Current patterns, habitat discontinuities and population genetic structure: the case of the kelp Laminaria digitata in the English Channel

Laminaria digitata is the dominant species of the dense, continuous kelp stands in the English Channel and on the Atlantic coasts of France, where it is harvested for its high quality alginates. However, in spite of its ecological and economic importance, our knowledge of the level and organisation of genetic diversity in this species is scant. Here, using comprehensive hierarchical sampling and 7 microsatellite loci, we explored the roles of dispersal strategies, current regimes and habitat discontinuities in shaping genetic structure of L. digitata populations. Our results show that continuous, non-fragmented forests of L. digitata were genetically differentiated at distances greater than 10 km, despite the absence of clear population boundaries. Furthermore, a pattern of isolation-by-distance indicated that gene flow occurred preferentially among adjacent populations following a stepping-stone model. In addition, we analysed the direction of migration using assignment tests and found that currents appeared to play a minor role in orienting gene flow, except in the Gulf of Saint Malo gyre. In contrast, habitat discontinuities were found to accentuate genetic differentiation and resulted in reduced genetic variation of isolated stands. In the context of a potential over-exploitation of kelp stands in Brittany, this study suggests that the existence of neighbouring populations can be vital to maintaining high levels of gene flow and thus, genetic diversity in this species.