Understanding connectivity patterns in endangered species living in fragmented habitats is fundamental to improve management and conservation actions. Such improvements can be particularly pressing at trailing edges where populations are facing the greatest challenges of climate change, and appear all the 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 located at the southern range limit of the oarweed kelp Laminaria digitata. Our populations were sampled in a roughly continuous manner, with the distance between neighboring populations ranging from a few km to about a few hundreds. By genotyping 11 microsatellite markers, our aim was to (1) refine analyses of population structure, (2) test whether on-shelf islands are genetically more differentiated compared to 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 relation 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 db-RDA 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 continuously distributed populations.