The amount of fresh water in the sea has decreased by almost 24 cubic kilometers per year in the last two decades. This is approximately the same size as Lake Peipsi.

"From the south, the Baltic Sea has become saltier, while from the north it has become more brackish. Due to differences in density, incoming saline water falls into the northern layers and freshwater carried by rivers remains in the surface layers. The current stratification will therefore become more pronounced, and oxygen will no longer reach the bottom layers as much," explained TalTech's Professor Urmas Raudsepp, the study's co-author.

This can lead to the expansion of so-called dead zones, where there is too little enough oxygen for most organisms to live. Raudsepp said further research can show how strong the effect of the increased salinity is.

The changes have not occurred uniformly across the Baltic Sea. For example, in the Gulf of Bothnia, the water is fresher due to increased rainfall and water flowing from rivers into the sea. Additionally, due to its location, less salt water reaches the gulf.

At the same time, seasonal freshwater changes in the Gulf of Bothnia and Finland are suppressed by the reduction of the ice cover. 

"Sea areas of importance for Estonia, such as the Gulf of Finland and the Gulf of Riga, are relatively stable compared to the overall changes in the Baltic Sea," the professor said.

In the south, you can see the effect of salt water reaching the Baltic Sea through the Danish strait. 

"We don't know why this salty water from the Danish strait is pressing in like this. One hypothesis is that global warming is causing ocean levels to rise, which could lead to more salt water. But this is the kind of question that anyone who can give a clear answer can write a good scientific article about themselves," said Raudsepp

"As a whole, with the changes seen, freshwater and the place of stratification are moving northwards," he added.

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