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 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. Plain Language Summary: Excess nitrogen in the environment can negatively impact both freshwater and coastal ecosystems. However, water bodies such as reservoirs have been shown to reduce the amount of nitrogen flowing downstream. This has led to the idea that if reservoirs are removed, there will be more nitrogen exported downstream. Many studies on reservoir influence on nitrogen flowing downstream have not explicitly included small reservoirs, transformations between forms of nitrogen, or seasonal variability of nitrogen processing. We found that while small reservoirs do decrease inorganic nitrogen during the warm season, organic nitrogen is also produced, leading to a lesser impact on total nitrogen. Dissolved inorganic nitrogen retention is highest during low flows. When flows are higher and nitrogen transport is greater, small reservoirs are not as biologically active, leading to very little nitrogen being transformed. We conclude that small reservoirs are not a nitrogen sink when accounting for transformations between nitrogen forms and accounting for nitrogen processing across all seasons. Removal of small reservoirs will likely not lead to increased annual nitrogen exports to coastal areas. Key Points: Small reservoirs were not a significant nitrogen sink over annual timescales when considering both inorganic and organic forms of NSeasonal variability in nitrogen processing led to small reservoirs acting as temporary sinks or sources for different forms of nitrogenIn contrast to previous predictions, small dam removals do not always lead to greater nitrogen exports to coastal ecosystems [ABSTRACT FROM AUTHOR]