Ebullition dominates methane emissions in stratified coastal waters

Coastal areas are an important source of methane (CH₄). However, the exact origins of CH₄ in the surface waters of coastal regions, which in turn drive sea-air emissions, remain uncertain. To gain a comprehensive understanding of the current and future climate change feedbacks, it is crucial to identify these CH₄ sources and processes that regulate its formation and oxidation. This study investigated coastal CH₄ dynamics by comparing water column data from six stations located in the brackish Tvärminne Archipelago, Baltic Sea. The sediment biogeochemistry and microbiology were further investigated at two stations (i.e., nearshore and offshore). These stations differed in terms of stratification, bottom water redox conditions, and organic matter loading. At the nearshore station, CH₄ diffusion from the sediment into the water column was negligible, because nearly all CH₄ was oxidized within the upper sediment column before reaching the sediment surface. On the other hand, at the offshore station, there was significant benthic diffusion of CH₄, albeit the majority underwent oxidation before reaching the sediment-water interface, due to shoaling of the sulfate methane transition zone (SMTZ). The potential contribution of CH₄ production in the water column was evaluated and was found to be negligible. After examining the isotopic signatures of δ¹³C-CH₄ across the sediment and water column, it became apparent that the surface water δ¹³C-CH₄ values observed in areas with thermal stratification could not be explained by diffusion, advective fluxes, nor production in the water column. In fact, these values bore a remarkable resemblance to those detected below the SMTZ. This supports the hypothesis that the source of CH₄ in surface waters is more likely to originate from ebullition than diffusion in stratified brackish coastal systems.