Résumé
Both historical and contemporary natural processes have a profound effect on the distribution of individuals within a species range and, as a consequence, on the distribution of the genetic diversity. In this study, we propose to use combined tools of population genetics, phylogeography and demography to investigate dynamics of dispersal and colonisation in two brown seaweeds with contrasting (annual vs perennial) life histories. The kelp, Laminaria digitata (Ld), is the dominant species along the Brittany coast where it is harvested for its high quality alginates. Its species range appears to be changing under the effect of the climatic changes and/or anthropogenic pressures. In Southern Brittany, the southern limit of its range, Ld populations are regressing in favour of the annual kelp species Saccorhiza polyschides (Sp). This later species found in warmer waters is thus suspected to out-compete Ld on northern European coasts. The historical approach reveals that the two species have different phylogeographic histories: the main refugia occurred in Brittany for Ld while several other refugia where found for the Sp. For both species, the present-day genetic diversity is partially explained by historical processes but also highly dependent on population connectivity (habitat availability, hydrodynamics). At its range limit, Ld is showing a significant decrease of genetic diversity while the diversity of the opportunistic species Sp is near its maximum. Interestingly, the highest variability of Ld is found in NW Brittany, close to its range limit and in places where it is highly harvested. In conclusion, global warming combined with human exploitation puts a risk in the maintenance of these forests and the development of long term monitoring is urgently needed to measure the impact of global change on the evolution of this algal resource.