Methane emission dynamics of a shallow dutch peat lake: Insights from long-term eddy covariance monitoring

Freshwater lakes, despite covering only ~ 3% of Earth’s surface, are among the largest natural sources of methane to the atmosphere. Current global estimates of lake methane emissions, however, remain uncertain and likely underestimated due to the scarcity of long-term and high resolution datasets and the spatial and temporal complexity of freshwater lakes. Methane fluxes from lakes are influenced by many factors such as lake depth, organic matter input, seasonal biogeochemical dynamics and vegetation composition, making upscaling from individual systems challenging. In this study, we investigate the temporal dynamics of methane emissions from a shallow peat lake in the Netherlands. The lake is characterized by rich submerged vegetation dominated by two species: Potamogeton perfoliatus and Nitellopsis obtusa. Continuous eddy covariance (EC) measurements of methane fluxes, collected since December 2021, were combined with various water quality data to assess seasonal and interannual variabilities. Our results show a clear seasonal pattern, with substantially higher methane emissions during summer months (average of 165 mg m^-2 d^-1), compared to autumn, winter and spring (average of 50, 15 and 68 mg m^-2 d^-1, respectively). Through strong positive correlations, both water and air temperature were identified as the main drives of methane emissions, with the redox potential at the lake water-sediments interface also showing strong negative correlation. Interestingly, two distinct emission peaks were observed each early summer and again in late summer to early autumn. These peaks are likely linked to the macrophyte life cycle, with an early-season peak preceding full plant development, a mid-summer decrease possibly associated with oxygen input from the vegetation, and a late-season increase due to plant decomposition. These findings highlight the role of aquatic vegetation in methane release from shallow peat lakes. Better quantifying these temporal drivers is important to improve regional and global methane budgets.