The Western Gotland Basin is located west of the Gotland island. In the west, it reaches mainland Sweden, and in the south the island of Öland. Between Lithuania and Sweden, within the Western Gotland Basin, lies Landsort deep, which at 459 m is the deepest point of the Baltic Sea.
The water in the depths is heavy and salty, and, because of the permanent stratification of waters in the Baltic Sea, it does not get mixed much. The narrow and shallow straits of Denmark are the only marine connection from the Baltic to the North Sea, and water inflows through them are also very slow. It has been calculated that it takes roughly 30 years for the entire water mass of the Baltic Sea to change. As a result of eutrophication, the deeps of the Baltic Sea suffer from chronic oxygen depletion, and there are vast areas of anoxic, dead seabed.
The oxygen status of the deeps of the Baltic Sea improves from time to time when large volumes of salty, oxygen-rich ocean water flow through the straits. These saline pulses require particular weather conditions, and do actually happen only approximately once a decade.