Extreme Methane Emissions from a Swiss Hydropower Reservoir: Contribution from Bubbling Sediments

Methane emission pathways and their importance were quantified during a yearlong survey of a temperate hydropower reservoir. Measurements using gas traps indicated very high ebullition rates, but due to the stochastic nature of ebullition a mass balance approach was crucial to deduce system-wide methanesourcesandlosses.Methanediffusion from the sediment was generally low and seasonally stable and did not account for the high concentration of dissolved methane measured in the reservoir discharge. A strong positive correlation between
water temperature and the observed dissolved methane concentration enabled us to quantify the dissolved methane addition from bubble dissolution using a system-wide mass balance.Finally,knowingthecontributionduetobubbledissolution, we used a bubble model to estimate bubble emission directly to the atmosphere. Our results indicated that the total methane emission from Lake Wohlen was on average >150 mg CH4 m-2 d-1, which is the highest ever documented for a midlatitude reservoir. The substantial temperature-dependent methane emissions discovered in this90-year-old reservoir indicate that temperate water bodies can be an important but overlooked methane source.