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Baltic Sea

The salinity of the Baltic Sea water is only 2-20‰ depending on the region and depth - about one fifth of the salinity of ocean waters (35‰).
The average depth of the Baltic Sea is only 55 meters - in the Mediterranean the average depth is 1500 meters.
It takes 30 years for the total water mass of the Baltic Sea to exchange.

A unique environment

The Baltic Sea is a small and shallow sea, but approximately 85 million people live in its exceptionally large catchment area and use the sea for various purposes. Unique feature of the Baltic Sea is its brackish water – a mixture of fresh water and saline seawater. Heavier saline water forms the deeper water layer in the Baltic Sea, whereas the surface water layer is diluted by rain and numerous rivers flowing into the sea. These two layers of water mix poorly with each other. The oxygen in the deepest bottom areas is replenished only from time to time, during heavy storms and when saline, oxygen-rich pulses of water flow to the Baltic from the North Sea through Skagerrak and Kattegat. These shallow and narrow Danish straits are the only connection between the Baltic Sea and the ocean, which means that the water in the Baltic exchanges slowly and harmful substances remain in the sea for decades.

Brackish water and cold wintertime create a challenging environment for organisms in the Baltic Sea. The number of flora and fauna, adapted to life in the brackish water, is small; however there may be large quantities of individual species. The food chains in the Baltic Sea are simple. Many species in the Baltic Sea live on the extreme limits of their adaptability and the flora and fauna of the Baltic Sea are very sensitive to changes in the environment. The International Maritime Organisation (IMO) designated the Baltic Sea as a Particularly Sensitive Sea Area in 2004.