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.

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.

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.

The basin of the Baltic Sea is like a very large dent on the Eurasian Plate. The dent was put there by the rocky ice blanket, as much as three kilometres thick in some places, that the latest ice age covered the Baltic Sea with. 

Around 13,000 years ago, the climate started getting warmer again and the meltwater from the glacier formed a little lake on the edge of the glacier. The lake’s salinity fluctuated depending on how much saltwater was able to flow into the area from the ocean next door.

The brackish water we all know has been lapping against the shores for only a few thousand years. That is a very short time from an ocean’s point of view.

As the climate warmed, a freshwater lake was formed from the meltwater right on the southern edge of the glacier 11 500 vuotta sitten.

The catchment area is four times larger than the Baltic Sea itself, so freshwater is continuously flowing into the small sea basin. The Baltic Sea is surrounded by eight EU Member States and Russia. In addition to these nine states, its catchment area comprises parts of five other countries as well.

Nearly 90 million people live in the catchment area, farming land, managing forests, running factories and just generally being busy. All of this puts a serious strain on the Baltic Sea.

Areas of the Baltic Sea. Photo from Itämeren fysiikka, tila ja tulevaisuus. Myrberg, Leppäranta, Kuosa. 2006.

Kuva 1_5 Alt-teksti: Map. Sea areas: The Bothnian Bay, the Kvarken, the Bothnian Sea, the Archipelago Sea, the Gulf of Finland, the northern parts of the Baltic Proper. West and South Finland have the largest number of fields; the many lakes and rivers in these areas affect the transportation of nutrients from the catchment area to the sea.

The shapes and substrate of the Baltic Sea’s seafloor are everything but dull. The underwater world has a big selection of hills, ridges, plains and deeps.

Where the seafloor is hard, it is covered either by boulders, rocks or gravel. Soft areas of the seafloor floor are in turn covered with sand, clay or mud. In certain places, the seafloor is formed of bedrock that can be up to two million years old. In the softer parts, the seafloor can be formed from substrate that has sunk to the bottom very recently.

If you sunk the tower of the Helsinki Olympic Stadion in a pool that is 54 metres deep – the average depth of the Baltic Sea – you would still have 18 metres of tower over the surface. The deepest point of Finland’s sea areas is in the Bothnian Sea. It is 293 metres deep, which means you could sink the Eiffel Tower in it.

The deepest point of the whole Baltic Sea is 459 metres deep. It is located in the Landsort Deep, in the Swedish sea area between Stockholm and Gotland. It is deep enough that you could sink the Eiffel Tower and stack two of the Helsinki Olympic Stadium’s towers on top of it. That’s pretty deep! However, the average depth of the great oceans – a whopping 4,000 metres – would hold a bit over 13 Eiffel Towers. That is why the Baltic Sea competes in a completely different race than the great oceans, when looking at how much water is in it (21,000 m3).

Bar chart on average depth: The Baltic Sea (54 m), tower of the Helsinki Olympic Stadium (72 m), the Eiffel Tower (300 m), the Mediterranean sea (1500 m) and the World Ocean (4000 m).

Fresh water, light in weight, flows continuously and in large quantities into the Baltic Sea from the hundreds of rivers in its large catchment area. On the other hand, only small volumes of saltwater flow in from the North Sea through the Danish straits.

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 water in the Baltic Sea has different layers based on water density. The upper layer consists of low-saline water. The deeper the water, the more saline and the more dense it is. The water that has the most salt is also the most dense or heaviest, so it sinks to the bottom.

The Baltic Proper has a layer of water with nearly constant salinity, i.e. a halocline, at 70 metres of depth. A halocline acts as a barrier and it prevents the oxygen-rich surface water from mixing in with the water in the deep layer even during the heavy storms in the autumn and winter.

The stratification in the Finnish sea areas is similar to the Baltic Sea’s connection to the World Ocean.

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 that is why it regulates the variations in the gulf’s halocline.

In the spring, the surface water becomes warmer and layers are formed depending on 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 in the deep layer more difficult. This means the deep layer is left without the oxygen-refill it needs from the oxygen-rich and light surface water.

Since the Baltic Sea is so far away from the equator, its ecosystems have adapted to the changing seasons. The alternation of summer and winter is due to the tilt of the Earth’s axis.

When the tilt is pointing towards the Sun, it is summer in the Baltic Sea region. The species living in the Baltic Sea have adapted to the rhythm of the seasons, which affects the weather, the amount of light, the length of the day, the temperature of seawater and the way water mixes in the sea, among other things.

Esimerkki kasviplanktonin määrän ja lajiston vuodenaikaisvaihtelusta Suomenlahdella. Määrä on huipussaan keväällä ja lajistossa vallitsevat piilevät ja panssarilevät. Kesän edetessä sinilevien osuus lajistossa kasvaa. Suomen ympäristökeskus / Sirpa Lehtinen
An example of how the amount and species of phytoplankton vary in the Gulf of Finland depending on the season. Images: Seija Hällfors. Source: Suomen ympäristökeskus / Sirpa Lehtinen

The Archipelago National Park is every nature buff’s dream destination. The glacial ice masterfully carved an ensemble of islands on the Baltic Sea’s area that is rare even on a global scale.

The islands on the open sea are beautifully rugged and bare islets that grow grass, weeds and small shrubs. On the open sea, trees only grow on the larger islands, but in the inner archipelago, they can grow on the small islets as well.

The plants on the barren outer islets have adapted to withstand the rough weather on the sea.

The pressure of the glacier that covered the area was so high that the dent it left on the tectonic plate is still bouncing back. As the ground rises, the archipelagos on the Baltic Sea keep changing. New islands rise from underwater, and others disappear as they are joined to the mainland or with other islands.

The speed at which the ground is currently rising, mm/year. Map: Vestöl, Ågren, Svensson. Background image: Metsähallitus

Learn more about the topic

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