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Abstract

The Baltic Sea is a semi-enclosed brackish sea located in Northern Europe. Due to some natural features (limited exchange with the World Ocean and permanent halocline) and anthropogenic forcing (nutrient loads from agriculture), the Baltic Sea experiences accelerating hypoxia and anoxia (bottom O2 concentrations less than 2 and 0 ml/l, respectively). Apart from the vertical oxygen fluxes across the pycnocline, the only natural ventilation mechanism in the Baltic Sea are the saltwater inflows from the North Sea transporting salty and oxygen-reach water to the central Baltic Sea. In this study, we utilized a 3-dimensional coupled ocean model to investigate the 29 biggest oxygen inflows within the 4 basins of the central Baltic Sea (the Bornholm Basin, the eastern Gotland Basin, the northern Gotland Basin, and the western Gotland Basin) happening from 1948 to 2018. We found a significant decline in inflows’ lifetimes for all basins, especially for the remote northern and western Gotland Basins, where oxygen supply via the inflows has been reducing since the 1970s and almost stopped during the last decade. Since no trend in the inflows’ strength was found, the overall decline in ventilation was attributed to the elevated oxygen consumption in the region, which was observed in the model. It was found that ventilation by inflows triggers the same processes consuming oxygen regardless of the system’s trophic state, mainly the mineralization of particulate organic matter (specifically detritus) in the sediments.