Dissertation Research: Quantifying the role of denitrification as a mechanism for inorganic nitrogen removal in Midwestern rivers

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Agricultural and urban development has resulted in increased nitrogen (N) loading to streams, lakes, rivers, and oceans across the globe. Excess N in water bodies causes rapid algal growth and these algae eventually die and decompose, with the decomposition removing oxygen from the water column, resulting in ?dead zones.? In addition to algal blooms, excess N contaminates drinking water and can decrease freshwater biodiversity. Denitrification is a microbially-mediated process that converts dissolved nitrate-N (NO3-) to N gases, and permanently removes N from ecosystems. The bottoms of streams are ideal locations for denitrification, where it has been shown to be an important mechanism for permanent N removal. In contrast, less is known about the role of denitrification in rivers; this N removal process has been overlooked mainly because of the challenges associated with field work in systems of this size. This study will quantify denitrification rates in the water column and river bottom of 5 rivers in the agricultural Midwest. Using a new technique developed in estuaries, this project will compare denitrification rates across an agriculturally induced gradient of NO3- . Additionally, this study will compare the N removal due to denitrification with other temporary N removal mechanisms like the incorporation of dissolved N into biomass (e.g. plants and algae). Small streams have been identified as "hotspots" for water quality improvement and have thus been the focus of many restoration projects. Rivers, on the other hand, have been treated as pipes, with their major function being to export water (and pollutants) downstream. We predict that rivers are similarly bioreactive, like streams, and play a significant role in improving water quality and should be managed to maximize bioreactivity to reduce the environmental effects of N pollution downriver.