The Influence of Floodplain Restoration on Stream Ecosystem Function in an Agricultural Landscape

Excess nitrogen (N) causes numerous water quality problems, including drinking water contamination, eutrophication, and loss of biodiversity. In the agricultural Midwest, the source of excess N is largely row crop agriculture, in which artificial drainage and N fertilizer additions facilitate the rapid movement of N from fields to downstream water bodies. One potential way to mitigate N pollution, while maintaining crop yields, is through two-stage ditch restoration, where floodplains are excavated adjacent to incised agricultural channels. During storm events, water flows onto the floodplains, spreads out, and slows down. My dissertation research examined the influence of the two-stage ditch on ecosystem function, including denitrification and whole-stream metabolism. I also assessed habitat constraints on the endangered clubshell mussel (Pleurobema clava), and evaluated the influence of plants on wetland denitrification.

I found that floodplain restoration increased reach-scale denitrification by increasing the time and space over which denitrification can occur. Denitrification rates increased over the course of a floodplain inundation event, and the presence of vegetation enhanced floodplain denitrification rates. In addition, gross primary production increased during storm events, although ecosystem respiration and assimilatory N uptake remained unchanged. I also compared the two-stage ditch to other N removal practices and found that wetlands are the most cost-effective practice (in $/kg N removed), followed by two-stage ditches and cover crops. In addition, I found that clubshell mussels appear to be limited by low pore water oxygen, likely as a result of interstitial sedimentation limiting the percolation of surface water oxygen. Finally, vegetation increased ammonium uptake rates in an urban wetland, but it did not influence denitrification. Overall, my dissertation research demonstrates that floodplain and wetland ecotones can enhance N retention, improving water resource management in anthropogenically-influenced landscapes.