1990; FONT et al., 1988; BETHOUX et al., 1988). </p><p>- - Freshwater discharge along the coast, mostly by the Rh6ne River, makes the water in the Gulf of Lions and over the shelf along the French and Spanish coasts less saline than the surface water further offshore (SALAT and CRUZADO, 1981). </p><p>The present paper deals with the nutrient distribution in the Gulf of Lions and adjacent areas, in relation to the water discharged from the Rh6ne River, and the processes that control such distribution. Uptake of nutrients by phytoplankton, particularly at the frontal zone, is also discussed although the phytoplankton data set available is limited in time and geographical coverage. </p><p>MATERIALS AND METHODS </p><p>Table 1 shows the periods in which the cruises were carried out and Fig. 1 the geographical coverage of the data sets used. Most of the stations were visited one or more times during each of the cruises. Cruise LEOPEL I failed completely, due to an unusually strong and lasting spell of Mistral and Tramontana; these are the dominant winds of the Gulf of Lions, particularly in winter. Phytoplankton samples were taken only at Stas 1, 2, 3, 17 and 23 during cruise PELAGOLION III. </p><p>Water samples for both nutrients and phytoplankton were taken with Niskin bottles, attached to a General Oceanics rosette sampler and incorporating a Guildline STD assembly. Most of the stations were shallow (</p></li><li><p>Nutrients and phytoplankton in the Gulf of Lions 933 </p><p>the deeper stations. Samples were never taken close enough to the bottom so that the effects of the sediment on the water column could be observed. </p><p>Samples for nutrient analyses were kept in 150 ml polyethylene flasks. When the analyses could not be made within less than 1 h, they were kept frozen at -18C, after addition of one drop of chloroform. Samples for phytoplankton analyses were kept in 150 ml glass jars and preserved with lugol (acetic acid containing potassium iodide/iodine solution). </p><p>The nutrient analyses were carried out on board, with a four-channel SKALAR segmented-flow analyser. The methods followed were those described by WHITLEDGE et al. (1981) with very minor modifications to avoid dilution (CRUZADO, 1989), in view of the usually low concentrations found in Mediterranean waters. Whenever concentrations, particularly those of nitrate, ammonia and silicate, were higher than about 10/~g-at. 1-1 the samples were diluted 10-fold with "double distilled water" (DDW). The water was prepared by double reverse osmosis (Milli-Ro), ion exchange (Milli-Q) and distillation in the presence of potassium permanganate. </p><p>DDW was used also to set the zero values which took into account the effect of salt on absorbance. Standards were prepared following WHITLEDGE et al. (1981). Running solutions were prepared daily at concentrations between 1 and 8/~g-at. l-1 and depending on the nutrient concentrations measured in the seawater samples. </p><p>Phytoplankton cell counts and species indentification were carried out with an Olympus inverted microscope, after settling of 100 ml of seawater over a 5 cm 2 counting plate. Identification was made whenever possible to the level of species. A few organisms could not be classified beyond the level of genus. </p><p>RESULTS </p><p>The data sets to which this paper refers are collated in three reports (CRUZADO, 1987a; CRUZADO and VARELA, 1988; CRUZADO and VELASQUEZ, 1989a). </p><p>The vertical distributions of nutrients, exemplified with those of nitrate, are shown in Fig. 2. During the summer, at stations not influenced by the Rh6ne River water discharges, surface concentrations are extremely low (Fig. 2a); they are often below the detection limits for the colorimetric methods used, indicating strong nutrient limitation of phytoplankton growth. At this time of the year, nitrate levels over the entire region are well below 0.4#g-at. NO3-N 1-1. As a consequence, a strong nutricline extends between 50 and 100 m depth. Below these depths all the nutrient concentrations slowly increase </p><p>Table 1. Cruises carried out within Projects PELAGOLION and PICS and dates on which they took place </p><p>Cruise Date </p><p>PELAGOLION I PELAGOLION II LEOPEL I LEOPEL II PELAGOLION III PELAGOLION IV </p><p>September 1986 December 1986 February 1987 July 1987 February 1988 May 1988 </p></li><li><p>934 A. CRUZADO and Z. R. VELASQUEZ </p><p>4330 </p><p>43* </p><p>42*30 </p><p>42* </p><p>3* 3*30 4* 4*30 5* 5*30 </p><p>.,.