Seasonal sampling and stable isotopes use to delineate seagrass phytodetritus macrofauna trophic ecology: baseline variation or actual diet change?

In Mediterranean exported seagrass macrophytodetritus accumulations, a diverse (more than 130 species) and abundant (up to 4900 id.m-2) macrofauna assemblage is found alongside meiofauna, microalgae, fungi and bacteria. Macrophytodetritus are mainly composed of poorly digestible yet highly colonized material: the dead leaves of the very productive (300 to 2000 g dry wt m-2 yr-1) endemic seagrass Posidonia oceanica. A key role may be played by macrofauna, and more particularly by litter vagile macroinvertebrates (invertebrates > 500µm), in the degradation, enrichment and carbon transfer from P. oceanica to coastal food webs. Indeed, results of gut content observations of the most abundant species show that even if only a few of these species ingest a large proportion of P.oceanica dead leaves fragments, most of the others ingest a small but non-negligible part, suggesting a potential role of the whole community in the mechanical fragmentation of the dead leaves. Mediterranean exported macrophytodetritus accumulations are very dynamic habitats with very variable food availability, quality, and composition. Such an inconstant habitat may result in drastic modifications of the invertebrate community but also of its trophic structure and ecology. To test this hypothesis of influence of pulsed availability, quality and composition of food sources on the vagile macrofauna diet, we took seasonal samples in Calvi Bay (Corsica, 8°45’E; 42°35’N), at two sites between August 2011 and May 2012. Gut content observations and C/N/S stable isotope analysis of bulk tissues were conducted on both the macrofauna and their potential food sources. Significant seasonal and spatial differences of ingestion patterns of the most abundant species were emphasised as were differences of isotopic signatures. “SIAR” Bayesian mixing model and “SIBER” package were used to analyse isotopic data and determine if these differences were due to actual diet modifications or only to baselines isotopic composition variations