Genetic structure in populations of Fucus vesiculosus (Phaeophyceae) over spatial scales from 10 m to 800 km

Recent studies showing consequences of species' genetic diversity on ecosystem performance raise the concern of how key ecosystem species are genetically structured. The bladder wrack Fucus vesiculosus L. is a dominant species of macroalga in the northern Atlantic, and it is particularly important as a habitat-forming species in the Baltic Sea. We examined the genetic structure of populations of F. vesiculosus with a hierarchical approach from a within-shore scale (10 m) to a between-seas scale (Baltic Sea-Skagerrak, 800 km). Analysis of five microsatellite loci showed that population differentiation was generally strong (average F-ST = 12%), being significant at all spatial scales investigated (10(1), 10(3), 10(4-5), 10(6) m). Genetic differentiation between seas (Baltic Sea and Skagerrak) was substantial. Nevertheless, the effects of isolation by distance were stronger within seas than between seas. Notably, Baltic summer-reproducing populations showed a strong within-sea, between-area (70 km) genetic structure, while Baltic autumn-reproducing populations and Skagerrak summer-reproducing populations revealed most genetic diversity between samples within areas (<1 km). Despite such differences in overall structure, Baltic populations of summer- and autumn-reproducing morphs did not separate in a cluster analysis, indicating minor, if any, barriers to gene flow between them. Our results have important implications for management and conservation of F. vesiculosus, and we raise a number of concerns about how genetic variability should be preserved within this species.