Zebra mussel filter feeding and food-limited production of Daphnia: recentchanges in lower trophic level dynamics of Oneida Lake, New York, USA

Exotic zebra mussels can alter lower trophic level dynamics in lakes that t hey colonize by consuming large quantities of phytoplankton. We simulated t he indirect effects of zebra mussel grazing on Daphnia by artificially redu cing phytoplankton concentration for in situ Daphnia reproduction experimen ts. The response of Daphnia reproduction to reduced phytoplankton was evalu ated for both the in situ experiments and field observations in Oneida Lake , New York, U.S.A. Oneida Lake has had an abundant population of zebra muss els since 1992. Our experiments revealed that fecundity of individuals from two species of Daphnia was positively related to phytoplankton concentrati on during the spring clearwater phase, although there was no discernible ef fect of food concentration on fecundity in summer cyanobacteria-dominated a ssemblages. The experimental results suggest that Daphnia fecundity respond s to chlorophyll a concentrations < 2 mu g l(-1). The years since zebra mus sels became abundant in Oneida Lake have been characterized by high water c larity, low chlorophyll concentrations, long clearwater phases, and low Dap hnia biomass compared with the previous 17 years. The food web effects of z ebra mussel grazing are complex and it will take more years for impacts at higher trophic levels to develop and be identified.