Methane dynamics in sediments of the eutrophic Stockholm Archipelago, Baltic Sea

Eutrophication of coastal systems stimulates the anaerobic degradation of organic matter in sediments, and thereby the production of the greenhouse gas methane. At present, the pathways and rates of removal of methane in coastal sediments are still incompletely understood. Here, we present pore water and sediment data from ten sites in the eutrophic Stockholm Archipelago capturing a range of bottom water redox conditions, from oxic to (seasonally) hypoxic (oxygen < 63 uM) and euxinic (no oxygen while hydrogen sulfide is present > 0 uM). Our data indicate high rates of methane production at all sites, with methane concentrations ranging up to 8 mM. The depth of the sulfate methane transition zone (SMTZ) is located between ~5 and 30 cm below the sediment-water interface at all sites, which is quite shallow. We will discuss the (potential) pathways of methane removal in the sediments at these sites and the possibility for escape of methane from the sediment to the overlying water.