Greenhouse gas emissions from rivers and streams along a steep gradient of biogeochemical constituents and land use

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Inland waters play a vital role in regional to global carbon cycling. Among the latest and least understood aspects in this context is the amount of carbon emitted from inland waters into the atmosphere as carbon dioxide (CO2) and as methane (CH4). Recent estimates suggest that less than half of the carbon that enters freshwater ecosystems from the terrestrial biome ultimately reaches the ocean. The remaining amount of terrestrial carbon, which is processed and eventually emitted into the atmosphere from inland waters, is a significant, though often neglected component of carbon cycling and greenhouse gas (GHG) budgets. While recent research effort focused mainly on lakes and reservoirs as GHG sources, major research gaps exist in respect to emissions from small streams and headwater systems, as well as in respect to the contribution of CH4 to the total GHG potential of emitted gases. The objectives of the proposed project are twofold: Our major goal is a quantitative assessment of the key processes controlling the intensity and flux paths of CO2 and CH4 emission from the fluvial network of a complete watershed. This process-based assessment will be obtained from extensive measurements within the core study area, the fluvial network of the 271 km2 watershed of the River Queich. This area consists of streams originating within the biosphere reserve Pfälzerwald and flowing through a contrasting landscape in terms of land use within less than 100 km distance. The measurements include GHG partial pressures in stream water, gas exchange velocity, bubble- and plant-mediated gas fluxes, stream metabolism, and surface area. A geographic information system will be used for up-scaling of the reach-scale measurements to the entire study area. As a second objective of this proposal, we will analyze the accuracy of this up-scaling, and there with the accuracy of regional-scale estimates of GHG fluxes and its dependence on quality and quantity of underlying data. These finding will be applied to estimate GHG emissions from an extended study area, where only low-level sampling will be performed. We expect land use and stream structural quality to be among major determinants for the key processes controlling local fluxes and for the magnitude of regional-scale emissions rates. Our measurement program and proposed analyses allows for a quantitative assessment of the impacts of anthropogenic activities on carbon processing and corresponding GHG emission from streams. Thus, this project may serve as a reference study for assessing GHG emissions from anthropogenically altered fluvial networks.