Contrasting temporal variation in habitat complexity and species abundance distributions in four kelp forest strata

In aquatic environments, habitat complexity influences community species composition at a local scale by partitioning physical niches, mediating water motion and retaining organic matter. Stratified into several microhabitats, kelp forests represent one of the most complex biotopes in coastal waters. These microhabitats are generally made of biogenic structures characterised by species-specific life cycles and their complexity is expected to change over time. In a Laminaria hyperborea forest near Roscoff (Brittany, France), we tested whether temporal changes differed among strata (lamina, stipe, holdfast and rock) in terms of habitat complexity, richness and species abundance distributions. At the scale of the study, the epiphytic habitat was relatively stable over four sampling dates, whereas epilithic structures of the understorey appeared particularly unstable in terms of habitat size and distribution among morpho-functional groups of habitat-formers. Unlike sessile species, mobile fauna experienced substantial temporal changes in richness and abundance distribution in all microhabitats studied. Although mobile fauna distribution varies in part with habitat complexity, additional factors likely come into play such as seasonal and stochastic variation in direct and indirect food resources.