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Patterns of diel variation in nitrate concentrations in the Potomac River

November 16, 2016

The 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 than estimates of total in-stream NO3 load loss across the basin that averaged 33% of the annual groundwater contribution to the river. Water temperature and discharge had stronger relationships to the daily magnitude of diel NO3 variation than did photosynthetically active radiation. Estimated diel areal NO3 loss rates were generally >1000 mg N m–2 d–1, greater than most published values because measurements in this large river integrate over a greater depth/unit stream bottom area than do those from smaller rivers. These diel NO3 patterns are consistent with the influence of photoautotrophic uptake and related denitrification, but we cannot attribute these patterns to assimilation alone because the magnitude and timing of diel dynamics were affected to an unknown extent by processes, such as evapotranspiration, transient storage, and hydrodynamic dispersion. Improvements to diel loss estimates will require additional high-frequency measures, such as dissolved O2, dissolved organic N, and NH4+, and deployment of 2 measurement stations.

Publication Year 2016
Title Patterns of diel variation in nitrate concentrations in the Potomac River
DOI 10.1086/688777
Authors Douglas A. Burns, Matthew P. Miller, Brian Pellerin, Paul D. Capel
Publication Type Article
Publication Subtype Journal Article
Series Title Freshwater Science
Index ID 70178393
Record Source USGS Publications Warehouse
USGS Organization California Water Science Center; New York Water Science Center