Trophic and biotic interactions in Laminaria Digitata beds: Which factors could influence the persistence of marine kelp forests in northern brittany?

In this paper, we first reviewed the trophic ecology studies carried out on Laminaria digitata beds, at two rocky areas of Northern Brittany (France), Batz Island and Ar Pourven, displaying contrasted ecological conditions. The general trophic structure did not vary between the two sites, with a wide diversity of filter-feeders and predators, and only 14% of grazers. The results of stable isotope analyses allow drawing a simplified model for the food web associated to L. digitata beds. Kelp-derived organic matter is channeled into two distinct trophic pathways, the particle feeding-based and the grazing-based, which are coupled with higher trophic levels. The L. digitata detritic pathway contributes most to the diet of filter-feeders inhabiting sheltered sites. Although detritus feeding seems to be the main pathway transferring matter from kelps to the upper trophic levels, we showed that the direct grazing of kelps by herbivores can be locally important. Indeed, the gastropod Helcion pellucidum based exclusively his diet on the consumption of L. digitata and therefore its grazing pressure was studied on L. digitata sampled in two sites of Bay of Morlaix. In both sites, the herbivory pressure was significantly higher on reproductive adult plants than juvenile algae and the distribution of grazing damages showed strong variations along L. digitata thalli. To study the specific algal responses against herbivory, a laboratory-controlled system was established using L. digitata / H. pellucidum model. Defense-related gene transcripts were monitored in grazed tissues of L. digitata, showing the induction of molecular responses upon grazing. Altogether, the functioning of kelp-associated food web and the contribution of L. digitata are likely to be affected by two important factors, hydrodynamism and nutritive values of the food source. However, our results suggest that specific induced defense responses upon grazing and chemical interactions in natural environment could also interfere with trophic links in kelp beds.