Trophic ecology in a Northern Brittany (Batz Island, France) kelp (Laminaria digitata) forest, as investigated through stable isotopes and chemical assays

This study aimed at characterizing the relationships between the food web's structure and the nutritive value of basal food sources in a Northern Brittany (France) Laminaria digitata bed. Stable isotopes were used to identify the food sources consumed by benthic invertebrates, and the nutritive value of primary producers was assessed according to four descriptors (total organic matter, C/N ratio, proteins content, lipids content).Although the food web appeared to be based on a wide diversity of food sources, only Rhodophyta (red algae) and biofilms (epilithic and epiphytic) were heavily consumed by grazers. In contrast, Phaeophyta (brown algae), which are dominant in this habitat, have no specialized grazer (with the exception of Helcion pellucidum, specialized grazer of Laminaria digitata). This selective consumption may be related to the higher protein content and lower C/N ratio of Rhodophyta and biofilms, in comparison with Phaeophyta. Fresh brown algae are thus of poor nutritive value, but processes associated with their degradation are likely to improve this nutritive value, leading in the assimilation of detritus by filter-feeders, revealed by high δ13C in these consumers. Our results thus suggest that the nutritive value of basal food sources may be an important factor involved in the structuration of kelp-associated food webs.