Eelgrass Zostera marina populations in northern Norwegian fjords are genetically isolated and diverse

Populations along the northern boundary of a marine species’ distributional range in the NE Atlantic are expected to harbor lower standing genetic variation as a consequence of postglacial expansion following the last glacial maximum. Founder events and marginal habitat availability may render the edge populations more vulnerable to anthropogenic stress and less capable of rapid adaptation to global climate change, a concern for conservation and management. We analyzed meadow architecture, persistence and connectivity within and among 15 locations (600 samples genotyped with 8 microsatellite loci) in 3 fjords in Troms County, Norway (69° N). Whereas global mean allelic diversity (standardized for sample size) was in accordance with previous studies using the same markers, more extensive sampling revealed a broader range of allelic richness (mean = 2.85; range = 1.84 to 4.21) in the regional pool. Genotypic diversity was typically high, whereas large genets were rare (2 out of 15 locations). Population differentiation (F ST) was 2 to 6 times higher between fjords than within fjords. A Bayesian (Structure) analysis also strongly supported the genetic distinctness of each fjord. Although 9 locations within the 60 km long Balsfjord were connected by gene flow, demographic connectivity may actually be low, as fixed differences were observed at 6 of the 9 locations, along with significantly positive inbreeding coefficients and strong substructure. Overall, our results suggest that these northern, leading-edge meadows are healthy, but vigilance is required to avoid further losses. Fjord-level management, especially of the larger fjords, will be sufficient to capture the range of variation.