Dinitrogen (N2) fixation and denitrification are two nitrogen (N) cycling processes that despite differences in environmental requirements and constraints, co-occur in aquatic ecosystems. The overall goal of this dissertation was to evaluate how spatial heterogeneity of environmental variables 1) drive hot spots of N2 fixation, denitrification rates and gene abundances in streams, 2) facilitate co-occurrence of these processes across wetland – stream – lake interfaces, and 3) affect differences in microbial community composition in streams across U.S. ecoregions. We found hot spots of both processes within 7 stream reaches in Michigan and Idaho, but rates of N2 fixation were not directly related to relative gene abundances of nifH, while denitrification rates were related to relative gene abundances of nirS. Spatial heterogeneity of organic matter and dissolved oxygen concentrations were important predictors of rates of both processes. In a survey across 5 wetland – stream – lake interfaces of Lakes Superior and Huron, we found that rates of N2 fixation and denitrification occurred across stream, wetland and shallow lake habitats and that phosphorus (P) availability was important for predicting rates of both processes, while N availability was an important predictor of denitrification and carbon (C) availability was important predictor of N2 fixation. Finally, in a survey of microbial assemblages from 30 streams across 13 U.S. ecoregions, we found that microbial community composition differed across ecoregions in alpha diversity and relative Class abundances, but little of this variation was explained by environmental variables. Together, these studies show that N2 fixation and denitrification co-occurred in stream and coastal ecosystems and across spatial scales from stream reaches to ecoregions. However, rates and microbial community composition are not explained fully xii by differences in environmental variables on the microhabitat, cross-habitat, or ecoregion scale. N alone was not always an important predictor of the processes despite N being thought of as the best indicator of these processes in the past. Overall, these studies highlight the need to include both N2 fixation and denitrification measurements in biogeochemical studies for a better understanding of the complexity of N cycling in aquatic ecosystems
