Streams provide a physical linkage between land and downstream river networks, delivering solutes derived from multiple catchment sources. We analyzed high‐frequency time series of stream solutes to characterize the timing and magnitude of major ion, nutrient, and organic matter transport over event, seasonal, and annual timescales as well as to assess whether nitrate ( NO3−) and dissolved organic carbon (DOC) transport are coupled in catchments, which would be expected if they are subject to similar biogeochemical controls throughout the watershed. Our data set includes in situ observations of NO3−, fluorescent dissolved organic matter (DOC proxy), and specific conductance spanning 2–4 years in 10 streams and rivers across New Hampshire, including observations of nearly 700 individual hydrologic events. We found a positive response of NO3− and DOC to flow in forested streams, but watershed development led to a negative relationship between NO3− and discharge, and thus a decoupling of the overall NO3− and DOC responses to flow. On event and seasonal timescales, NO3− and DOC consistently displayed different behaviors. For example, in several streams, FDOM yield was greatest during summer storms while NO3− yield was greatest during winter storms. Most streams had generalizable storm NO3− and DOC responses, but differences in the timing of NO3− and DOC transport suggest different catchment sources. Further, certain events, including rain‐on‐snow and summer storms following dry antecedent conditions, yielded disproportionate NO3− responses. High‐frequency data allow for increased understanding of the processes controlling solute variability and will help reveal their responses to changing climatic regimes.

Little is known about the regional extent and variability of nitrate from atmospheric deposition that is transported to streams without biological processing in forests. We measured water chemistry and isotopic tracers (δ18O and δ15N) of nitrate sources across the Northern Forest Region of the U.S. and Canada and reanalyzed data from other studies to determine when, where, and how unprocessed atmospheric nitrate was transported in catchments. These inputs were more widespread and numerous than commonly recognized, but with high spatial and temporal variability. Only 6 of 32 streams had high fractions (>20%) of unprocessed atmospheric nitrate during baseflow. Seventeen had high fractions during stormflow or snowmelt, which corresponded to large fractions in near-surface soil waters or groundwaters, but not deep groundwater. The remaining 10 streams occasionally had some (<20%) unprocessed atmospheric nitrate during stormflow or baseflow. Large, sporadic events may continue to be cryptic due to atmospheric deposition variation among storms and a near complete lack of monitoring for these events. A general lack of observance may bias perceptions of occurrence; sustained monitoring of chronic nitrogen pollution effects on forests with nitrate source apportionments may offer insights needed to advance the science as well as assess regulatory and management schemes.

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 15 years 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 (NO 3 À ) concentration-discharge behavior from enrichment in the years prior to and including the floods to chemostasis after the floods, but for both time periods NO 3 À 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. Key Points:• Small decreases in concentrations and fluxes of salts and major ions were observed over 12 years despite an increase in the human footprint • Small (<1% of mean annual) decreases in DOC concentrations were observed over a 15-year period, but DOC did not respond to discharge • Occurrence of two 100-year flood events had little, if any, effect on multiyear trends in concentrations, fluxes, or C-Q relationships Supporting Information:• Figure S1