Fine-scale genetic structure and relatedness in the eelgrass Zostera marina

The genetic composition of groups of individuals can significantly influence the productivity, resilience, and functioning of communities and ecosystems. For example, the re latedness of individuals within a group often dictates whether their interactions are competitive or cooperative. It is therefore necessary to characterize the genetic structure of populations at spatial scales relevant to these interactions and to determine the distribution of genetic diversity at those scales. Using microsatellite data, we assessed fine-scale population structure of Zostera marina, an important habitat-forming seagrass, within and between Bodega Harbor and Tomales Bay in northern California, USA. Despite the potential for long-range dispersal, we found significant population structure at all hierarchical scales (among bays, among sites, among tidal heights), corresponding to distances ranging from meters to tens of kilometers. The pattern of genetic differentiation that emerged at local scales differed between bays, with Tomales Bay being more structured even though the Euclidean distances among sites were similar in each bay. The relatedness of genets within a tidal height also differed among bays: in Bodega Harbor most genets occurred in proximity to unrelated individuals, whereas in Tomales Bay, genets were mixed with their close relatives, likely due to decreased dispersal. These contrasting kin structures, coupled with highly variable levels of clonal diversity, underscore the importance of examining variation at multiple scales, as this reveals genetic factors which might play an important role in many ecological processes.