Trophic ecology of macrofauna inhabiting seagrass litter accumulations is related to the pulses of dead leaves

Accumulation of exported macrophytodetritus (AEM) represent unique habitats formed by the dead material originating from macrophyte ecosystems (e.g., seagrass, kelp, other seaweeds). AEM can be found everywhere, from the littoral zone to the deepest canyons, and from high to low latitudes. Seagrass AEMs are among the most common detrital accumulations found in marine environments, and sometimes include macroalgae wrack that has been ripped from the substrate. In the Mediterranean Sea, Posidonia oceanica (L.) Delile litter accumulations undergo pulses of new necromass all year, particularly in autumn, when dead leaves are shed. Here, macrofauna inhabiting AEM of Calvi Bay (Corsica, France) was sampled troughout an annual cycle (four seasons). By combining gut content examination and stable isotope analysis, we aimed to assess the effect of seasonal litter pulses on the trophic ecology of the dominant macrofauna species. Litter composition showed drastic variations throughout the sampling period, with the highest leaf litter quantity and contribution to AEMs in November. Dominant detritivores, herbivores, and omnivores responded positively to this increase by ingesting more seagrass material. A Bayesian stable isotope mixing model showed that the assimilation of carbon originating from seagrasses also increased. Additionally, isotopic niche modelling showed that consumer niches shifted towards seagrass isotopic composition in November. Predators did not shift their diet, but their isotopic composition was affected by the isotopic shift of their prey, demonstrating the transfer of seagrass carbon to higher trophic levels and the shift towards dead leaf material in the entire community. This response was, therefore, a rapid (days to weeks) parallel to that of the slow (months to years) decomposition of detrital material via physical alteration and microbial decomposition. This seemingly underestimated transfer route should be better characterised to understand the role of these seagrass beds in carbon sequestration in the marine environment.