Effects of oyster farming on macrofaunal assemblages associated with Lanice conchilega tubeworm populations: A trophic analysis using natural stable isotopes

The macrobenthic assemblages associated with aggregations of Lanice conchilega polychaetes under and near oyster culture tables were investigated with regard to the food web. Samples were collected from a reference site without oyster influence and from a site beneath oyster tables. While no changes in species diversity were observed, we showed a profound effect of shellfish structures both on the composition of macrobenthic assemblages, and on the trophic structure of the food web. Predators predominated in sediments beneath oyster tables, both in number of individuals and in biomass, and other suspension-feeders were excluded. Oysters are seen here as key species in exerting a top-down control on water quality and food resources. Stable isotopic analysis allowed calculation of trophic position for organisms and provided an in-depth understanding of interspecific relationships, especially among polychaetes. We showed two distinct trophic pathways, one dominated by Nephtys hombergii in reference sediments, and the other dominated by Lumbrineris tetraura beneath oyster tables. Microphytobenthos and detritus of macroalgae seemed to contribute most to the suspension-feeders' diets, but we observed high $δ$15N values in surface and sub-surface deposit-feeders, which indicated they utilized sedimentary organic matter after bacterial processing and subsequent isotopic fractionation.