MEPS 306:191-200 (2006)  -  doi:10.3354/meps306191

ABSTRACT: Exposure to wave action is an important factor determining the distribution and abundance of marine species on rocky shores. We studied the primary and secondary effects of wave exposure on sublittoral blue mussel Mytilus edulis populations in a fragmented archipelago in the tideless northeastern Baltic Sea. Triplicate bottom samples were collected by SCUBA divers from 4 depths at 30 locations along a wave exposure gradient defined by an exposure index. From 15 of these locations, water samples were collected in order to measure physicochemical and biological conditions at each site. Results showed that mussel densities increased steadily with increasing wave exposure. Biomasses, however, were highest at areas with intermediate exposure. Water chemistry, seston and chlorophyll a concentrations did not differ between locations. Recruitment and growth experiments on artificial substrata carried out at the same 30 locations showed that exposure neither influenced the abundance of recruits on ropes nor markedly influenced the growth of adult mussels. This suggests that lack of mussels at less exposed localities may originate from problems in recruitment processes to adult populations or early post-recruitment mortality. We hypothesize that even thin films of stagnant sediment on rocks may have a profoundly negative effect on mussel recruitment. Results suggest that increasing sedimentation of rocky bottoms may limit the spatial distribution of mussels, pushing stable mussel beds towards outermost areas of the archipelago where strong wave action keeps bottoms free of sediments.

KEY WORDS: Physical stress · Predation · Recruitment · Rocky shore · Sedimentation · Artificial substrata · Wave exposure index · Depth distribution