Bacterial Activity and Organic Matter Turnover in Oxygen Deficient Waters of the Baltic Sea

The Baltic Sea is an enclosed marine system that suffers from expanding zones of oxygen deficiency due to limited ventilation by the episodic inflow of oxygenated North Sea water combined with high anthropogenic nutrient loads. In particular coastal areas are strongly influenced by eutrophication that leads to enhanced microbial oxygen consumption and sporadic anoxia even at shallow sites. It has been shown that oxygen availability is a powerful determinant of the taxonomic composition of prokaryotic communities in deep waters of the Baltic Sea. However, it remains unclear if community changes in response to low oxygen impact carbon remineralization or if functional redundancy prevents effects on major biogeochemical processes driven by bacterial activity. Our study includes monthly samplings at a coastal time series station over three annual cycles with recurring anoxic periods in late summer. Furthermore, sampling was accomplished in the deep Gotland Basin, where a permanent pycnocline prevents vertical mixing. We determined rates of extracellular glucosidase, aminopeptidase and phosphatase, as well as bacterial protein production using fluorescent and radioactive labelled substrate analogues, respectively. The rate measurements were combined with the analysis of organic matter concentration and composition by different analytical tools. Field data and experimental work show that enzymatic polymer hydrolysis, bacterial biomass production and growth rates in oxygen deficient waters of the Baltic Sea are not inherently lower than in oxic waters. Instead, results reveal that the reactivity of organic carbon and the availability of inorganic nutrients are more powerful constraints on organic matter turnover in oxygen deficient zones of the Baltic Sea. Our results imply that oxygen availability alone is not the decisive factor for heterotrophic bacterial activity in deep waters, instead it is part of a multiple environmental control of carbon remineralization.