Understanding connectivity patterns exhibited by endangered species living in fragmented habitats is fundamental to improving management and conservation actions. Such improvements can be particularly pressing at the trailing edges of these habitats, where populations are facing the greatest challenges from climate change, and appear even more crucial if the species is commercially harvested. Seascape genetics have been increasingly used to meet these needs. In this study, we examined connectivity patterns among 32 populations of the oarweed kelp Laminaria digitata located at the species’ southern range limit. The distance (or sampling gap) between neighboring populations ranged from a few km to a few 100s of km. By genotyping 11 microsatellite markers, we aimed to (1) refine analyses of population structure; (2) test whether on-shelf islands are genetically more differentiated than mainland populations; (3) evaluate the relative importance of various abiotic conditions in shaping the genetic structure; and (4) evaluate if the relative importance of each environmental factor varied according to sampling schemes. Our analyses revealed a positive relationship between connectivity links and genetic diversity: populations with high levels of connectivity were genetically enriched while isolated populations showed signs of genetic erosion. The genetically impoverished populations corresponded to the southernmost populations as well as populations along the northern coast of Brittany (Locquirec, Saint-Malo Bay) and the northernmost population in Pas-de-Calais. By performing distance-based redundancy analysis on various sampling schemes, geographic distance appeared as the dominant factor influencing connectivity between populations separated by great distances, while hydrodynamic processes were the main factor when analyzing at a final spatial resolution.
