Changes in the plankton composition of estuarine systems are often driven by freshwater flow. These changes in species composition and abundance have the potential to affect trophic dynamics within the plankton community. In order to quantify the effects of freshwater flow in estuaries, the structure of the spring plankton community and copepod grazing were examined in an extreme dry (2002) and wet (2003) year in two tributaries of Chesapeake Bay. Increases in phytoplankton and copepod biomass in the wet year were large in comparison to the increase in microzooplankton biomass. Ample abundance of prey and high copepod community grazing potentials indicate that microzooplankton biomass was influenced by strong top-down control in the high flow year. While no evidence of a copepod-microzooplankton-phytoplankton trophic cascade was found, increased top-down control by grazers in combination with increased nutrient supply in wet years may be important in establishing spring phytoplankton blooms in Chesapeake Bay.