The Effect of Sediment Gas Storage on Methane Ebullition Dynamics in a Freshwater Reservoir

Freshwater reservoirs are important sources of the greenhouse gas methane to the atmosphere. Methane is mainly produced in the bottom sediment, from where bubble mediate transport is the most efficient way of transporting methane to the atmosphere. The sediment characteristics shape net methane production, gas mobility in the sediment matrix, and in combination with external forcings also the release of gas bubbles. We combine methane flux monitoring (diffusive, ebullitive, and oxidation) with measurements of potential methane production rates, vertical distributions of gas voids in the sediment, and sediment gas content, to investigate the effect of sediment gas storage on methane ebullition dynamics from a freshwater reservoir. The study site is the pre-dam of Wupper Reservoir located in the western region of Germany. The extensive data set allowed to establish a methane balance for the pre-dam. The system is a methane source to the atmosphere in which ebullition was the main emission pathway (49.2% of methane emissions). Potential methane production was higher at the top 20 cm sediment layer and sediment freeze cores are being analysed to determine the vertical distribution of bubbles in the sediment. In a rough estimation, an equivalent of 8 – 28 days of potential methane production is stored as free gas in the sediment. The interplay between gas storage in the sediment and ebullition flux will be discussed.