Genetic diversity and connectivity remain high in eelgrass Zostera marina populations in the Wadden Sea, despite major impacts

Beginning in the 1930s, eelgrass meadows declined throughout the Wadden Sea, leaving populations susceptible to extinction through patchiness, low density and isolation. Additional anthropogenic impacts have altered current regimes, nutrients and turbidity — al l of which affect eelgrass. Recent abiotic modeling studies suggest that poor recovery is the result of a regime shift caused by the loss of positive feedbacks between seagrass meadows and their capacity to mediate turbidity. Additionally, it is hypothesized that genetic and demographic factors — in particular, the loss of genetic diversity and patch connectivity — have contributed to lower fitness of eelgrass, thereby further diminishing recovery potential. We assessed genetic diversity and connectivity of Zostera marina among 19 locations, covering some 950 km of coastline between Zeeland, Netherlands and Langhölmen, Sweden. Both allelic and genotypic diversity were high. A Bayesian analysis of population structure revealed 6 significant clusters of subpopulations that are connected by varying degrees of dispersal. Although population divergence was significant at as little as 5 km, isolation by distance was very weak, indicating high connectivity at scales of 150 km. A demographic interpretation of these data suggests that realized gene flow is strong and predominantly northward, leaving the western Wadden Sea relatively isolated. The failure of eelgrass to recover in the western Wadden Sea is, therefore, due to both unsuitable physical conditions and low incoming gene flow. Nonetheless, the greater Wadden Sea can be considered a seed transfer zone providing source material for restoration efforts in any areas where abiotic conditions are more favorable.