Impact of introduced juvenile mussel cultures on the pelagic ecosystem of the western Wadden Sea, The Netherlands

Pelagic mussel collectors provide an alternative to fishing for mussel seed from natural beds. These collectors, which have been recently introduced in the Dutch part of the Wadden Sea, facilitate the settlement and survival of blue mussel Mytilus edulis larvae. We assessed the removal of plankton by juvenile mussels and the recovery of plankton after filtration. A mesocosm experiment, using natural sea water, was executed on 12 occasions from June to October in 2010and 2011. Mussel filtration resulted in large reductions in nanophytoplankton, ciliate and total chlorophyll biomasses (65−62%), while picophytoplankton and bacterial biomasses were reduced to a lower extent (38 and 18%). After filtration, mussels were removed and the plankton community was allowed to recover for 8 d, which is the average residence time of water in the area. During this recovery period, net growth rates of bacteria, pico- and nanophytoplankton increased initially in the mussel-filtered mesocosms, but at the end of the recovery period, growth rates were similar in mussel-filtered and control mesocosms. At the end of the recovery period, plankton concentrations between control and mussel mesocosms were not statistically different despite the initial large reduction due to mussel filtration. Our results suggest that nutrients released by musselsduring filtration might have stimulated the filtered plankton community, enabling recovery to filtration within 8 d