Ventilation of the Northern Baltic Sea

Abstract. The BalticSea is a semi-enclosed, brackish watersea in northern Europe. The deep basins of the central BalticSea regularly show hypoxic conditions. In contrast, the northern parts of the BalticSea, the Bothnian Sea and Bay, are well oxygenated. Lateral inflows or a ventilation due to convection are possible mechanisms for high oxygen concentrations in the deep water of the northern BalticSea. Owing to the high latitudes of the northern Baltic, this region is regularly covered by sea iceduring the winter season. In March 2017, the RV Maria S. Merian was for two days in the Bothnian Bay collecting ice coresamples, brinewater, and CTDprofiles. The bulk sea icesalinity was on average 0.6 g/kg and in brinesamples, a salinity of 11.5 g/kg and 17.8 g/kg have been measured. At one station, the CTDprofiles indicated a recent ventilation event of the deep water. A water massanalysis showed that the ventilation is most probably due to mixing of Bothnian Sea and Bothnian Bay surface water which results in sufficient dense water able to replace older bottom water. However, the high salinity of brineprovides the potential for forming dense bottom watermasses as well.