Quantifying nutrient fluxes and composition during extreme climate events in the Susquehanna River: Application for continuous water quality monitoring

Abstract:

Extreme high flow events in the Susquehanna River basin export a disproportionate mass of water, nutrients, and sediment into Chesapeake Bay. The magnitude and composition of the exported flux can influence the response in the Bay ecosystem. These events are often sparsely sampled because of logistical difficulties when sampling in dangerous flow conditions. Continuous water quality (CWQ) monitors were installed to improve flux estimates during these critical flow periods. We developed regression models relating discrete samples with CWQ constituents, including nitrate (NO₃), over a 2-year period. Daily concentrations were estimated for suspended sediments (SSC), total nitrogen (TN), and nitrogen species, including particulate nitrogen (PN). We quantified fluxes for individual events, spring, and water years, and compared regression-based fluxes to a USGS discrete sample model (Weighted Regressions in Time, Discharge and Season, WRTDS). With CWQ monitoring, NO₃ is measured at high frequencies and provides complete coverage of the hydrograph. Since a majority (>70%) of the TN flux in the Susquehanna is composed of NO₃, TN fluxes estimated with CWQ regression models are improved relative to those estimated with WRTDS. The CWQ and discrete model fluxes differed for individual flow events because CWQ monitoring records event-specific constituent-discharge relationships. The CWQ model results suggest that the geographic extent of storm events may play an important role in activating regions with differing availability of SSC and TN. Given that only moderately high flows (max. 170,000 cfs) occurred during the 2-year period, we conducted a retrospective examination of TN fluxes and their composition for 9 historic flow events ( > 300,000 cfs) using discrete samples. Despite their magnitude, TN fluxes during most extreme events were dominated by NO₃; event fluxes shifted from predominantly NO₃ to PN only during the two most recent tropical storms. This shift is likely due to reduced storage capacity in the reservoir system upstream of the monitoring site. Since PN was well predicted using CWQ constituents, our approach can capture this shift in TN composition. These results illustrate how CWQ monitoring expands our understanding of how extreme flow events transport sediments and nutrients to Chesapeake Bay.