|Description:||In this thesis I studied the clonal structure in dense eelgrass (Zostera marina) meadows in the Baltic Sea in the context of the mating system, inbreeding depression, local adaptation, kinship structure and genet dynamics. I used microsatellite markers to access the fine scale (1-m) clonal structure in four 15-m x 15-m plots located within dense eelgrass meadows in two populations on the Baltic Coast (1.5 – 3.5 m water depth). These plots served as templates for the selection of replicated transplants for laboratory and field experiments, as permanently marked areas for resampling and tracking genets and as database for the calculation of genetic parameters. The key questions investigated and their answers were the following: (i) Can flowering ramets recognize their genetic neighbourhood through pollen and/or growth interactions? - Addition of self versus cross pollen affected the inflorescence sex ratio. This can only be explained by the presence of a cryptic self-incompatibility system. (ii) Does inbreeding depression influence the size distribution of eelgrass clones? - The level of heterozygosity was higher in larger clones. Together with measures of reproductive output this suggests that large dominant clones outcompete their relatively inbred neighbours in an environment with low levels of disturbance. (iii) Are eelgrass clones locally adapted? - Transplantation of replicated genets between two populations showed significant local adaptation and dominance at one site. (iv) Does limited gene flow lead to kinship structure beyond the spatial spread of eelgrass clones? - Spatial autocorrelation, modified for a clonal species, revealed significant coancestry (fij) for the neighbourhood of ramets, clone fragments and entire clones. (v) What are the demographic parameters of eelgrass genets? - Eelgrass clones showed surprisingly high turnover across genets but almost constant patterns within genets between the years 2000 and 2001. This suggests a genetic component to flowering intensity and the production of vegetative shoots. This last study was designed to continue for several more years.