- ':::;" ~:"'~ ..TOU LOb </p><p>ii) ~'! O 24 35 @ </p><p>::"! (D 1 o 21 8 ~7 </p><p>ERPIGNAN 31 </p><p>BANY 0 25 </p><p>'~:.. </p><p> 26 </p><p>34 @ </p><p>(2) 20 </p><p>32 (~) 18 I~_~ FBANCE I 33 (D </p><p>O 27 @ 19 </p><p>Fig. 1. Geographical layout of the Gulf of Lions and location of the stations visited during the six cruises. </p><p>downwards until they reach maximum values at depths between 100 and 800 m, depending on the nutrient and the season. </p><p>During the winter (Fig. 2b), the surface nutrient concentrations reach higher values. Deep vertical convective cells (CRUZADO and KELLEY, 1974) developing in the deep-water formation area and upwelling at the thermohaline frontal zone existing at the outer boundary of this area (CRLIZADO, in preparation) bring about this fertilization, which is little utilized by the winter light-limited phytoplankton (VELASQUEZ and CRUZADO, in preparation). </p><p>Parts of the surface waters of the Gulf of Lions are enriched over the offshore concentrations, with respect of all nutrients; this is due to the Rh6ne River discharge, forming a tongue of low-salinity seawater in the eastern part of the Gulf (Fig. 3a). The figure shows nitrate concentrations reaching 26Mg-at. NO3-N 1- t near the river mouth with salinity 33%0. The area affected by this tongue varies with depth from about 1100 km 2 at 10 m depth to 3300 km 2 at 5 m depth and 6000 km2 at the surface. </p><p>In the winter, the nitrate levels are high all over the area (1-2/~g-at. NO3-N 1- t) (Fig. 2b) reaching values over 35 Mg-at. NO3-N 1-l at salinity just above 31%o (Fig. 2d). The tongue of river water shows areal coverages which are not too different from those measured in the summer (Fig. 3b). However, such coverage depends upon both the river flow regime and the meteorological conditions. </p><p>The vertical distributions of nutrient concentrations in the neighbourhood of the river outflow, because of the strong salinity gradients, vary drastically according to the </p></li><li><p>Nutrients and phytoplankton in the Gulf of Lions 935 </p><p>Nitrate ( / zg -o t . NO3-N/L ) o . . . 5 lO o 5 lO </p><p>I ) " </p><p>= 50 1 Q e= </p><p>o o </p><p>= I </p><p>100 </p><p>Nitrate (/~g-at. NO 3 -N/ t ) o 5 lO 5 lO </p><p>| -~ - , -~ i I I I l </p><p>~ 50 </p><p>t00 Fig. 2. Vertical distribution of nitrate in the top 100 m of water. (a) Stations not influenced by the Rh6ne River discharges during the summer cruise PELAGOLION I (September 1986). (b) Stations not influenced by the Rh6ne River discharges during the winter cruise PELAGO- LION III (February 1988). (c) Stations under the influence of the Rh6ne River discharges during the summer cruise PELAGOLION I (September 1986). (d) Stations under the influence of the </p><p>Rh6ne River discharges during the winter cruise PELAGOLION III (February 1988). </p><p>meteorological conditions. The wind plays an important role in controlling the vertical distributions of salinity and thus of nutrients. During calm weather conditions, a shallow layer 2-10 m thick, containing a very large proportion of freshwater, lies on top of a subsurface layer containing almost undiluted seawater (Fig. 2c). This stratification breaks </p></li><li><p>936 A. CRUZADO and Z. R. VELASQUEZ </p><p>down in winter, due to frequent wind events (Fig. 2d). Under these circumstances, the transition from the fresher surface water to the saline subsurface water is less sharp. Mixing of fresh and salt waters takes place to depths of 40 m and more. </p><p>The nutrient concentrations and salinity, within the salinity range found during the various cruises, show (Fig 4) linear relationships; these are indicative of mixing processes predominating over those that add or remove inorganic dissolved nutrients to the seawater. This feature has been confirmed for the entire range of salinities in another study of the same area (CRUZADO and VELASQUEZ, 1989c). </p><p>The distribution and composition of the phytoplanktor, populations during the winter cruise PELAGOLION III, the only one studied so far, show the dominance of Bacillario- phyceae in most of the samples (Fig. 5). Maximum cell densities occur just above and below the halocline with minimum density right at the halocline (Fig. 6) Dinoflagellates, coccolithophorids and cyanobacteria, possibly of the genus Synechococcus, were ob- served. The latter were sometimes present in large numbers, but they were not counted. A large population of heterotrophic microorganisms, such as cilliates and tintinnids, were also present; they showed a positive correlation with the photosynthetic organisms (Fig. 7). </p><p>DISCUSSION </p><p>From the above results, various features of the Gulf of Lions appear clear. Some of these features contribute to the understanding of the general function of the northwestern Mediterranean Sea. </p><p>The influence of the Rh6ne River water extends over a large part of the Gulf, being confined over the shelf by the density front which is in dynamic equilibrium with the Liguro-Provenqal Current. As far as water budget is concerned, the Gulf of Lions is a net exporter of water. Seawater moves horizontally into the Gulf from the open sea, mixes with the river water, flowing out along the southwestern coastal area. If Si is the salinity of the incoming seawater and So that of the outgoing mixed water, the amount (F) of seawater required for the dilution of the river water discharge (R), is given by the crude expression reflecting the salt balance </p><p>F= R So Si - So" </p><p>For a salinity lowering of about 0.4?/00, the amount of seawater required to mix with the river water would be approximately 100 times the Rh6ne discharge, i.e. about 150,000 m 3 s -1. For salinity lowerings smaller than 0.4?/o0, the fluxes of seawater advected from the open sea would be much greater because of the reciprocal relationship existing. Such a strong flow of...</p></li></ul>