Abstract
Abstract. The water column of the Landsort Deep, central Baltic Sea, is stratified into an oxic, suboxic, and anoxic zone. This stratification controls the distributions of individual microbial communities and biogeochemical processes. In summer 2011, particulate organic matter was filtered from these zones using an in situ pump. Lipid biomarkers were extracted from the filters to establish water-column profiles of individual hydrocarbons, alcohols, phospholipid fatty acids, and bacteriohopanepolyols (BHPs). As a reference, a cyanobacterial bloom sampled in summer 2012 in the central Baltic Sea Gotland Deep was analyzed for BHPs. The biomarker data from the surface layer of the oxic zone showed major inputs from cyanobacteria, dinoflagellates, and ciliates, while the underlying cold winter water layer was characterized by a low diversity and abundance of organisms, with copepods as a major group. The suboxic zone supported bacterivorous ciliates, type I aerobic methanotrophic bacteria, sulfate-reducing bacteria, and, most likely, methanogenic archaea. In the anoxic zone, sulfate reducers and archaea were the dominating microorganisms as indicated by the presence of distinctive branched fatty acids: archaeol and pentamethylicosane (PMI) derivatives, respectively. Our study of in situ biomarkers in the Landsort Deep thus provided an integrated insight into the distribution of relevant compounds and describes useful tracers to reconstruct stratified water columns in the geological record.
Highlights
The Baltic Sea is a brackish marine marginal sea with a maximum depth of 459 m in the Landsort Deep
The Landsort Deep in the western central Baltic Sea is characterized by a stratified water column
7-methylheptadecane, different alkenes, BHT cyclitol ether, and BHT glucosamine were indicative of the presence of bacterial primary producers, namely cyanobacteria
Summary
The Baltic Sea is a brackish marine marginal sea with a maximum depth of 459 m in the Landsort Deep (western central Baltic Sea; Matthäus and Schinke, 1999; Reissmann et al, 2009; Fig. 1). A positive freshwater budget and saltwater inflows from the North Sea through Skagerrak and Kattegat lead to a permanent halocline that stratifies the water column of the central Baltic Sea at about 60 m water depth (Reissmann et al, 2009). As detected in 1993 and 2003, sporadically disturb the stratification in the eastern central Baltic Sea and oxygenate the suboxic zone and deep water. These inflows, rarely reach the western central Baltic Sea. Even the strong inflow from 1993 had only minor effects on Landsort Deep, where stagnating conditions prevailed throughout (Bergström and Matthäus, 1996). The Landsort Deep offers stable environments for microbial life within the oxic, suboxic, and anoxic zones and provides an excellent study site for the investigation of biomarker inventories that specify stratified water columns
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