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 massive decline in phytoplankton biomass concurrent with the invasion of an exotic benthic bivalve, the zebra mussel (Dreissena polymorpha), and demonstrates that the zebra mussel actually caused this decline. In the 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 was over 10-fold greater than it had been prior to the zebra mussel invasion. This increased grazing was associated with an 85% decline in phytoplankton biomass. Between 1987 and 1991 (pre-invasion), summertime chlorophyll averaged 30 mg/m3; during 1993 and 1994 summertime concentrations were <5 mg/m3. Over this same period, light availability increased, phosphate concentrations doubled, some planktonic grazers declined, and average flow was not different from the pre-invasion period. Thus, changes in these other factors were not responsible for phytoplankton declines. We developed a mechanistic model that reproduces the spatial and temporal dynamics of phytoplankton prior to the invasion of the zebra mussel (1987–1991). The model accurately predicts extreme declines in phytoplankton biomass after the invasion (1993–1994). The 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 benthic grazers. The model suggests that the fate of light-scattering inorganic particles (turbidity) is a key feature determining the impact of benthic grazers in aquatic systems.