• The Baltic Sea is unique in terms of its location, salt concentration, islands and living organisms.
• The water type of the Baltic Sea is called brackish water. It is a mixture of salt water and fresh water. This water type accommodates a unique mixture of saltwater and freshwater species.
• The most important threats to the Baltic Sea's ecosystem include pollution and nutrient-caused eutrophication, extensive fishing, lack of oxygen and frequent ship and boat traffic. 
• Key words: the Baltic Sea, the Gulf of Finland, the Gulf of Bothnia, salt concentration, brackish water, eutrophication.