Nitrogen Removal via Denitrification in Two Small Reservoirs in Central Wisconsin, U.S.A.

Abstract. Aquatic ecosystems with long hydraulic residence times (e.g., wetlands and reservoirs) can be important nitrogen (N) sinks via denitrification. The objective of this study was to examine denitrification rates of two small reservoirs (Springville and McDill) in central Wisconsin. Sediments, water chemistry, and discharge data were collected once per month between May and September of 2014 to achieve these objectives. Denitrification rates and microbial biomass carbon were not different between Springville and McDill; however, organic matter was significantly higher in McDill. Average denitrification rates were low at both sites, but ranged widely in Springville (0–23.72 mg N m–2 h–1) and less so in McDill (0.32–12.16 23 mg N m–2 h–1). Low denitrification rates in Springville may be the result of several locations being organic matter limited, whereas the McDill site was likely nitrate limited. Results from this study suggest reservoirs in central Wisconsin that are groundwater fed with sandy substrate have the potential to be nitrate sinks, but variation in the landscape (e.g. land use) and within each reservoir is influencing the magnitude of realized denitrification capabilities.