ABSTRACT: Exposure to wave action is an important factor determining the distribution and abun-dance of marine species on rocky shores. We studied the primary and secondary effects of waveexposure on sublittoral blue mussel Mytilus edulis populations in a fragmented archipelago in thetideless northeastern Baltic Sea. Triplicate bottom samples were collected by SCUBA divers from4 depths at 30 locations along a wave exposure gradient defined by an exposure index. From 15 ofthese locations, water samples were collected in order to measure physicochemical and biologicalconditions at each site. Results showed that mussel densities increased steadily with increasing waveexposure. Biomasses, however, were highest at areas with intermediate exposure. Water chemistry,seston and chlorophyll a concentrations did not differ between locations. Recruitment and growthexperiments on artificial substrata carried out at the same 30 locations showed that exposure neitherinfluenced 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 recruit-ment processes to adult populations or early post-recruitment mortality. We hypothesize that eventhin films of stagnant sediment on rocks may have a profoundly negative effect on mussel recruit-ment. Results suggest that increasing sedimentation of rocky bottoms may limit the spatial distri-bution of mussels, pushing stable mussel beds towards outermost areas of the archipelago wherestrong wave action keeps bottoms free of sediments.KEY WORDS: Physical stress · Predation · Recruitment · Rocky shore · Sedimentation · Artificialsubstrata · Wave exposure index · Depth distribution