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The Baltic Sea is a young and fragile northern sea

The Baltic Sea is a unique inland sea, and there is no other like it. Its unique qualities – like its short history, shallowness and low salinity – make it more susceptible to negative change than the great oceans, for example.


When the continental glacier of the latest ice age began to melt, the great pressure it was imposing on the area began to lift as well. This made the bedrock underneath the glacier rise. During the next millennia, the sea level of the World Ocean went up and down, and as the bedrock in the Baltic Sea region was ‘breathing’, the meltwater’s route to the ocean was opened and closed as well. The salinity of the meltwater varied depending on how much saltwater was able to flow in from the ocean.

The glacial ice furrowed the ancient bedrock in the Baltic Sea area.
Eija Rantajärvi

Today, the water from the Baltic Sea comes into contact with the water from the Atlantic Ocean only in the narrow Danish straits. This causes the low salinity of the Baltic Sea, since there is comparatively little water transfer. The shallow Baltic Sea rests entirely on the Eurasian Plate and it has a very small amount of water when compared to the great oceans.

The Baltic Sea gets highly saline water only though the narrow Danish straits.
Danish straits on the left and the Baltic Sea on the right. Kuvat: NASA.
NASA

Seawater and freshwater species live side-by-side in this young sea, with even a few genuine brackish water species mixed in. Brackish water is not suitable for most lifeforms, and the Baltic Sea’s variety of species is rather restricted. The Baltic Sea freezes over at least partly during winters, which makes life in it even more challenging. These characteristics make the ecosystem of the Baltic Sea so much more vulnerable than the ecosystems of the great oceans.

On rare and random occasions, a strong pulse of saline water gushes in from the North Sea into the Baltic Sea. The saline pulse pushes hundreds of thousands of litres of fresh saltwater with plenty of oxygen from the North Sea. Water from an ocean is heavier than the low saline brackish water of the Baltic Sea, so the ocean water moves from one part of the Baltic Sea to the next along the seafloor.

The saline pulse increases the amount of oxygen especially in the deeper parts of the Baltic Proper’s seafloor. However, this reduces the amount of oxygen in the Gulf of Finland, as the old water from the Baltic Proper is pushed into the gulf.

The high ridge on the seafloor in the Sea of Åland very effectively prevents the water from the deep layer in the main basin from entering the Gulf of Bothnia. This is why the water in the Gulf of Bothnia only has weak stratification.

The saline water from the main basin can freely flow into the Gulf of Finland, and it regulates the variation in the gulf’s salinity stratification.

In the spring, the surface water becomes warmer and water stratifies according the temperature as well. The upper layer is warmer and lighter than the water in the deep layer. Strong layers which form according to water density, known as pycnoclines, make water transfer into the deep layer more difficult. This means that the deep layer is left without the oxygen-refill it needs from the oxygen-rich and light surface water.

Did you know?


  • Approximately one-fifth of the surface area of the Baltic Sea belongs to Finland.

  • The catchment area of the Gulf of Finland is around 14 times larger than the bay itself.

  • There are some 80,000 islands in the Finnish sea areas alone!

  • The fossils on the coast of Estonia and the Åland Islands originate from the tropical ancient sea that existed 450 million years ago. In those times, the area we know as the Baltic Sea was located near the equator!

Learn more about the topic

The Baltic Sea is geologically diverse(siirryt toiseen palveluun)
Younger seabed deposits(siirryt toiseen palveluun)
Characteristics of the seafloor in Finnish marine areas (siirryt toiseen palveluun)
Climate in the Baltic Sea Region(siirryt toiseen palveluun)
Ice has many forms in the Baltic Sea(siirryt toiseen palveluun)
Salinity, temperature and stratification(siirryt toiseen palveluun)
Species of the Baltic Sea(siirryt toiseen palveluun)
Alien species(siirryt toiseen palveluun)