An analysis of phytoplankton variations in the Occoquan Reservoir, 1970-1988

The phytoplankton data from Occoquan Reservoir surface samples collected during 1970-1988 were entered into a database. The data were analyzed to determine the relationships between changes in phytoplankton succession patterns and the various physical and chemical water-quality variables. The investigation considered the variations in both the phytoplankton and the water-quality variables with respect to sampling station location along the length of the reservoir. The variables included rainfall, season, pH, alkalinity, nitrogen, phosphorus, copper sulfate, odor intensity and type, and chlorophyll-a.

The discharge from the Upper Occoquan Sewage Authority advanced wastewater treatment plant (UOSA AWT) had a noticeable influence on both the phytoplankton division dominance and the genera occurrence. Before the AWT plant was built, Chlorophycota was usually dominant; afterwards, Bacillariophycota dominance increased. The dominant genera were a subset of the genera that occurred frequently. Of the original 72 genera, an analysis of 9 prevalent genera would have provided virtually the same information. Neither the intensity nor type of odors observed were predictable on the basis of the phytoplankton genera present in the reservoir. No statistically valid conclusion could be reached about the impact that copper sulfate applications had on reservoir algae.