Storm effects on nitrogen flux and longitudinal variability in a river–reservoir system

Sustainable management of the nitrogen (N) cycle remains a considerable global challenge that has major implications for aquatic ecosystems. Dams play a critical yet often neglected role in addressing this challenge as they increase hydraulic residence time and denitrification potential. However, during storm events when the majority of N loading occurs, less is known about the effects dams have on N fate and transport processes. Here, we examined the flux of N species and phosphorus (P) and longitudinal profiles of nitrate (NO 3 − ) along a sixth‐order river–reservoir system at baseflow and across the falling limb of a multiday, 1‐year storm hydrograph. During the storm event, the reservoir reduced total NO 3 − flux by 19.1% and P flux by 12.7%. On the contrary, ammonium (NH 4 + ) fluxes were 200% higher downstream of the dam in comparison with fluxes into the reservoir, indicating that there was a substantial net export of NH 4 + from the reservoir during the storm event. Longitudinally along the river to reservoir transition, a breakpoint of statistical significance was identified, highlighting the sharp contrast between NO 3 − concentrations within the river and reservoir. Results suggest that frequent storm events such as the one presented here can greatly alter N removal processes in river–reservoir systems. Overall, this study highlights the need to better understand the role that storm events play in river–reservoir N cycling dynamics.