The brackish nature of the Baltic Sea

Salinity, temperature and oxygen are physical background parameters, constraining biodiversity, fish recruitment and water quality in a semi-enclosed water body as the Baltic Sea. For example, cod larvae are dependent on water with salinity and oxygen levels above 11 psu and 1 ml/l, respectively. It is only since the most recent inflow in 2003 that they may survive east of the western Southern Baltic Proper.

Low salinity

The brackish water of the Baltic Sea is a mixture of sea water from the North Sea and fresh water from rivers and rainfall. The salinity of its surface waters varies from around 20 PSU (parts per thousand) in the Kattegat to 1–2 PSU in the northernmost Bothnian Bay and the easternmost Gulf of Finland, compared to 35 PSU in the open oceans. Baltic surface waters are strongly influenced by land run-off of freshwater. Changes in run-off alter the surface salinity while inflows through Öresund and the Belt Sea control the salinity of the deeper waters.

Salinity levels also vary with depth, increasing from the surface down to the sea floor. Saltier water flowing in through the Sound and the Belt Sea does not mix easily with the less dense water already in the Baltic, and tends to sink down into deeper basins. At the same time, the less saline surface water flows out of the Baltic. The boundary between these two water masses, known as the halocline, consists of a layer of water where salinity levels change rapidly. In the Baltic Proper and Gulf of Finland, for instance, the halocline lies at a depth of around 60–80 m. Like a lid, the halocline limits the vertical mixing of water. This means that the oxygen content of the deep basins consists of a layer of water where salinity levels change rapidly. In the Baltic Proper and Gulf of Finland, for instance, the halocline lies at a depth of around 60–80 m. Like a lid, the halocline limits the vertical mixing of water. This means that the oxygen content of the deep basins of the Baltic Proper is mainly  replenished by oxygen-rich saltwater flowing in from the North Sea along the sea floor. In the Gulf of Bothnia, the halocline is very weak or absent.