Feeding rate of the oyster Crassostrea gigas in a natural planktonic community of the Mediterranean Thau Lagoon

The Mediterranean Thau Lagoon is an important oyster farming area in Europe . Oyster growth rates are among the highest in France, although chlorophyll a concentration is low. Previous studies have demonstrated that picophytop lankton, nano-microphytoplankton, dinoflagellates and loricate ciliates suc h as tintinnids are abundant. However, heterotrophic flagellates and aloric ate ciliates have not been investigated. The aim of this study was to asses s picophytoplankton, protist and zooplankton abundances in the Lagoon and t o investigate the particular structure of the microbial food web, which may explain such paradoxical oyster growth. In oligotrophic waters in the Thau Lagoon, the picoeukaryote Ostreococcus tauri is the dominant autotrophic p icoplankter, with an abundance maximum in summer. On 17 August 1998, follow ing a rainfall event, pico- and nanophytoplankton abundances were not as hi gh as expected and we observed a high abundance of large diatoms. At this t ime, the available carbon resources were produced by microphytoplankton (84 .5%), and picoplank-tonic cells represented only 1.27 % in terms of carbon. Heterotrophic cells were low in abundance and constituted <14% of carbon r esources. In order to evaluate the importance of the 'protozoan trophic lin k' for energy transfer from the microbial food web to large benthic suspens ion feeders, the oyster Crassostrea gigas was offered a planktonic communit y as potential prey. In the grazing experiment, all >5 mum flagellates, mic rophytoplankton, dinoflagellates, ciliates and large zooplankton were retai ned by the oyster gills. Only flagellates <5 pm and O. tauri were not very well retained (45 and 2% respectively). The high clearance rates of C. giga s found in this experiment can be explained by a low concentration of suspe nded particulate matter (0.65 mg l(-1)). The oysters adapted their retentio n mechanism when they Lived in oligotrophic waters. These results indicate that, under the given experimental conditions, picophytoplankton did not re present a valuable trophic resource for farmed oysters because (1) C. gigas cannot retain picoparticles and (2) the picoplankton represented a poor ca rbon resource capable of being transferred via a weak heterotrophic protist community. In the oyster pens of the Thau Lagoon during this study, microp hytoplanktonic primary producers, in particular diatoms, were the main food sources for bivalve suspension feeders.