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Denitrification in small reservoirs: understanding nitrogen removal across an agricultural watershed



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As the world’s increasing human population demands greater agricultural intensity, concerns regarding environmental nutrient loading are also rising. Landowners in the Tobacco Creek Model Watershed (TCMW) in southern Manitoba, Canada, have implemented beneficial management practices to reduce nutrient loading to surface waters. Among these practices is a network of small reservoirs that have been shown to reduce nutrient concentrations downstream. It is unclear what is contributing to this ecosystem service, but bacterial denitrification is thought to be important. To understand the role of these reservoirs in excess nitrate removal and assess how this ecosystem service can be optimized, denitrification activity in reservoirs and stream pools of the watershed was measured via the chloramphenicol-amended acetylene block technique. Results indicated that denitrification activity was positively correlated with measured nitrate concentrations and sediment organic carbon (SOC), and negatively correlated with sediment particle size and pH. Regression trees typically identified nitrate as the primary node in partitioning denitrification activity. Importantly, reservoirs exhibited higher denitrification activity and were at times less likely to be nitrate-saturated than stream pools. This finding was attributed mainly to higher levels of SOC, and lower concentrations of dissolved oxygen found in reservoirs. Nitrate was added to a set of samples to test for nitrate saturation, which is an indicator of poor ecological status, as nitrate concentrations exceed the denitrification capacity of microbes. Almost half of measurements (49%) demonstrated nitrate saturation, indicative of the need for additional remediation activity in the watershed. Classification trees suggested a threshold for nitrate saturation across sites at 0.68 mg L-1 NO3 + NO2. Findings from this research reveal that reservoirs not only improve nitrate removal capacity, but could inform the proposed construction of additional reservoirs in the TCMW. Understanding nutrient retention in this system could also have implications for downstream ecosystems such as Lake Winnipeg; an intensely eutrophic lake that has become a high priority area for remediation.



denitrification, nitrogen, beneficial management practices, reservoirs



Master of Environment and Sustainability (M.E.S.)


School of Environment and Sustainability


Environment and Sustainability


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