FILTRATION OF HUDSON RIVER WATER BY THE ZEBRA MUSSEL (DREISSENA-POLYMORPHA)

Zebra mussels (Dreissena polymorpha) graze on phytoplankton, and decre ased phytoplankton concentrations have been associated with zebra muss els in lakes. It is not known, however, how the zebra mussel will affe ct phytoplankton in turbid systems such as rivers and the freshwater p ortions of estuaries. To determine whether zebra mussels can effective ly remove phytoplankton in these turbid systems, and to determine what components of the suspended material are removed and at what rates, w e conducted a series of grazing and size-selection experiments using a mbient Hudson River water and its natural phytoplankton community. Zeb ra mussels removed both phytoplankton and total suspended weight (TSW) at comparable rates (similar to 115 ml mussel(-1)h(-1)). Variation in filtration rates were not correlated with TSW or chlorophyll a (chl a ) concentration, and did not appear to depend on relative proportions of either component, Mussels removed particles with approximately equa l efficiency in all particle size classes measured (0.4 mu m to >40 mu m). Zebra mussels appear to remove Hudson River phytoplankton effecti vely in the presence of suspended sediment and do so at rapid rates. B ased on our measurements and unpublished estimates of the size of the population, zebra mussels filter a volume equivalent to the entire vol ume of the tidal freshwater portion of the Hudson River about every 2 d.