Changes in the plankton community passing a Mytilus edulis mussel bed

Abstract

The changes in phytoplankton community in water passing a large natural population of the blue mussels Mytilus edulis L. in the Oresund strait between Malmo and Copenhagen were studied. Changes in both the biomass and composition of the phytoplankton community were detected and an explanation invoking intense grazing pressure from the mussels is proposed. The biomass, measured as biovolume, decreased from 0.42 to 0.11 mm3 l-1 (74 % decrease) as the non-stratified turbulent water passed the mussel bed over a distance of 20 km. Beyond the mussel bed, biomass increased to 0.39 mm3 l-1 within 27 km. A benthic filter-feeder model was adapted to correlate the theoretical maximum filtration with observations. The model was in accordance with observed values if a filtration efficiency of 25 % was applied. During the passage the phytoplankton community was shifted to smaller phytoplankton cells: phytoplankton with a diameter of 2 to 12 μm increased from 65 to 89 % of the total phytoplankton community and all other plankton taxa declined. This observed shift in community was significant correlated with a predicted shift due to the mussel filtration.