The effect of flow velocity on methane production and oxidation in aquatic sediments.

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Inland waters are important components of the global carbon cycle. Emissions of the greenhouse gas methane (CH4) from inland water bodies are of growing global concern, because of their impact on climate change. Recent research efforts aim at improving the process-based understanding of the spatial and temporal dynamics of CH4 emissions from inland waters. Among the open research questions are: What are the driving factors of CH4 emission dynamics and how are they influenced by global change and anthropogenic alterations of aquatic systems, like river damming or reservoir construction? Many of the factors that are currently considered to affect the rates of methane production, oxidation and emission from aquatic sediments are directly or indirectly related to flow velocity. The flow-dependence of the factors and underlying processes, however, has not been considered explicitly. In this project we will use novel experimental mesocosm systems to study the flow-dependence of these processes in a series of targeted laboratory experiments. The experimental setup simulates the environmental conditions to which aquatic sediments are exposed in a hydraulic gradient from fast-flowing (lotic) to still water (lentic) ecosystems. Such transitions occur, for example, along longitudinal gradients in river impoundments as a consequence of damming. The experiments aim to detangle the effects of flow velocity from the processes that contribute to the overall methane budget in the sediment and the sediment-water interface. The results will be implemented into a process-based model. Besides relevant biogeochemical parameters, also flow velocity (near-bed turbulence) will be considered as an explicit boundary condition of the model. While the model will be validated using the data obtained from laboratory experiments, the conceptual framework by which flow velocity affects methane emissions from different types of aquatic ecosystems will be analyzed with a system-analytical approach.