Small Reservoirs as a Beneficial Management Practice for Nitrogen Removal

There are few beneficial management practices (BMPs) with demonstrated efficacy in snowmelt-dominated regions. Small reservoirs are a BMP that can help mitigate flooding and reduce sediment transport, while reducing export of dissolved nutrients. To understand controls on nitrate removal and assess how this ecosystem service can be optimized, denitrification activity was measured in reservoirs and stream pools of the Tobacco Creek Model Watershed (Manitoba, Canada) via the chloramphenicol-amended acetylene block technique. Denitrification activity was positively correlated with nitrate and sediment organic carbon (SOC), and negatively correlated with sediment particle size and pH. Reservoirs exhibited higher denitrification activity than stream pools and were associated with higher levels of SOC, higher nitrate in early summer, and lower concentrations of dissolved oxygen. Nitrate was added to a set of samples to test for nitrate saturation, an indicator of poor ecological status, where nitrate concentrations exceed the denitrification capacity of microbes. Forty-nine percent of measurements demonstrated nitrate saturation, indicative of the need for additional remediation activity. Findings from this research suggest this BMP has higher capacity for nitrogen removal than stream pools because of higher denitrification rates and a higher apparent threshold for nitrate saturation, coupled with increased residence times. Results also inform the construction of additional reservoirs, which have been identified as a priority BMP in this region. Siting reservoirs in areas where conditions contribute to buildup of fine sediments and planting riparian vegetation to foster high organic C availability may help optimize denitrification, although tradeoffs in terms of other ecosystem services must be considered.