A study on the drivers of methane emissions in a eutrophic lagoon in the Baltic Sea (Darß-Zingst-Bodden chain)

Coastal shallow water areas are important carbon dioxide sinks, but their sink strength is significantly reduced by the simultaneous emission of other greenhouse gases such as methane (CH₄). These areas are often characterized by strong anthropogenic pressure from adjacent agricultural land use, which leads to increased nutrient input, high biological production, oxygen consumption through remineralization of the organic material produced, and ultimately to increased greenhouse gas production. Despite their outstanding importance for marine greenhouse gas emissions, these areas have been little studied to date and the drivers of the spatial and temporal variability of greenhouse gas distribution are poorly understood. To address this problem, we study a lagoon on the German Baltic Sea coast (Darß-Zingst Bodden chain) using a multidisciplinary approach that combines gas chemical and observational oceanographic methods with modeling. Our investigations in the summer of 2024 and 2025 show that the spatial and temporal variability of CH₄ concentration in the water and emissions into the atmosphere are primarily caused by wind-driven oceanographic processes, such as water mass transport and mixing. Notably high CH₄ concentrations were recorded primarily in protected reed belts and adjacent drainage ditches, indicating the particular importance of these areas as CH₄ sources. The high-frequency measurements of CH₄ concentrations (Equilibrator-CRDS) provided evidence that changes in water level and the associated pressure change on the sediment have an impact on the CH₄ concentration in the water column. Measurements at the water surface with a floating chamber and an eddy covariance flux tower have shown that gas bubble fluxes play a significant role in atmospheric CH₄ fluxes and that the intensity of gas bubble release is influenced by water level fluctuations. Our study thus provides a rare CH₄ data set from shallow water areas of the German coast and, through its high-frequency data acquisition, reveals the highly dynamic variability of CH₄ concentration development and underscores the importance of oceanographic processes in this context.