Top-down control from the bottom: Regulation of eutrophication in a large river by benthic grazing

We use a 15-yr record from a 100-km stretch of the tidal, freshwater Hudson River to examine the controls of phytoplankton biomass. Across years, seasonal mean chlorophyll a (Chl a) and maximum Chl a varied by approximately 15-fold, with mean growing-season Chl a as high as 24 mu g L-1 and maximum Chl a as high as 120 mu g L-1. For 3 of the 15 yr the river would be classified as eutrophic on the basis of lake standards for Chl a. Year-to-year variation in Chl a was not closely related to either nutrients (phosphorus or nitrogen concentrations) or hydrologic flow. Annual variation in grazing by the invasive zebra mussel (Dreissena polymorpha) explained 90% of the variation in mean Chl a; however, the maximum Chl a reached during the growing season was not significantly related to grazing and, even with high grazing, blooms of phytoplankton that contained high proportions of potentially toxic cyanobacteria occurred. Further, zebra mussels decreased dissolved oxygen concentrations even while increasing production of submersed aquatic vegetation. The results from the Hudson add to a growing literature that suggests that ecosystem changes linked with high phytoplankton biomass depend on a diverse range of system characteristics as well as whether phytoplankton are controlled by top-down or bottom-up mechanisms.