Multiyear Trends in Solute Concentrations and Fluxes From a Suburban Watershed: Evaluating Effects of 100‐Year Flood Events

Anthropogenic activities have increased solute concentrations and fluxes in rivers globally. Increases in salt, dissolved organic carbon, and inorganic nutrients have been attributed to changes in atmospheric deposition, road salt and fertilizer application, and urbanization. Extremely large flood events, which are increasing in frequency and duration, could also alter river chemistry by flushing various solute reservoirs within watersheds and effectively resetting the linkages between land and river. We evaluated changes in concentrations and fluxes of inorganic nutrients, dissolved organic matter, and salts and major ions across 12 to 15years in a rural-suburban watershed in New Hampshire, USA. During this period, the human imprint on the landscape (impervious surfaces and population density) increased, but two 100-year flood events also occurred. We found that concentrations and fluxes of chloride and potassium, fluxes of all other major ions, and concentrations of dissolved organic carbon declined over time, but concentrations and fluxes of inorganic nutrients and dissolved organic nitrogen were unchanged. The minor declines in concentrations were of similar magnitude to declines previously observed in other watersheds that did not experience extreme flood events. We observed a shift in nitrate (NO3-) concentration-discharge behavior from enrichment in the years prior to and including the floods to chemostasis after the floods, but for both time periods NO3- concentrations were weakly related to discharge. Our results suggest a limited role of extreme events in the biogeochemistry of urbanizing watersheds and emphasize the importance of long-term records and multiple analytical approaches to understand the dynamics of complex watersheds.