Composition of biofouling communities on suspended oyster cultures: an in situ study of their interactions with the water column

This study was based on in situ experiments conducted in a shellfish lagoon (Thau, France) to determine the interactions between suspended oyster Cras sostrea gigas Thunberg cultures and their environment at a seasonal scale. Three groups of descriptors were considered simultaneously during the cours e of the study: the taxonomic composition of an oyster culture unit (OCU), the nutrient and oxygen exchanges at the OCU-water interface (fluxes) and t he water-column characteristics (water). We used a multiple factorial analy sis (MFA) to assess in a single reference frame: (1) the seasonal organisat ion of the taxonomic groups which compose the OCU and (2) the interactions between this multispecific assemblage and the water column. We found strong seasonal variability in the OCU composition and the OCU interactions with the water column. Taxonomic richness of the biofouling was at a maximum in spring and minimum in August after a bottom anoxia. Among biofouling specie s, ascidians always constituted the dominant group. The annual pattern of c oexistence observed between ascidians and oysters might be explained by res ource partitioning, and also by a beneficial trophic interaction between th ese 2 groups. The structure of the OCU was based on a complex system of int eractions, particularly in spring. During this season, the development on t he OCU of macrofauna of soft substratum such as polychaetes reflected the p resence of a suspended sediment compartment. The influence of the OCU on th e water column was maximum in summer and minimum in winter. In the shellfis h zone, the OCU plays a central role in nutrient renewal, particularly in s ummer when benthic fluxes are insufficient to satisfy phytoplankton nitroge n requirements. We estimate that the multispecific assemblages occurring on the oyster ropes (oysters and biofouling) have a potential annual DIN prod uction of 2 x 10(7) mol yr(-1) and thus oyster cultures could have a determ ining influence on nitrogen recycling in the water column in the Thau lagoo n.