Temporal Variability in Nitrate-Discharge Relationships in Large Rivers as Revealed by High-Frequency Data

Little is known about temporal variability in nitrate concentration responses to changes in discharge on intraannual time scales in large rivers. To investigate this knowledge gap, we used a six-year data set of daily surface water nitrate concentration and discharge averaged from near-continuous monitoring at U.S. Geological Survey gaging stations on the Connecticut, Potomac, and Mississippi Rivers, three large rivers that contribute substantial nutrient pollution to important estuaries. Interannually, a comparison of nitrate concentration-discharge (c-Q) relationships between a traditional discrete grab sample data set and the near-continuous data set revealed differing c-Q slopes, which suggests that sample frequency can impact how we ultimately characterize hydrologic systems. Intraannually, we conducted correlation analyses over 30-day windows to isolate the strength and direction of monthly c-Q relationships. Monthly c-Q slopes in the Potomac were positive (enrichment/mobilization response) in summer and fall and negative (dilution response) and weakly chemostatic (nonsignificant near-zero c-Q slope) in winter and spring, respectively. The Connecticut displayed a dilution response year-round, except summer when it was weakly chemostatic. Mississippi c-Q slopes were weakly chemostatic in all seasons and showed inconsistent responses to discharge fluctuations. The c-Q dynamics in the Potomac and Connecticut were correlated (R>0.3) to river temperature, flow percentile, and calendar day. Minimal correlation in the Mississippi suggests that the large basin area coupled with spatiotemporally variable anthropogenic forcings from substantial land use development created stochastic short-term c-Q relationships. Additional work using high-frequency sensors across large river networks can improve our understanding of spatial source input dynamics in these natural-human coupled systems.