The Anacostia River, a Chesapeake Bay tributary running through Washington, D.C., is small but highly polluted with nutrients and contaminants. There is currently a multi-billion dollar tunnel project underway, being built in several phases, aimed at diverting effluent to sewage treatment, especially during high flow periods, and improving water quality of the Anacostia and the river into which it flows, the Potomac. Here, 4 years of biweekly to monthly nutrient and phytoplankton data are analyzed to assess pre-tunnel eutrophication status and relationships to flow conditions. Under all flow conditions, nutrients prior to tunnel implementation were well in excess of values normally taken to be limiting for growth, and hypoxia was apparent during summer. Chlorophyll a was higher in summer (averaging 26.9 μg L-1) than in spring (averaging 14.8 μg L-1), and based on pigment composition, summer communities had proportionately more cyanobacteria (> 2-fold higher zeaxanthin to chlorophyll a ratios) compared to spring, which had proportionately more diatoms (> 2-fold higher fucoxanthin to chlorophyll a ratios). When all data from all years and sites were considered, there was a decrease in diatoms and increase in cyanobacteria with decreasing NO3- and increasing NH4+ concentrations, increasing ratios of NH4+ to NO3-, and increasing temperature. Tunnel implementation and associated nutrient reductions may reduce the severity of summer blooms but reductions of spring assemblages may be even greater because river flows are typically higher at that time of year.
Keywords: Combined sewer overflow; Cyanobacteria; Flow relationships; Nutrient pollution; Wastewater treatment.