The Effect of Submersed Aquatic Vegetation on Phytoplankton and Water Quality in the Tidal Freshwater Potomac River

ABSTRACT

Phytoplankton biomass and water quality were determined at two sites in the tidal freshwater Potomac River. At each site three plant treatments were sampled twice each during the summer and the fall. Phytoplankton biomass, as measured by chlorophyll a, was negatively related to macrophyte density. High density Hydrilla beds harbored the least phytoplankton chlorophyll, while low density mixed beds were most similar to the open water areas. This effect was more pronounced in summer than in fall. Variation in dissolved oxygen and pH reflected the standing crop of both phytoplankton and macrophytes. Phytoplankton were the dominant influence in the open water; macrophytes were more important in the dense weedbeds. Greatest variation in temperature was observed in the dense weedbeds.