DIFFERING EFFECTS OF EELGRASS ZOSTERA-MARINA ON RECRUITMENT AND GROWTH OF ASSOCIATED BLUE MUSSELS MYTILUS-EDULIS

I studied the effects of habitat structure, provided by an eelgrass Zo stera marina canopy, on shell growth rate and recruitment of co-occurr ing blue mussels Mytilus edulis in the Western Baltic Sea. M. edulis i n clumps consisting of 10 and 30 individuals were tagged and placed in unvegetated areas and in the centers of small, medium and large eelgr ass patches (0.8 to 1.6 m, 1.6 to 3.2 m, and >4 m across, respectively ). Inside eelgrass, M. edulis growth was approximately one-third of th at in adjacent unvegetated areas, regardless of vegetated patch size a nd mussel clump size. In contrast, mussel recruitment, assessed as abu ndance of 1 to 5 mm long juveniles, was enhanced by the presence of ee lgrass, and was highest in medium-size eelgrass patches where juvenile s were similar to 3x as dense as in clumps on the sand flat. Populatio ns of animals associated with seagrasses can thus be enhanced and depr essed simultaneously by the plant canopy, depending on the response va riable. Moreover, the spatial structure of the vegetation, in this cas e eelgrass patch diameter, may be important for one response Variable (recruitment) but irrelevant for another (shell growth).