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

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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 post- glacial expansion following the last glacial maximum. Founder events and marginal habitat avail- ability may render the edge populations more vulnerable to anthropogenic stress and less cap - able 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 pre- vious 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 (FST) 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 Bals- fjord were connected by gene flow, demographic connectivity may actually be low, as fixed differ- ences were observed at 6 of the 9 locations, along with significantly positive inbreeding coeffi- cients 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.