Changes in the plankton community passing a Mytilus edulis mussel bed

The changes in phytoplankton community in water passing a large natural pop ulation of the blue mussels Mytilus edulis L.. in the Oresund strait betwee n Malmo and Copenhagen were studied. Changes in both the biomass and compos ition of the phytoplankton community were detected and an explanation invok ing intense grazing pressure from the mussels is proposed. The biomass, mea sured as biovolume, decreased from 0.42 to 0.11 mm(3) 1(-1) (74 % decrease) as the non-stratified turbulent water passed the mussel bed over a distanc e of 20 km. Beyond the mussel bed, biomass increased to 0.39 mm(3) 1(-1) wi thin 27 km. A benthic filter-feeder model was adapted to correlate the theo retical maximum filtration with observations. The model was in accordance w ith observed values if a filtration efficiency of 25 % was applied. During the passage the phytoplankton community was shifted to smaller phytoplankto n cells: phytoplankton with a diameter of 2 to 12 mu m increased from 65 to 89 % of the total phytoplankton community and all other plankton taxa decl ined. This observed shift in community was significant correlated with a pr edicted shift due to the mussel filtration.