The water in the Baltic Sea is brackish with a low salinity – a unique mixture of salty and fresh waters. The salinity of the Baltic Sea water is only approximately 20% of the salinity of the oceans (35 per mille). Furthermore, the salinity of the Baltic Sea’s surface water reduces towards the North, and the water in the Bay of Bothnia and at the bottom of the Gulf of Finland is almost fresh.
Compared to the oceans, the Baltic Sea is a small and shallow body of water. The average depth of the Baltic Sea is only 54 metres, whereas the average depth of the Atlantic, for example, is around four kilometres, and even the Mediterranean’s average depth is one around 1.5 kilometres. The deepest basin in the Baltic Sea is 450 metres deep. The Baltic Sea is linked to the North Sea via the narrow Danish straits, and the turnover of the water is very slow. Theoretically, it has been estimated that the turnover of the entire volume of water of the Baltic Sea takes approximately 30 years. Due to the slow water turnover, environmental toxins and eutrophying nutrients remain in the Baltic Sea and cause long-term effects.
The water of the Baltic Sea is permanently stratified according to the level of salinity. Salty seawater flowing from the North Sea is heavier and sinks to the bottom and deep basins of the Baltic Sea. The surface layer consists of water diluted by rainwater and numerous rivers flowing into the Baltic Sea. The stratification hinders water turnover between the bottom and the surface layer. Oxygenic surface water cannot reach the deeper water layers, and there are recurrent oxygenless periods in the deep basins of the Baltic Sea. The oxygen situation improves periodically, about once per decade, with the help of pulses of saline water (infrequent currents of ocean water), when salty and oxygenic ocean water flows from the North Sea into the basins of the Baltic Sea, mixing with the Baltic Sea’s water layer near the bottom.
The population of the Baltic Sea is an unusual combination of freshwater and ocean flora and fauna. The number of flora and fauna adapted to life in the brackish water is small, but there may be large quantities of individual species. Compared to the oceans, the food chains in the Baltic Sea are simple. The number of species reduces from the southern Baltic Sea to the North. The low salinity of the northern Baltic Sea, the cold winters and the sea freezing over set challenges for the adaptation of organisms. Many species in the Baltic Sea live on the extreme limits of their adaptability. The flora and fauna of the Baltic Sea are very sensitive to changes in the environment.
Eutrophication is the biggest problem for the Baltic Sea, and the Gulf of Finland is its most badly affected basin. Though eutrophying nutrient discharges have decreased lately, there has been an increase in the visible symptoms of eutrophication, such as blue-green algae, water opacity, beach mucilage and the number of anoxic seafloor areas. According to current estimates, climate change will further precipitate the eutrophication of the Baltic Sea.
Eutrophication is caused by nitrogen and phosphorus emissions, which nourish the growth of algae in the water. With respect to the occurrence of blue-green algae, phosphorus, in particular, plays a key part. Nitrogen and phosphorus drift into the sea in wastewater from cities and along with rainwater from fields, for example. In addition, part of the nitrogen emissions caused by traffic ends up in the Baltic Sea as fallout from the air. With regard to Finland’s nutrient load, the proportion of agriculture is large: its share of the nitrogen load is approximately one third, and almost half of the phosphorus load.
Ample growth of small algae, i.e. phytoplankton, in the surface waters cause the oxygen to run out in the water areas near the bottom. Dead algae sink to the bottom, and their decomposition consumes oxygen from the seafloor. Under anoxic circumstances, the process of decomposition changes and begins to produce poisonous hydrogen sulphide, which kills the animal populations at the bottom of the area. In addition, phosphorus is released from the seafloor. This is called internal loading of the sea. If the water mixes very powerfully, the phosphorus released from the seafloor reaches the water surface layers and promotes the extensive growth of blue-green algae.
In addition to microscopic algae, the large amount of nutrients in the water also precipitates the growth of annual filament algae on the stony beaches and cliffs. The more frequent filament algae suffocate the perennial algae of the Baltic Sea, such as bladder wrack, which has a significant role in the ecosystem of the Baltic Sea. Bladder wrack forms rich growths in shore waters, which act as an important spawning area and a ‘nursery’ for many fish species, including financially important ones.
Eutrophication is changing life in the Baltic Sea. The effects also reach financially important fish populations, not to speak of the detrimental effect the eutrophication symptoms have on tourism, for example. In order to curtail emissions, all countries around the Baltic Sea should start cooperating and take concrete action.
BAltic Sea Portal