Biodiversity, Trophic Structure and Functioning in Laminaria hyperborea forests and associated communities, among pristine and harvested areas in Brittany

Kelp forests are emblematic habitats of temperate rocky reefs. In Brittany, these ecosystems are dominated by Laminaria hyperborea, this species is harvested for its alginic acid content. The aim of this thesis work is to characterise the biodiversity, structure and trophic functioning that feature these communities in natural and harvested conditions. The incidence of at least 572 species (110 algae and 462 fauna) was reported from sampling performed within two stations off the coast of Brittany. Diverse assemblages were highlighted according to the stratification of kelp forests in various microhabitats, with contrasting complexities. The link between habitat stratification and associated biodiversity suggests that considering this stratification is necessary for a sustainable management of this ecosystem. Within local food web, while the resource was partitioned at micro-scale, trophic pathways were based on phytoplankton, kelp and associated seaweeds. The seasonal cycle (i.e. the alternation of growth and senescence phases) of food sources proved to influence consumption by associated communities (both indirect consumption through detrital pathways and direct consumption through grazing). The results of this thesis highlighted the importance to ascertain the overall functional diversity within kelp forests. This diversity needs to be taken into consideration at a multi-scale level in order to comprehend ecosystem stability.