Patterns of diel variation in nitrate concentrations in the Potomac River

Abstract

AbstractThe Potomac River is a large source of N to Chesapeake Bay, where reducing nutrient loads is a focus of efforts to improve trophic status. Better understanding of NO3– loss, reflected in part by diel variation in NO3– concentrations, may refine model predictions of N loads to the Bay. We analyzed 2 y of high-frequency NO3– sensor data in the Potomac to quantify seasonal variation in the magnitude and timing of diel NO3– loss. Diel patterns were evident, especially during low flow, despite broad seasonal and flow-driven variation in NO3– concentrations. Diel variation was ~0.01 mg N/L in winter and 0.02 to 0.03 mg N/L in summer with intermediate values in spring and autumn, equivalent to <1% of the daily mean NO3– concentration in winter and ~2 to 4% in summer. Maximum diel NO3– values generally occurred in mid- to late morning, with more repeatable patterns in summer and wider variation in autumn and winter. Diel NO3– loss reduced loads by 0.7% in winter and 3% in summer. These losses were less th...