Biogeochemical Controls on Greenhouse Gas Dynamics in Streams and Rivers

Streams and rivers are biogeochemical “hot spots”, simultaneously processing and transporting both terrestrially and locally fixed carbon and nitrogen. The greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) are (by)products of some of these biogeochemical processes, making streams and rivers significant natural sources of greenhouse gases to the atmosphere. The magnitude of greenhouse gas emissions from streams and rivers is highly uncertain, in part, because of the numerous controls on greenhouse gas production and export across spatial and temporal scales. This dissertation examines biological, physical, and chemical controls on greenhouse gas dynamics in nested streams and rivers, resulting in improved understanding of temporal variability at the watershed scale. The first chapter reports concentration-discharge relationships for CO2 and CH4, providing insights into reaction and transport controls. This analysis shows that CO2 exhibits chemostatic behavior while CH4 exhibits source-limited behavior, which results high CO2 evasion but moderate CH4 evasion during hydrologic events. The second chapter explores in situ controls on CO2 at low flows and examines the potential for CO2 limitation of aquatic primary productivity. Results show that the highest levels of aquatic gross primary productivity in a watershed occur after all available CO2 is consumed, illustrating the importance of non-CO2 photosynthesis at the ecosystem scale for the first time. The third chapter presents four-year time series of dissolved N2O concentrations and emissions and identifies controls on in-stream production. This chapter shows that a storm-induced hydrologic event pushes the watershed from a N2O source to a N2O sink, likely due to disturbance of in-stream nitrogen processing. In sum, this dissertation contributes significant and novel steps forward in understanding processes controlling lotic greenhouse gas production and emissions