Small Reservoirs as Nitrogen Transformers: Accounting For Seasonal Variability in Inorganic and Organic Nitrogen Processing

Anthropogenic nitrogen (N) inputs to the landscape have serious consequences for both inland and coastal waters. However, river networks, and reservoirs in particular, have been found to be effective at mitigating downstream N fluxes, reducing the impact of these N inputs. Previous studies of reservoir N removal have focused on larger reservoirs and do not always include both seasonal variability and the complete N pool. In this study, we conducted an N mass balance in eight small reservoirs in coastal New England of various ages and physical characteristics over annual time periods, including both inorganic and organic forms of dissolved N. We found that small reservoirs have high capacity for dissolved inorganic N (DIN) removal with the highest DIN removal occurring at the lowest hydraulic loads (HL), continuing through moderate HL before exporting DIN at higher HL. This high DIN removal generally took place during the low flow growing season when small reservoirs are the most biologically reactive but also when N inputs were at their lowest. This suggests that when small reservoirs have the greatest capacity for DIN removal, they are receiving low DIN inputs and therefore, their effect on reducing downstream DIN fluxes is negligible. Similarly, rates of dissolved organic N (DON) production in small reservoirs were greatest during the growing season but due to low flows, DON exports were marginal. Accounting for DON production, the net effect on total dissolved N (TDN) was diminished compared to DIN removal alone. Accounting for the effect of small reservoirs on only DIN fluxes during the low flow growing season has the potential to overemphasize their importance in reducing N fluxes to downstream ecosystems. Organic N, and transformations between inorganic and organic N, need to be considered when estimating the effect these small reservoirs have on TDN fluxes during not just the growing season but rather on annual timescales. As the value of N removal is an aspect of reservoir function that is considered when evaluating dams for removal, seasonality and the complete dissolved N pool are important factors to account for amid site-specific studies for such small reservoirs. It is possible that the benefits of dam removal may vastly outweigh the consequences in terms of N fluxes when these additional factors are considered.