Impact of the blue mussel Mytilus edulis on the microbial food web in the western Wadden Sea, The Netherlands.

To study the impact of juvenile blue mussels Mytilus edulis on the microbial food web in the Dutch Wadden Sea, natural sea water was first exposed to mussel filtration. Subsequently, filtered plankton communities were used in a dilution experiment to establish mussel-induced changes in bacterial, pico- and nanophytoplankton growth rates as well as heterotrophic nanoflagellates (HNAN) and ciliate-induced grazing mortality rates. During the experimental period, from July to September, mussel filtration had a size-selective impact on the plankton community; on average, nanophytoplankton, HNAN and ciliates biomasses were removed at equal rates, while bacterial and picophytoplankton biomasses were affected to a much lower extent. The reduction in HNAN predators by mussels significantly lowered the grazing mortality rates for picophytoplankton. For bacteria, grazing mortality did not change, while specific growth rates almost doubled (from 0.65 to 1.16 d^-1). There was an increase in HNAN biomass following the enhanced bacterial production. Single exposure to mussel filtration thus led to a stimulation of the bacterial-HNAN pathway. HNAN biomass, although seriously reduced by mussel filtration, recovered to pre-filtration levels within 24 h. Nanophytoplankton and ciliates did not recover completely within 24 h. The results from this study reveal potentially important effects of mussel filtration on the pelagic food web not disclosed when considering phytoplankton biomass alone.