ZEBRA MUSSEL INVASION IN A LARGE, TURBID RIVER - PHYTOPLANKTON RESPONSE TO INCREASED GRAZING

Changes in the biomass of benthic bivalves can cause dramatic changes in total grazing pressure in aquatic systems, but few studies document ecosystem-level impacts of these changes. This study documents a mass ive decline in phytoplankten biomass concurrent with the invasion of a n exotic benthic bivalve, the zebra mussel (Dreissena polymorpha), and demonstrates that the zebra mussel actually caused this decline. In t he fall of 1992 the zebra mussel became established at high biomass in the Hudson River Estuary, and biomass of mussels remained high during 1993 and 1994. During these 2 yr, grazing pressure on phytoplankton w as over 10-fold greater than it had been prior to the zebra mussel inv asion. This increased grazing was associated with an 85% decline in ph ytoplankton biomass. Between 1987 and 1991 (pre-invasion), summertime chlorophyll averaged 30 mg/m(3); during 1993 and 1994 summertime conce ntrations were <5 mg/m(3). Over this same period, light availability i ncreased, phosphate concentrations doubled, some planktonic grazers de clined, and average flow was not different from the pre-invasion perio d, Thus, changes in these other factors were not responsible for phyto plankton declines. We developed a mechanistic model that reproduces th e spatial and temporal dynamics of phytoplankton prior to the invasion of the zebra mussel (1987-1991). The model accurately predicts extrem e declines in phytoplankton biomass after the invasion (1993-1994). Th e model demonstrates that zebra mussel grazing was sufficient to cause the observed phytoplankton decline. The model also allows us to test which features make the Hudson River sensitive to the impact of benthi c grazers. The model suggests that the fate of lightscattering inorgan ic particles (turbidity) is a key feature determining the impact of be nthic grazers in aquatic systems.