The Baltic Sea is a unique, northern brackish sea – it is one of a kind in the entire world. The small and shallow Baltic Sea is particularly sensitive to discharges and changes in the environment.
A vulnerable inland sea
Water in the Baltic Sea is low-saline brackish water – a unique mix of salty water from the ocean and fresh water from the rivers. The Baltic Sea has only a narrow connection to the ocean via the Danish straits, and the salinity of its waters is only about a fifth compared to that of the oceans. Moreover, the salinity of the water diminishes towards the north, and in the Bay of Bothnia and the eastern end of the Gulf of Finland, the water of the Baltic Sea is actually almost fresh. The Baltic Sea is the world’s second largest brackish sea, beaten only by the Black Sea.
Compared to oceans, however, the Baltic Sea is small, shallow, and low on water. The average depth of the Baltic Sea is only 55 m, whereas the depth of the oceans is counted in kilometres, with the average depth of even the Mediterranean reaching approximately 1.5 km. At its deepest, in the Landsort Deep, the Baltic Sea reaches 459 metres.
The waters of the Baltic Sea are renewed slowly via the narrow Danish straits. One can calculate it would take roughly 30 years for the entire water mass of the Baltic Sea to change. Because of the slow turnover of the water, environmental toxins and nutrients that cause eutrophication in the sea will stay in the Baltic Sea for a long period of time.
The catchment area of the Baltic Sea is the land area from which all surface and ground waters run towards the Baltic Sea. The status of the other waterways in the catchment area will eventually also impact the Baltic Sea, as nutrients and harmful substances will flow to the sea from lakes via rivers. Dozens of large rivers flow to the Baltic Sea, carrying with them discharges that can also originate in regions far from the coastline.
Compared to the area of the Baltic Sea, the catchment area of the Sea is approximately four times larger, which is why nutrients and other discharges are carried to the Sea from quite a distance. Almost 90 million people, from 14 different countries, live in the catchment area. The Baltic Sea has nine coastal states: Finland, Sweden, Denmark, Germany, Poland, Lithuania, Latvia, Estonia, and Russia.
The Baltic Sea is stratified
Water in the Baltic Sea is stratified by temperature. Cold water sinks to the bottom, and warmer layers of water are closer to the surface. Autumn storms mix the water layers up, bringing oxygen-rich surface water also to the depths. The Baltic Sea is, however, permanently stratified also by water salinity. The salty seawater, entering from the North Sea, is denser, and sinks down to the seabed and deeper areas of the Baltic Sea basin. A layer of water with low salinity, diluted by rain and the numerous rivers that run to the Baltic Sea, remains on top. The ground and surface layers of water are separated by a salinity cline, i.e. a halocline.
Storm winds are not felt below the halocline. Oxygen-rich surface waters are not mixed with waters all the way in the depths, and the basins of the Baltic Sea are indeed often anoxic, a state made worse by the Sea’s eutrophication.
The oxygen situation does improve from time to time with saline pulses, which occur irregularly, approximately once in a decade: a saline pulse introduces great volumes of saline water with high levels of oxygen from the North Sea to the Baltic Sea abysses. If the seabed is anoxic in a large area, the oxygen-rich water flowing to the abysses is, however, not always able to sufficiently improve the oxygen status of the depths of the Baltic Sea. Read more on anoxic waters.
Unique marine nature
The Baltic Sea is home to a very special mix of species from both salty oceans and fresh waters. Very few species, however, have adjusted to life in brackish waters, as the water is too salty for freshwater species, and too fresh for ocean dwellers: in fact, many species are constantly living on the edge of their adaptive capability. Adapting to a cold, low-on-salt environment takes up a lot of energy, and many ocean species are actually smaller in size in the Baltic Sea than they would be if living in an ocean. Marine species are more easily found in the southern Baltic Sea, close to the Danish straits, whereas freshwater species are more at home in the eastern parts of the Baltic Sea where salinity is lower.
Even though the number of species in the Baltic Sea is low compared to other seas, individual species can be found in great numbers in the Baltic Sea. Food chains in the Baltic Sea are simple, making the Sea’s ecosystem vulnerable to changes in the environment. A marine community that continues to be as diverse as possible helps the Baltic Sea to sustain changing circumstances, caused, for example, by climate warming. Read more on the impact of climate change.
Baltic Sea protection
The most severe environmental problem faced by the Baltic Sea is eutrophication, caused to a large extent by the excessive nutrient load the Sea receives. Even though nutrient discharges that eutrophicate the Sea have been halved in the past few decades, the visible signs of eutrophication, such as blue-green algae blooms, murky waters, more thread algae, and anoxic seabeds, all continue to pester the Baltic Sea. The consequences of climate change also further accelerate the eutrophication of the Sea.
Warming seawaters change the species communities of the Baltic Sea, and impact all of underwater nature in various ways. As the water warms, southern non-indigenous species have an easier time of making the Baltic Sea their home, and some of the newcomers may pose a threat to the indigenous species of our Sea. Some of the northern species that are well-adjusted to the conditions in the Baltic Sea cannot, on the other hand, adapt to the fast-occurring changes. This will lead to changes in the biome of the Baltic Sea.
The goal of our work at the John Nurminen Foundation is to reach a good ecological status for the Baltic Sea, i.e. no excess nutrients or algae, sufficient oxygen in the seabed, and a diverse and robust ecosystem. Even though protecting the Baltic Sea is often a large-scale operation, private individuals can also make a difference for the wellbeing of the sea. Read more on ways to protect the Sea.