The varying salt concentration of different parts of the Baltic Sea results in different species favouring different areas. In the southern parts of the Baltic, where the salt concentration is as high as one percent, the species are mostly the same as in the North Sea. In these parts, ocean-dwelling species such as starfish and crabs can be found. In the Gulf of Finland and the Gulf of Bothnia the salt concentration is under 0,5 %. These gulf areas are the home of lake-dwelling species that have adapted to the brackish water of the Baltic. 

The conditions of the Baltic Sea are challenging for living organisms. For example, most fish species of the Baltic are not adapted to high concentrations of salt, which means that they are often in the danger of acquiring too much salt in their systems. The long winter and the frozen gulfs also make life in the Baltic Sea quite challenging. 

The Baltic Sea freezes over partially during the winter.