Baltic Sea

• Category: Marine and Oceanic Biomes.
• Geographic Location: Northern Europe.
• Summary: The Baltic Sea is an enclosed brackish-water sea with low species diversity, threatened by eutrophication and pollution from its catchment area.

The Baltic Sea, a mostly enclosed sea in northern Europe, comprises several branches and basins, which include, from northeast to southwest, the Gulf of Bothnia, the Gulf of Finland, and the Gulf of Riga; the central-southern area known as the Baltic Proper; and the Danish straits of Kattegat and Skagerrak, which connect to the North Sea, an arm of the Atlantic Ocean. To the east, the White Sea-Baltic Canal, opened by Russia in 1993, allows traffic from the Baltic—via the Gulf of Finland and a chain of rivers and major lakes—to the White Sea, itself an arm of the Arctic Ocean. To the southwest, the Kiel Canal opened in 1895, cuts through the Jutland peninsula to connect the Baltic with the North Sea.

The total surface area of the Baltic is approximately 163,000 square miles (422,000 square kilometers) and its volume is roughly 5,040 cubic miles (21,000 cubic kilometers). The Baltic Sea is small compared to other global seas. However, it is among the world's largest brackish water bodies, in the same league as the Black, Caspian, and Hudson Bay. Connected to the Atlantic Ocean by the narrow Danish straits, the salinity in the Baltic varies between 20 practical salinity units (PSU) in the southern Kattegat to less than 1.0 PSU in the innermost areas of the Gulf of Finland, which makes it a unique, but also a very sensitive, environment. The Baltic Sea is bordered by nine countriesFinland, Sweden, Denmark, Germany, Poland, Lithuania, Latvia, Estonia and Russia. More than eighty-five million people live in its catchment area.

The history of the Baltic Sea is strongly linked with the Ice Age and the land uplift after the ice had melted. Fossil history clearly demonstrates an alternating dominance of typical freshwater and marine species since the last glaciation period. Approximately 4,000–7,500 years ago, during the so-called Littorina Sea stage, the connection through the Danish straits to the Atlantic was formed.

The average salinity of the Baltic Sea was then about eight times higher than it is today. Consequently, the species that could be found during the Ice Age differed drastically from those found in the Baltic Sea today. The Baltic Sea reached its current form and salinity level about 3,000 years ago, a very short period in geological terms. Due to its young age, ecological niches are still available to fill in the Baltic Sea ecosystem.

Species and Stresses

Not all marine animals and plants can tolerate low salinity. Fifty-nine species and sixteen biotopes are considered threatened and declining in the Baltic Sea. The species distribution depends on the geographic location, with more marine species encountered in the southern Baltic Sea and more brackish and freshwater species in the northern Baltic Sea. There are only twenty zoobenthic animal species in the benthicdeepwaterecosystem of the Bothnian area. There are around 500 species in the southwestern Baltic Sea and about 1,600 in the open Skagerrak waters. The number of macroalgae reduced from 250 in the Skagerrak area to 40 in the Bay of Bothnia.

The low salinity of the Baltic Sea suppresses the growth of some species. For instance, the blue mussel species Mytilus edulis is, at its longest, only one inch (2.5 centimeters) long in the Baltic Sea but reaches up to nearly three inches (seven centimeters) in length in the oceanic environment of the North Sea. On the other hand, some freshwater fish species, such as pike, grow faster and larger in the Baltic Sea than they typically do in lakes or rivers.

Apart from its low salinity, the lack of a proper intertidal zone or a deep sea ecosystem is another contributing factor to the low number of species in the Baltic Sea. Indeed, the Baltic Sea overall is a very shallow sea, with an average depth of only 177 feet (54 meters). The average depth of the world's oceans is 12,465 feet (3,800 meters), about eight times deeper than the maximum depth of some 1,505 feet (459 meters) of the Baltic Sea.

The Baltic Sea is divided into subbasins with different topographic, chemical, and biological traits. The inflow from the catchment area is larger than the inflow of seawater through the Danish straits, which means that, for the most part, the water in the Baltic Sea remains in the basins for decades. This leads to a strong stratification of the water column. The heavy saline water sinks to the bottom, forming a distinct halocline, a layer of saltier water. Vertical mixing and ventilation of the benthic ecosystem are quite constricted. Hence, some areas on the seafloor are anoxic or depleted of oxygen.

