Seagrass detritus as marine macroinvertebrates attractor

Seagrasses colonise coastal areas worldwide. Despite their high primary production, a considerable proportion becomes detritus that can be used as food, physical habitat and occasional or permanent shelter by several benthic macroinvertebrates. In turn, macroinvertebrates can contribute to regulating seagrass decomposition, and represent an important trophic link between primary producers and higher consumers. Nonetheless, several factors could modify colonizer responses to this habitat. In this study, we tested if colonisation of the seagrass detritus of Zostera noltei Hornemann, 1832 was related to substrate availability rather than food and whether the colonising assemblages were similar according to the structural complexity of the meadow. We used artificial seagrass detritus to mimic the physical structure of the natural detritus while disentangling the effect of food attractiveness vs. physical habitat availability. Litterbags were filled with natural or artificial detritus and deployed within a seagrass meadow in Thau lagoon (Étang de thau, France) in areas of different structural complexity. During two field experiments, detritus decomposition and litterbag colonisation were analysed. A total of 11270 individuals belonging to 26 taxa were identified (including polychaetes, crustaceans, molluscs, and chironomids larvae). Habitat structural complexity shows no effects on the colonisation of the detritus, but there were clear differences between empty and filled litterbags, which had a higher number of species and individuals, but without a general preference for the natural or artificial detritus substrate. In conclusion, colonisation appeared to be driven by the presence of debris material, and not by its type. The natural and artificial detritus acts as an attractor for macroinvertebrates, which use opportunistically one or the other type, with no differences according to the seagrass habitat complexity, probably indicating a supply of individuals from further distances. These findings show that the detritus, acting as a faunal magnet, can be colonised by a rich and diverse benthic community, highlighting its important role in maintaining the biodiversity within the seagrass meadows.