International audienceThe Mediterranean Thau Lagoon is an important European oyster farming area. Oyster growth levels are among the highest in France, although chlorophyll a concentration is low. Previous studies have demonstrated that picophytoplankton, nano-microphytoplankton, dinoflagellates and loricate ciliates such as tintinnids are abundant in the Thau Lagoon. Moreover, heterotrophic flagellates and aloricate ciliates have not been investigated. The aim of this study was to assess picophytoplankton, protist and zooplankton abundances in water columns of the Thau Lagoon and to understand the particular structure of the Thau microbial food web, which may explain such a paradoxical oyster growth. In oligotrophic waters in the Thau Lagoon, the picoeukaryote Ostreococcus tauri is the dominant autotrophic picoplankter with a maximum Summer abundance. On 17 August 1998, the picophytoplankton and nanophytoplankton abundances were not as high as expected and we observed the development of large diatoms. At this time, available carbon resources arose from microphytoplankton (84.5 %) and picoplanktonic cells represented only 1.27 % in terms of carbon. The heterotrophic cells were few in abundance and constitued only < 14 % of carbon resources. In order to evaluate the importance of the "protozoan trophic link" for energy transfer from "microbial food web" to large benthic suspension feeders, the oyster Crassostrea gigas was offered a planktonic community as potential prey. In the grazing experiment, all > 5 μm flagellates, microphytoplankton, dinoflagellates, ciliates and large zooplankton were retained by the oyster gills. Only < 5 μm flagellates and picoeukaryotic cells, Ostreococcus tauri, were not very well retained (45 % and 2 %). The high clearance rates of Crassostrea gigas found in this experiment can be explained by a low suspended particulate matter (0.65 mg l-1). Oysters adapted their retention mechanism when they lived in oligotrophic waters. These results indicate that, under the given experimental conditions, picophytoplankton did not represent a valuable trophic resource for farmed oysters because (1) Crassostrea can not retain picoparticles and (2) the picoplankton represented a poor available carbon resource to be transferred via a weak heterotrophic protist community. In the oyster pends of the Thau Lagoon during this study, which followed a rainfall event, microphytoplanktonic primary producers, in particular diatoms, could be considered as the main food sources for bivalve suspension feeders

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Daily zooplankton samples were obtained from the Etang d e Thau, France, a 70 km2 coastal lagoon of the northwestern Mediterranean Sea. The survey was monitored at 2 stations inside the lagoon and at 1 station on the seaside during 4 periods in 1994. The microzooplankton, towed with a 40 pm mesh size net, included tintinnids, rotifers, anthozoan larvae, and crustacean and mollusk larvae. The tintinnids and rotifers have not been studied in the lagoon yet. Over all the sampling periods, their mean biomass represented between 26 and 4 5 YO of the net collected mlcrozooplankton biomass, which was estimated by image analysis. Their biomass in the central part of the lagoon free of anthropic activity was close to the level observed at the nearby seaside. However, at the shellfish culture area, the occurrence of oysters and of epibiotic fauna resulted in a drastic biomass reduction of these taxocoenoses by a factor of 10 compared to values measured in the middle of the lagoon. Samples collected during the fall indicated well-developed tintinnid and rotifer populations in this season. lnside the lagoon these populations seemed to be already abundant in early spring (March and June samples). An ordination of taxa biomasses showed 2 main factors which might have contributed to the organization of the tintinnid and rotifer assemblages: the geographical position and the thermal period. The geographical position integrated the lagoon-sea water exchange. The thermal period reflected both the populations' development cycles and the env~ronmental constraints. The resulting effects appeared in assemblages structured in a space and time gradient. The capacity of the tintinnids and rotifers to quickly colonise the habitat (reproduction modes, resistance forms) and to use a wide range of food resources (organic matter, bacteria, pico-and nanoplankton) likely enhances their ability to serve as trophic links between primary and secondary producers. Hence, their function in the lagoon ecosystem is not minor.

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