Effects of an invasive bivalve on the zooplankton community of the Hudson River

1. Previous studies documented that zebra mussels became abundant in the Hudson River during 1992 causing an 80–90% reduction in phytoplankton biomass. This study used intervention time series analysis of abundance, biomass and reproduction over the period 1987–95 to assess changes in zooplankton in response to the invasion. 2. Zebra mussels caused a size‐dependent decline in zooplankton. Microzooplankton, including tintinnid ciliates, rotifers and copepod nauplii all declined in 1992 and were scarce thereafter. Mean abundances of post‐naupliar copepods and of cladocerans were also lower following the invasion but these changes were not statistically significant (P > 0.05). Egg ratios and clutch sizes for the dominant cladoceran, Bosmina freyi, were not significantly related to zebra mussels, even though relatively low egg ratios were observed after the invasion. 3. The strong declines in microzooplankton were probably caused by direct zebra mussel predation. Estimated consumption rates by mussels were roughly equivalent to maximum microzooplankton growth rates. 4. The total biomass of zooplankton in the Hudson River declined by more than 70% following the invasion. Annual average zooplankton biomass was correlated with chlorophyll, but biomass per unit chlorophyll in the Hudson River was much lower than in lakes. The present study hypothesizes that this lower biomass reflects limitations by riverine flow and by predation during summer.