EXPERIMENTAL MEASUREMENTS OF ZEBRA MUSSEL (DREISSENA-POLYMORPHA) IMPACTS ON PHYTOPLANKTON COMMUNITY COMPOSITION

1. To investigate direct effects of zebra mussel (Dreissena polymorpha ) feeding activities on phytoplankton community composition, short-ter m microcosm experiments were performed in natural water with complex p hytoplankton communities. Both gross effects (without resuspension of mussel excretions) and net effects (with resuspension) were studied. 2 . Gross clearance rates were not selective; essentially all taxa were removed at similar rates ranging from 24 to 63 mt mussel(-1) h(-1). Ne t clearance rates were highly selective; different plankton taxa were removed at very different rates, ranging from 12 to 83% of the gross r ates, leading to consistent changes in the phytoplankton community com position. Thus, although zebra mussels can cause most phytoplankton to decline, there is considerable variation among taxa in either pre-dig estive selection or post-digestive survival. 3. The direct, short-term effects of zebra mussels on phytoplankton community composition are c onsistent with some of the major changes observed in the Hudson River since establishment of zebra mussels. 4. We show, with simple calculat ions, how zebra mussel filtration rate, its selective efficiency on va rious taxa, and phytoplankton growth rates interact to produce changes in the phytoplankton composition.