Posidonia oceanica is the most abundant seagrass of the Mediterranean Sea. It can cover extensive areas with monospecific formations, called meadows. These meadows, whose extent is estimated to about 40,000 km2, are critical features of the Mediterranean coastal zones. Moreover, they shelter important biomass and biodiversity of vagile invertebrates. Among these invertebrates, amphipod crustaceans are, alongside gastropod mollusks and polychaete annelids, one of the dominant groups.
Amphipods are key-features of other temperate seagrass systems. As they are generally primary consumers, they are important in the transfers of organic matter from producers to higher rank consumers. In addition, their grazing activity on the epiphytes that grow on the seagrasses influence the dynamics of the epiphytic cover, and therefore the functioning of the whole meadow as an ecosystem.
However, the situation in Mediterranean Posidonia oceanica meadows is still unclear. In particular, several lacks of information limit the comprehension of actual trophic ecology of amphipods, and of the impact of their feeding activity on the meadow functioning. In this context, the main goal of this work was to enhance the knowledge of the trophic diversity and the functional role of amphipods associated to Posidonia oceanica meadows. To achieve this, we structured our research in three main tasks. For each of these tasks, we chose Calvi Bay (NW Corsica, France) as study site, and all sampling and experimentation was undertaken from the STARESO research station (University of Liège).
The first task (chapter 3) was the study of the precise composition of the amphipod community structure at our study site, and its temporal variation at day/night and seasonal scale. Our results show that the fauna of Posidonia oceanica meadows of Calvi Bay is abundant and diverse. The density and the structure of the community were different in each season (November, March and June), probably in relation with meadow parameters such as foliar surface, epiphytic biomass and abundance of litter in the meadow. Moreover, day/night variations were very important. Most amphipods performed vertical migrations that could be a mechanism to avoid predation and/or competition for food and habitat. The comparison of three sampling techniques (hand-towed net, litter collection and light traps) yielded deeply different results, suggesting that each of them only collects a subset of a complex assemblage. Combination of several sampling methods is therefore advised to have a holistic and accurate view of the community. These faunistic data also allowed highlighting the most abundant and/or representative species of the studied community. These include Apherusa chiereghinii, Aora spinicornis, Dexamine spiniventris, Amphithoe helleri, Caprella acanthifera, Gammarella fucicola and Gammarus aequicauda. These species were therefore chosen as target species for the second task.
The second task (chapter 4) was the assessment of the extent of interspecific trophic diversity among the studied community. This phenomenon could indeed be important to limit food competition. We tried to perform a full reconstruction of the diet of the dominant species of the community and to evaluate the contribution of each of the potential food items offered by the meadow (animal and vegetal epiphytes from the leaves, rhizomes and litter fragments, SPOM, BPOM, living and dead P. oceanica material). To have an accurate view of the dietary habits of the dominant species, we used a triple strategy based on the joint use of traditional methods (gut content examination) and trophic markers (stable isotopes of C & N, fatty acids).
The combination of these three methods proved to be successful, as each method had specific strengths and weaknesses. Overall, results indicate that all dominant species relied on macroalgal epiphytes for a large part of their diet. Our insights were unfortunately limited by the poor discrimination between potential food items, due to high inter-source similarity. Considerable interspecific differences could nonetheless be highlighted, notably concerning preferences of epiphytes from leaves or litter fragments vs. epiphytes from rhizomes. In addition, most species had a mixed diet, and relied on several food items. None of the examined species seemed to graze on their seagrass host, but Gammarus aequicauda partly relied on seagrass leaf detritus. Contribution of microepiphytes (e.g. diatoms) to the diet of amphipods was apparently anecdotical. Our data also suggested the existence of a certain extent of intraspecific trophic diversity that should be taken into account for future work.
In the third and final task (chapter 5), we aimed to put the data obtained in the first two parts of this study in the wider context of the functioning of the Posidonia oceanica meadow as an ecosystem. We used in vitro and in situ microcosms experiments to characterize the interaction between epiphytes and amphipods from a triple point of view (resource depletion, resource assimilation by the consumer and secondary production), and to understand how amphipod grazing could influence the dynamics of the epiphytic cover of the leaves of P. oceanica.
Amphipod grazing had no effect on the total epiphytic biomass, or on the encrusting epiphytes’ biomass. However, all three taxa (A. chiereghinii, D. spiniventris and Gammarus spp.) consumed significant amounts (45 to 90 % of total biomass) of erected epiphytes, both vegetal and animal. This selective top-down control might influence the structure and biomass-specific productivity rates of the epiphytic cover. In addition, amphipod grazing caused an increase in N availability and residence time. Through epiphyte removal and N enrichment, amphipods could boost seagrass production. Overall, amphipods of Posidonia oceanica meadows could be seen as ecosystem engineers. Assimilation of the consumed epiphytes was clear for all taxa. However, the utilization of this biomass for secondary production was hard to measure, due to low survival rates of animals.
In fine, by combining in situ sampling and microcosm experimentation, and trough the joint use of traditional and innovative techniques, we showed that feeding activity of amphipods influence their biotope through several effects, and that they could be pivotal items of Posidonia oceanica meadows. In doing so, we improved, to some extent, the understanding of these critically important, yet endangered ecosystems.