Etude multidisciplinaire de la diversité trophique et du rôle fonctionnel des crustacés amphipodes associés aux herbiers à Posidonia oceanica

Posidonia oceanica is a seagrass endemic to the Mediterranean Sea, and is able to form large monospecific areas, called meadows. These meadows are critical features of the Mediterranean coastal zones, and are of great ecologic and economic importance. Posidonia oceanica meadows shelter high biomasses and biodiversities of amphipod crustaceans. In other temperate meadows, such as Atlantic Zostera marina meadows, the amphipods play an important part in the functioning of the ecosystem, notably in organic matter transfers from producers to higher level consumers. However, the situation in Posidonia oceanica meadows remains unclear, due to the lack of precise studies, and little is known about the trophic ecology of amphipods. In this context, our research is structured in three main tasks. We chose the Calvi Bay (NW Corsica, France) as a study site, and sampling is undertaken from the STARESO research station (University of Liège, Belgium). First, we study the exact composition of the amphipod community, as well as its temporal variation at diel and seasonal scale. This task is based on in situ collection of samples using three methods: the hand-towed net, litter collection and light traps. Completion of this task will allow us to have accurate and reliable data, taken on our study site, concerning the abundance and specific diversity of amphipods associated with P. oceanica meadows. The second task is a reconstruction of the diet of the studied animals. Indeed, amphipods from P. oceanica meadows are usually regarded as vegetal epiphytes grazers, or generalist detritivores, but few studies focus on the interspecific trophy diversity, or on the importance of alternative food sources. Our approach relies on in situ sampling of amphipods and potential food items. The techniques used combine traditional methods (gut content observation) and use of trophic markers, such as measurements of C and N stable isotope rations and fatty acid composition analysis. Finally, we use in vitro and in situ microcosms experiments to evaluate the impact of amphipod feeding activity on the ecosystem functioning, and more precisely on the dynamics of the epiphyte cover. By quantifying this interaction, our purpose is to put back the results obtained in the first two parts into a wider context, the functioning of the Posidonia oceanica meadow as an ecosystem. Thus, by combining in situ sampling and in vitro experimentation, and by combining traditional and innovating techniques, we hope, at the end of this research, to enhance the knowledge of the trophic diversity and the functional role of amphipod crustaceans associated with Mediterranean Posidonia meadows