Why does the Baltic Sea have much less salt than most oceans?

The Baltic Sea is a semi-enclosed basin in northern Europe, so fresh water from rivers and rainfall dilutes the salt water inside it. Its connection to the Atlantic Ocean is through the Danish straits, which restrict the exchange of water. As a result, the Baltic is brackish, meaning it is saltier than freshwater but much less salty than the open ocean. Salinity varies a lot from one part of the sea to another, with the north and east being especially fresh. This unusual chemistry influences the region’s marine life, favoring species that can tolerate changing salinity.