Geobiology of the stratified central Baltic Sea water column

The central Baltic Sea water column is permanently density stratified leading to stagnating bottom waters and a zonation into an oxic, suboxic and anoxic zone. The anoxic zone of the central Baltic Sea is characterized by strong methane gradients that indicate methane consumption in the suboxic zone. This thesis comprises five papers that are based on investigations of water column and sediment samples from the central Baltic Sea. The aim of these investigations was to assess the geobiology of this stratified system using biomarkers with a focus on bacteriohopanepolyols (BHPs). First, three different extraction methods were compared with respect to BHP yields. Microwave, ultrasound, and Bligh & Dyer extraction revealed similar results for total extracted BHPs and studies were continued using microwave extraction. In the Landsort Deep, biomarkers were used to assess the geobiology in the whole water column. They mirrored the water column stratification and revealed different microbial communities. An initial multidisciplinary study including biomarker analysis revealed the presence of type I aerobic methanotrophic bacteria in the suboxic zone of the Gotland Deep. A following detailed investigation of Gotland Deep biomarkers in the oxic and suboxic zone and the underlying sediment showed high BHP concentrations in the suboxic zone and suggested that this zone is an important source layer for these compounds. The BHP signal found in the suboxic zone is also mirrored in the surface sediments. Subsequently, the Holocene geological record of BHPs was analyzed in a Gotland Deep sediment core covering all stages of the Baltic Sea development. BHPs reflected the onset of the stratification during Littorina stage and are of particular abundance during periods of pronounced anoxia. The studies presented in this thesis demonstrate biomarkers to be a versatile tool to assess microbial communities in present and past stratified environments and contribute to the general knowledge on the geobiology of stratified water columns