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Long term response of water quality to changes in nutrient loading at a mainstem site in the tidal freshwater Potomac River

Like many freshwater systems in urbanized areas, the tidal Potomac River was subjected to extreme cultural eutrophication in the early to mid-20'h century. Phosphorus (P) loads to the river increased rapidly, reaching 32 200 kg/day in 1968, predominantly due to domestic wastewater input (CLARK et al. 1980). This increase coincided with major population increases in the metropolitan Washington area as well as increased collection and concentrated delivery of wastewater. Resulting degradation ofwater quality, including massive cyanobacterial blooms, led to a major commitment to enhanced wastewater treatment by officials from the U.S. President to localleaders in the midto late 1960s. Advances in phosphorus removal and phosphate detergent bans resulted in a rapid decrease, with point-source P loading declining to 7 700 kg/day by 1978, and <400 kg/day by 1984 where it has remained (CLARK et al. 1980, CBP 2007). Although generally not the limiting nutrient in the tidal freshwater Potomac, nitrogen (N) loads have also been reduced. As part of the Chesapeake Bay system, the tidal Potomac River continues to be the focus of restoration efforts. In addition to the decline in nutrient loading, submerged macrophytes have recolonized the shallow embayments and mainstem flanks, but remain much more restricted than early historical records suggest. This study reports the results of long-term monitoring since 1984 at an index station located in the mainstem channel area of the river.