Data on the hydrography, nutrient chemistry, primary production, standing crops of phytoplankton and zooplankton, and the effects of entrainment on the phytoplankton-bacteria community by a power plant in the upper Patuxent estuary are reported for the period August 1968 to August 1970. Calculated primary production ranged from 0.01 to 5.77 g per sq. cm per day and the spacial pattern was inversely related to turbidity and the volumetric measurements of productivity. Maximum penetration of high nutrient levels into the estuary occurred during the winter. Nutrient concentrations were high and comparisons with earlier published data indicate accelerated rates of nutrient input. Published information on intake-effluent studies showed that the power plant can cause a significant reduction in the standing crop and photosynthetic rates of entrained organisms. Preliminary studies were made on the species composition, chemical content, biomass and production of marsh vegetation.