Dissolved methane during hypoxic events at the Boknis Eck time series station (Eckernförde Bay, SW Baltic Sea)

Abstract

Dissolved CH₄ was measured in the water column at the Boknis Eck (BE) time series station in the Eckernförde Bay (SW Baltic Sea) on a monthly basis from June 2006 to November 2008. The water column at BE was always supersaturated with CH₄ and, therefore, CH₄ was released to the atmosphere throughout the sampling period: the mean CH₄ surface (1 m) saturation at BE was 554±317%. A pulse of enhanced CH₄ emissions occurs when the CH₄ accumulation in the hypoxic bottom layer during summer is terminated in late summer/autumn. We did not detect a straightforward relationship between periods of enhanced CH₄ in the bottom layer and hypoxic events at BE: the sedimentary release of CH₄ seemed to be mainly triggered by sedimenting organic material from phytoplankton blooms. We conclude that future CH₄ emissions from BE will be determined by the intensity of phytoplankton blooms, which in turn will be influenced by eutrophication. However, hypoxic events seem to have only a modulating effect on the enhancement of sedimentary methanogenesis and the subsequent release of CH₄ to the water column.