Who would want to live in there? A history of Posidonia oceanica detritus accumulations, the associated invertebrate community, and its food web…

Most seagrasses are rarely consumed directly by herbivore organisms. The "detrital pathway" thus represents a potentially important way of transfer of the seagrasses production to the coastal food webs. The case of Posidonia oceanica is particularly interesting since up to 90% of its foliar primary production may constitute extensive and highly dynamic exported litter accumulations. Preliminary studies concerning these detritus accumulations suggest that an abundant community of vagile macro invertebrates (size > 500μm) lives inside them. We characterized for the first time this community in an exhaustive way (multi-site, seasonal and multi-year study), we linked the observed density and diversity variations to environmental parameters, but also described the trophic web these invertebrates compose. We sampled an abundant (up to 5000 organisms/m²) community composed of 115 species. We showed that crustaceans were massively dominant, followed by annelids and mollusks, and that one single amphipod species Gammarella fucicola represented from 20 to 85% of the whole sampled community. Observed variations appeared to be mostly linked to litter oxygen water concentration in a very species specific way. Most species were linked to no measured environmental parameter at all, but several dominant species were demonstrated (observation and in situ experimentation) to be linked positively or negatively to litter oxygen concentration. The described food web was composed of more than 3 trophic levels, indicating the presence of a trophic web composed of primary consumers/detritivore species, of omnivore species, but also of first and second order predators, each level occupying a distinct isotopic niche. From a specific point of view, we highlighted several different feeding preferences, with SIAR mixing model runs indicating that some species feed mostly on detrital material, other species feed on a mixture of detrital and algal material, other species feed on both animal and vegetal material and finally predator species feed exclusively on animal material. The fact that P. oceanica detritus constituted a non-negligible food source for some dominant species confirmed the importance of this macrofauna community as a key transfer link of seagrass organic matter from P. oceanica to the coastal food webs