In recent decades rising temperatures have led to the continuous global expansion of severely reduced oxygen levels and hypoxia (<63 µM) in aquatic habitats. These effects predominantly result from increasing nutrient inputs and reduced oxygen solubility due to increasing water temperatures. Microbially driven organic matter degradation represents the main driver of oxygen consumption in seafloor habitats. In oxygen-depleted sediments sulfate reduction and methanogenesis are two of the most important metabolic pathways, which result in the formation of toxic hydrogen sulfide and the greenhouse gas methane. These compounds may be released from the sediments, posing a threat to marine life, affecting global warming and resulting in potentially severe socio-economic impacts.

In order to gain a deeper understanding of the mechanisms involved in the expansion of hypoxic and sulfidic zones in coastal regions of the Baltic Sea, we analyzed geochemical parameters and microbial communities of sediments underneath a seasonally hypoxic water column at the time series station Boknis Eck (Eckernförde Bay). First results of the porewater geochemistry, modeled turnover rates and 16S rRNA gene assessments suggest an intensification and upwards shift of sulfate reduction and methanogenesis in warmer periods and after hypoxic events compared to the winter period. This trend is also reflected in the respective microbial community compositions, especially the abundances of typical sulfate reducers, sulfide oxidizers and methanogens. In comparison to prior campaigns over the last 15 years, our data also indicate an overall increase of reducing processes in sediments of the Eckernförde Bay. Our findings will be further evaluated during future sampling campaigns to Boknis Eck and other areas in the southwestern Baltic Sea. Our ultimate goal is to develop a sediment biogeochemical model, which can be used to spatially predict the onset of anoxic-sulfidic bottom waters and the associated risk to ecosystems and local stakeholders.