Differing effects of eelgrass Zostera marina on recruitment and growth of associated blue mussels Mytilus edulis

Abstract

I studied the effects of habitat structure, provided by an eelgrass Zostera marina canopy, on shell growth rate and recruitment of co-occurring blue mussels Mytilus edulis in the Western Baltic Sea. M. edulis in clumps consisting of 10 and 30 individuals were tagged and placed in unvegetated areas and in the centers of small, medium and large eelgrass 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 that in adjacent unvegetated areas, regardless of vegetated patch size and mussel clump size. In contrast, mussel recruitment, assessed as abundance of 1 to 5 mm long juveniles, was enhanced by the presence of eelgrass, and was highest in medium-size eelgrass patches where juveniles were ~3x as dense as in clumps on the sand flat. Populations of animals associated with seagrasses can thus be enhanced and depressed simultaneously by the plant canopy, depending on the response variable. Moreover, the spatial structure of the vegetation, in this case eelgrass patch diameter, may be important for one response variable (recruitment) but irrelevant for another (shell growth).