Inland waters are an important component of the global carbon cycle. Emissions of the potent greenhouse gas methane (CH₄) from inland water bodies are of growing global concern due to their impact on climate change. Recent research efforts have been aiming to improve the process-based understanding of the spatial and temporal dynamics of CH₄ emissions from inland waters. Among the open research questions are the driving factors of CH₄ emission dynamics and how they are influenced by global changes and anthropogenic alterations of aquatic systems, such as river damming and reservoir construction. Many of the factors currently considered to affect the rates of methane production, oxidation and emission from aquatic sediments are directly or indirectly related to flow velocity. However, the flow-dependence of these factors and underlying mechanisms have not been explicitly considered. 

In this project we developed a novel experimental mesocosm facility to study the flow-dependence of these processes in a series of targeted laboratory experiments. The experimental setup was used to simulate the environmental conditions to which aquatic sediments are exposed in a hydraulic gradient from fast-flowing (lotic) to still water (lentic) ecosystems. The dynamics of CH₄, CO₂ and O₂ were studied over prolonged periods in the gastight mesocosms, allowing for the establishment of mass balances and facilitating a process-based understanding of CH₄-related metabolism. In these budgets, we also took into account several unavoidable shortcomings of the experimental setup, including gas leakage from the flumes and the sorption and desorption of CH₄ to the flume walls, which were quantified in additional experiments. Nevertheless, the variability of observed CH₄ dynamics among flumes with identical sediments exposed to the same flow velocity was high, possibly due to divergent microbial communities that had developed during the initial phase of the experiments. However, by combining mass balances of measured CH₄, CO₂ and O₂ dynamics, we found that increasing flow velocities led to both increasing gross CH₄ production in the sediment and increasing ecosystem CH₄ oxidation rates. Under the studied conditions, both opposing effects were of comparable magnitude, resulting in an insignificant net effect of flow velocity on CH₄ emissions. The flume system that was developed and extensively characterized in this project has proven to be a valuable tool for studying sediment metabolism under environmental flow conditions. However, considerable effort is required in preparing and in analysing such experiments.

Project PI: A. Lorke