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

Abstract

Anthropogenic nitrogen (N) inputs to the landscape have serious consequences for inland and coastal waters. Reservoirs are effective at mitigating downstream N fluxes but measurements have generally focused on large reservoirs and have not considered seasonal variability or all N forms. In this study, we conducted an N mass balance in eight small reservoirs (surface area <0.55 km2) in coastal New England over annual time periods, including both inorganic and organic forms of N. We found that small reservoirs have high capacity for dissolved inorganic N (DIN) retention during low and moderate discharge, but are roughly in balance for DIN at higher discharge. Because proportional DIN retention occurred when N inputs were at their lowest, their effect on downstream N fluxes is small over annual time frames. Further, dissolved organic N (DON) was also evident during low flow late in the warm season. Accounting for DON production, the net effect of reservoirs on total dissolved N (TDN) fluxes was limited. These transformations between inorganic and organic N should be considered when evaluating the effect of small reservoirs on TDN fluxes over seasonal and annual timescales. With dam removal becoming a common solution to aging, unsafe dams, their ability to retain or produce N must be scrutinized at longer time scales while accounting for the complete N pool to better comprehend the effect their reservoirs have on downstream waters.