Methane dynamics and thermal response in impoundments of the Rhine River, Germany

River impoundments have been identified as important emission hot spots of the greenhouse gas methane. In this study, we investigated methane dynamics of five river impoundments within a two year period using a variety of methods ranging from sediment incubations for measuring methane formation rates (MF), automatic bubble-traps and echo-sounding surveys to assess ebullition fluxes, and estimated diffusive methane fluxes via dissolved concentrations in the water and calculated transport coefficients via wind speed. MF was found to be predominantly acetoclastic, and higher porewater acetate concentrations were associated with higher MF. Moreover, sediment MF showed consistent depth profiles, and when depth-integrated, MF was comparable to bubble-trap ebullition time-series measurements. Thermal response analysed for our systems and a wide range of literature data demonstrated a consistent mean value, but a large range of temperature coefficient Q₁₀ (1.6 to 7.0) for different studies. Annual mean ebullition rates varied over more than one order of magnitude from site to site (0.03 to 1.85 mgCH₄ l^-1 d^-1), demonstrating that river impoundments are not all hot-spots. Future work should investigate the role of sediment delivery, deposition patterns and management on methane emissions by ebullition.