Seasonal and spatial methane dynamics in the water column of the central Baltic Sea (Gotland Sea)

•The methane inventory of the eastern Gotland Basin showed a seasonal pattern.•The western Gotland Basin is mainly influenced by lateral weak intrusions.•The varying transport rate of methane caused an adapting methanotrophic community.•Methane production in oxygenated waters was identified in the central Baltic Sea.

The influence of hydrodynamic events on the distribution of methane and its microbial turnover was investigated during the period from August 2011 to August 2013 along a transect from the eastern (EGB) to the western Gotland Basin (WGB), central Baltic Sea. The water column was characterized by a pronounced methane concentration gradient between the methane-rich deep anoxic and the methane-poor upper oxic water layer. In both basins, enhanced vertical turbulent diffusivities in fall (November 2011) and winter (February 2012) lead to an enhanced flux of methane from the deep anoxic water towards the oxic–anoxic transition zone (redox zone). In both basins, the increased vertical transport of methane in fall/winter was mirrored by reduced methane turnover times measured within the redox zone. Moreover, specific biomarkers indicative for aerobic methanotrophic bacteria implied an increase in the microbial population size from August 2011 till February 2012, indicating a methanotrophic community adapting to the variable methane fluxes. The deep water methane inventory of the EGB showed a seasonal pattern, with concentrations increasing during spring (May) and summer (August) and decreasing during fall (November) and winter (February) as a direct result of the seasonality of the vertical turbulent diffusivity. In contrast, the WGB showed no clear correlation between the seasons and the observed deep water methane variability. Here, the impact of lateral weak intrusions penetrating the deep water layer was identified as the main factor controlling the variability of the deep water methane concentration.Moreover, methane concentration and carbon stable isotopic data (δ13C CH4) demonstrate that the previously reported production of methane in the oxic water column below the thermocline occurs in the entire central Baltic Sea from May through November, and despite the large methane pool in the underlying anoxic deep water, might govern the moderate methane flux to the atmosphere in this area in summer.