Distribution of methane in the water column of the Baltic Sea

Abstract

[1] The distribution of dissolved methane in the water column of the Baltic Sea was extensively investigated. A strong correlation between the vertical density stratification, the distribution of oxygen, hydrogen sulfide, and methane has been identified. A widespread release of methane from the seafloor is indicated by increasing methane concentrations with water depth. The deep basins in the central Baltic Sea show the strongest methane enrichments in stagnant anoxic water bodies (max. 1086 nM and 504 nM, respectively), with a pronounced decrease towards the pelagic redoxcline and slightly elevated surface water concentrations (saturation values of 206% and 120%, respectively). In general the more limnic basins in the northern part of the Baltic are characterized by lower water column methane concentrations and surface water saturation values close to the atmospheric equilibrium (between 106% and 116%). In contrast, the shallow Western Baltic Sea is characterized by high saturation values up to 746%.

One year of continuous measurements constraining methane emissions from the Baltic Sea to the atmosphere using a ship of opportunity

“…As a consequence, a methane gradient towards the water surface is usually observed in the Baltic, with high concentrations in the deeper layers and low concentrations at the sea surface (Schmale et al, 2010). During the transport through the water column towards the sea surface, methane is substantially reduced by methane oxidation especially in the oxic/anoxic transition layer, as has been shown for the Black Sea (Schmale et al, 2011).…”

“…On the other hand, methane concentrations and carbon dioxide partial pressures do not have to be directly related. Though both properties generally increase underneath the mixed layer (Thomas and Schneider, 1999;Schmale et al, 2010), the pCO 2 increase, mostly resulting from organic matter degradation, will start immediately underneath the mixed layer, while stronger enhancement of methane is generally observed at greater depth. The oval shaped characteristic of the plot of methane concentration versus sea surface temperature and methane (Fig.…”

“…GB, BB, WoG, GoF), the deepening of the mixed layer leads to entrainment of methane-enriched deeper waters into the surface layer, increasing methane concentrations. Despite the deepening of the mixing depth in the Gotland Basin, the stratification of the water column sustains, and the deep water volume with high methane values as described in Schmale et al (2010) is not mixed to the surface. Cooling of the mixed layer partly dampens the effect of increasing concentrations on the oversaturation.…”

“…Usually, all parameters show elevated values for the same time and at the same locations for all recorded upwelling events. The findings reflect the transport of deeper water towards the surface, which are characterized by colder temperatures (Umlauf et al, 2010), enhanced pCO 2 from organic matter remineralization (Thomas and Schneider, 1999) and often enhanced methane concentrations (Schmale et al, 2010). Methane saturation values (Fig.…”

“…7, blue lines). In general, the stratification of the water column hampers the methane transport between deeper layers and the sea surface in the Baltic Sea (Schmale et al, 2010). The temperature-driven solubility decrease leads to an increase in methane saturation values in all regions of the Baltic Sea (Fig.…”