Patterns of Diel Variation in Nitrate Concentrations in the Potomac River

Abstract:

The Potomac River is the second largest source of nitrogen to Chesapeake Bay, where reducing nutrient loads has been a focus of efforts to improve estuarine trophic status. Two years of high frequency sensor measurements of nitrate (NO₃^-) concentrations in the Upper Potomac River at the Little Falls gage were analyzed to quantify seasonal variation in the magnitude and timing of the apparent loss of NO₃^- from the water column that results from diel-driven processes. In addition to broad seasonal and flow-driven variation in NO₃^- concentrations, clear diel patterns were evident in the river, especially during low flow conditions that follow stormflow by several days. Diel variation was about 0.01 mg N/L in winter and 0.02 to 0.03 mg N/L in summer with intermediate values during spring and fall. This variation was equivalent to <1% of the mean daily NO₃^- concentration in winter and about 4% in summer; however, variation >10% occurred during some summer days. Maximum diel concentrations occurred during mid- to late-morning in most seasons, with the most repeatable patterns in summer and wider variation in timing during fall and winter. Diel NO₃^- loss diminished loads by about 0.6% in winter and 1.3% in summer, and diel-driven processes were minor compared to estimates of total in-stream NO₃^- loss that averaged about one-third of the inferred groundwater NO₃^- contribution to the river network. The magnitude of diel NO₃^- variation was more strongly related to metrics based on water temperature and discharge than to metrics based on photosynthetically active radiation. Despite the fairly low diminishment of NO₃^- loads attributable to diel variation, estimates of diel NO₃^- uptake were fairly high compared to published values from smaller streams and rivers. The diel NO₃^- patterns observed in the Potomac River are consistent with photosynthesis of periphyton as a principal driver which may be linked to denitrification through the release of labile carbon. The extent to which these diel patterns are related to measures of aquatic metabolism are unknown as is the role of dispersion in obscuring diel patterns. Improvements to these diel estimates will require additional measures such as dissolved oxygen and ammonium, and the use of a second upstream measurement station to better constrain NO₃^- uptake values.