Dissolved Organic Carbon and Nitrate Concentration‐Discharge Behavior Across Scales: Land Use, Excursions, and Misclassification

Abstract

Abstract High‐frequency in situ sensors have enabled researchers to measure solute concentrations at a time scale that captures the variability in stream discharge. We analyzed discrete samples and high‐frequency time series of solutes to characterize how nitrate (NO 3 − ) and fluorescent dissolved organic matter (fDOM; a proxy for dissolved organic carbon) respond to changes in discharge at annual and intra‐annual timescales across a stream network in New Hampshire, USA. NO 3 − and fDOM exhibited highly variable concentration‐discharge (c‐Q) behavior at intra‐annual scales. Transport limitation, source limitation, and chemostatic behavior were observed to occur within and among years in all our study watersheds. Annual assessment of c‐Q misclassified streams 31% of the time, as the annual time step missed seasonal and event‐induced shifts in c‐Q dynamics. In some instances, anomalous events lasting less than 5% of the year determine the annual c‐Q behavior for a site. Catchment land use appeared to drive some of the variability among watersheds in c‐Q relationships and their temporal variability. Forested streams had highly variable NO 3 − c‐Q behavior and streams draining watersheds with more development had greater variability in fDOM c‐Q behavior. Sample frequency impacts how hydrologic systems are characterized and extrapolating c‐Q behavior from discrete samples alone can bias interpretations of c‐Q dynamics and our understanding of solute transport.