About the Baltic Sea

A Mixture of Fresh and Salt Water

Today, very little saltwater enters the Baltic Sea from the North Sea and the Atlantic. Freshwater flows in from the land, creating the Baltic Sea’s brackish environment—a mixture of fresh and salt water. Since all animals originally come from either saltwater or freshwater, they have had to adapt over thousands of years to survive in the Baltic Sea. The brackish water is challenging for animals to live in, which means they have less energy left for growth. As a result, animals in the Baltic Sea are usually smaller than their relatives in either truly salty or truly fresh water.

The salinity is not the same everywhere. In the far south, where the Baltic Sea connects to the Atlantic, the water is saltier than up north in the Bothnian Sea. This means there are more marine species in the southern parts of the Baltic Sea. In addition to varying geographically, salinity also changes with depth. Saltwater is denser than freshwater, which makes it heavier, so it sinks to the bottom. Freshwater floats on top of the saltwater, and the boundary between them is called the halocline. In the Baltic Sea, the halocline is very strong, which means there is little mixing between the oxygen-rich surface water and the oxygen-poor bottom water. Combined with eutrophication, this has led to oxygen-free bottoms in many parts of the Baltic, where almost no animals can live.

The Baltic Sea and Humans

The land area from which freshwater flows into the Baltic Sea is called the catchment area. The Baltic Sea’s catchment area is more than four times larger than the surface area of the sea itself, and includes 14 countries. Water that falls as rain anywhere in the catchment area will eventually flow into the Baltic Sea. This means that even people who do not live right on the coast still have an impact on what ends up in the sea. About 90 million people live in the Baltic Sea catchment area.

Because of the Baltic Sea’s peculiar shape, with many surrounding land areas and only a very small exchange of water with the ocean outside, the Baltic is often compared to a bathtub. You could say that everything that ends up in the Baltic Sea stays there for a very long time. It actually takes almost 30 years, or even longer, for all the water in the Baltic Sea to circulate and be replaced before it flows out to the West Coast again.

Throughout history, humans have used the Baltic Sea—for recreation, for food in the form of fish, for waging war, as a recipient of waste we do not want on land, and for transporting ourselves and goods. On the sea floor there are many traces of human use—old munitions, rubbish, chemicals, radioactive fallout from Chernobyl, ghost nets, and countless wrecks.

The Baltic Sea is therefore heavily impacted by humans, and the brackish water is already a challenging environment for plants and animals. But contrary to what many believe, the Baltic Sea is neither dead nor dying. There is still much beauty left to care for and a unique mix of species! This also means we need to think carefully about how we treat the Baltic Sea if we want to be able to enjoy this wonderful sea in the future.