Methanogenic Rates in Sediments Explain Long-Term Trends on Spatial and Seasonal Variation of CH4 Cycling in the Littoral Zone of Lake Constance

Lakes are significant sources of the greenhouse gas methane (CH₄) to the atmosphere, with littoral zones recognized as hotspots for CH₄ emissions. However, the specific pathways driving the enrichment of CH₄ in the littoral zones of lakes remain poorly understood, contributing to uncertainties in global lake CH₄ budgets. To address this gap, our study investigates the role of littoral sediments as the main source of dissolved CH₄ in lakes and the drivers behind the high yet variable CH₄ concentrations in the water column along the different seasons. We specifically focus on the impact of temperature fluctuations on methanogenic rates in sediments over different seasons.

The CH₄ dynamics in lake sediments exhibit significant spatial variability driven by differences in sediment properties. In this study, we aimed to explain the seasonal variability of dissolved CH₄ concentrations in the water column by linking them to sediment fluxes, which are driven by methanogenic activity within the sediments. To achieve this, we conducted seasonal field campaigns in Lake Constance, Germany, during 2024 and complemented them with data obtained ten years ago. We measured sediment-to-water CH₄ fluxes, dissolved CH₄ concentrations in water and sediments, and potential methanogenesis rates under varying temperature conditions in sediment cores from diverse littoral sites. Stable isotope analysis of CH₄ and CO₂ provided further insights into the origin and fate of methane from the sediments into the water column.

Our findings reveal that CH₄ cycling varies spatially across sites and among seasons, as expected, yet long-term trends remain relatively stable over several years despite short-term seasonal fluctuations. Methanogenic rates represent a good proxy for explaining the spatial heterogeneity of CH₄ dynamics in the littoral zone and remain as the main source of littoral dissolved CH₄. These results highlight the importance of site-specific and seasonal variations in regulating CH₄ cycling in Lake Constance, providing valuable insights into the drivers of CH₄ dynamics in lake littoral zones.