Seasonal and spatial dynamics of carbon dioxide and methane fluxes from a reservoir in Western Germany

Dams play a critical role in water management and hydropower across Europe, yet the greenhouse gas (GHG) emissions from these reservoirs remain largely understudied, particularly in Germany. While hydropower is often considered a clean energy source, reservoirs can play a significant role in global carbon cycling and the global GHG budget, acting as both sources and sinks of carbon dioxide (CO₂) and methane (CH₄), depending on their ecological and management conditions. In this study, we measured CO₂ and CH₄ fluxes from the Rur Reservoir, located in the Eifel district of western Germany, in a catchment area characterized by a mix of agricultural and forested land. Land use practices in the area influence water quality, nutrient levels, and greenhouse gas emissions, particularly through organic matter input. Using floating chambers, we conducted weekly measurements during the summer and autumn and biweekly measurements in winter, starting in August 2024.

CO₂ fluxes displayed a distinct seasonal pattern, with the reservoir acting as a sink during summer (negative fluxes) before transitioning to a source in autumn, peaking mid-fall (~2600 mg CO₂ m⁻² d⁻¹). Fluxes declined to near-zero levels in early winter, with a transient spike (~6000 mg CO₂ m⁻² d⁻¹) in late November. Throughout the study period, the reservoir consistently emitted methane (CH₄), with fluxes ranging from 1 to 3 mg CH₄ m⁻² d⁻¹ with a slight peak observed mid-fall. CH₄ emissions were significantly higher in the pelagic zone compared to the dam and shoreline areas, whereas CO₂ fluxes showed no discernible spatial pattern across these zones. CO₂ fluxes were strongly correlated with water temperature (r =-0.82, p < 0.01), highlighting the sensitivity of emissions to thermal conditions.

These findings underscore the dynamic nature of GHG emissions in reservoirs and emphasize the importance of considering both seasonal and spatial variability to accurately quantify their contribution to regional and global carbon budgets.