Methane oxidation along oxygen gradients in the Baltic Sea

Eutrophication and anoxic and hypoxic conditions can drive substantial CH₄ production in sediment and potentially in the water column. However, the extent of CH₄ oxidation in the water column of marginal seas remains poorly quantified, leading to a possible overestimation of CH₄ fluxes to the atmosphere. Here, we investigate the fate of CH₄ in the deep-water column analysing its concentration and stable isotope (δ¹³C-CH₄) along a 5000-km cruise track in the Baltic Sea. CH₄ concentrations increased with water column depth, more so under low oxygen conditions. The median CH₄ concentration in the bottom layer in different basins ranging from 4 to 1300 nM. δ¹³C-CH₄ values ranging from -82 to -46‰ with respect to VPDB indicates benthic CH₄ production. Methane oxidation causes isotopic fractionation, resulting in a more ¹³C-enriched CH₄. Here oxidation in the water column removed 1% to 90% of benthic-produced CH₄ before it reaches the surface. Large differences in CH₄ concentrations and δ¹³C-CH₄ were observed between basins related to oxygen concentrations, reflecting distinct biogeochemical dynamics. For instance, benthic CH₄ concentrations in the anoxic, deep Baltic Proper were 2 to 295 times higher than those in the oxygenated, shallower Gulf of Bothnia. Our results underline the importance of CH₄ oxidation in the water column, mitigating CH₄ emissions to the atmosphere. Accurate regional CH₄ budgets should consider oxidation processes and the unique characteristics of different basins.