Unique Baltic Sea
The water in the Baltic Sea is brackish with a low salinity - a unique mixture of salty and fresh waters. The salinity of the Baltic Sea water is only approximately 20% of the salinity of the oceans (35 per mille). Furthermore, the salinity of the Baltic Sea's surface water reduces towards the North, and the water in the Bay of Bothnia and at the bottom of the Gulf of Finland is almost fresh.
Compared to the oceans, the Baltic Sea is a small and shallow body of water. The average depth of the Baltic Sea is only 55 metres, whereas the average depth of the oceans totals several kilometres, and even the Mediterranean's average depth is one kilometre. The deepest basin in the Baltic Sea is 450 metres deep. The Baltic Sea is linked to the North Sea via the narrow Danish straits, and the turnover of the water is very slow. Theoretically, it has been estimated that the turnover of the entire volume of water of the Baltic Sea takes approximately 30 years. Due to the slow water turnover, environmental toxins and eutrophying nutrients remain in the Baltic Sea and cause long-term effects.