The inflow of oxygenated Atlantic Ocean seawater into the deep bottom habitats is essential for maintaining a healthy benthic ecosystem in the Baltic Sea. These saltwater pulses, however, are relatively infrequent, occurring about every two years. About thirty-three percent of the bottom of the Baltic Sea is anoxic.

Principal negative habitat pressures acting on the Baltic Sea are eutrophication—the overabundance of nutrient inflow, destabilizing ecosystem balance—and loss of biodiversity. The heaviest anthropogenic pressure in the Baltic Sea is experienced in the Gulf of Finland, the Gulf of Riga, the southeastern part of the Baltic Proper, and the southern Baltic Sea. Riverine input of nutrients, organic matter, and heavy metals are among the main challenges in the Gulf of Finland and the southeastern part of the Baltic Proper. The southern parts of the Baltic host the most dense human population, the heaviest fishing pressure, and large inputs of nitrogen and heavy metals through atmospheric deposition.

Nutrients entering the Baltic Sea environment have altered the food-web structure, changed species composition, and altered population dynamics. Nutrients can increase the growth of phytoplankton, often leading to severe blooms of cyanobacteria. The Baltic Sea is remarkable for these major cyanobacteria blooms in a marine environment, as elsewhere in the world such phenomena tend to be found in freshwater ecosystems. Excess biomass of dead blooms sinks onto the seafloor and leads to an increased growth of microbes there, which eventually leads to anoxic conditions on the seafloor and, if prevailing for a prolonged period, may result in die-offs of benthic organisms.

The main sources of nutrients and organic matter in the Baltic Sea are the rivers that flow into it: Vistula from Poland, Neva from Russia, Oder from Germany, Daugava from Latvia, and Nemunas from Lithuania. The most severe nutrient enrichment pressure is found in the Baltic Proper, the Gulf of Finland, and the Gulf of Riga. Sewage discharges from ships account only for one percent of the total inputs of nutrients, but this nutrient enrichment may increase the phytoplankton blooms.

The average waterborne nitrogen and phosphorus inputs have remained roughly steady since 1990, as coordinated efforts have been made to reduce output from municipal and agricultural sources. However, the draining of wetlands in the catchment areas of rivers in the Gulf of Bothnia has increased the flow of organic matter into the Baltic Sea. Natural background input of organic matter is high in the Gulf of Bothnia compared to the other areas of the Baltic. Runoff from farmlands and forests still represents the primary input of manufactured organic matter into the Baltic Sea.

Physical disturbance to the seafloor can be very destructive on a local scale. However, researchers suggest these inputs make the least impact among stressors on the overall health of the Baltic Sea. Nevertheless, dumping dredged material on the seabed can smother benthic communities and lead to significant losses in biodiversity, favoring opportunistic species. Harbors, offshore wind farms, cables, bridges, coastal structures, and oil platforms occur along the Baltic Sea coastline. Additional seafloor areas will likely be disturbed in the future since more construction and wind farms are expected to be built. Construction may alter the water flow and increase erosion in coastal areas.

Fishing Industry

Overfishing seriously threatens the world's oceans; there is no exception in the Baltic Sea. Industrial fishing has removed large quantities of predatory fishes such as cod, pikeperch, pike, salmon, and the grey seal. The removal or depletion of these top-of-the-chain species has contributed to ecosystem regime shifts, increased nuisance species, and further eutrophication.

The stocks of cod (Gadus morhua) and salmon (Salmo salar) are of particular concern from both economic and ecological standpoints. The eastern and the western stocks of cod have been overexploited for centuries. At the same time, the reproductive capacity of salmon in rivers flowing into the Baltic has also dwindled, even as stocks of Baltic herring (Clupea harengus membras) and sprat (Sprattus sprattus) have been harvested sustainably. The eastern stock of cod has only recently—through various fishing moratorium efforts—returned to what is thought to be a sustainable level; it is anticipated that the western stock can also recover in the near future.

In recent history, two species of fish have become extinct in the Baltic Sea. One is the Atlantic sturgeon (Acipenser oxyrinchus), which has suffered from overexploitation and the obstruction of its migratory pathways. The other is the bluefin tuna (Thunnus thynnus), which was overfished in the Kattegat area in the early 20th century.

Climate change will continue to challenge the health of the Baltic Sea. In the early twenty-first century, the average annual sea surface temperature in the southern Baltic has increased by approximately 1.8 degrees Fahrenheit (F) (one degree Celsius(C)). Latest measurements show this warming has accelerated to nearly 2.5 degrees F (1.4 degrees C) above historical averages, with particularly severe impacts on ice coverage and cod spawning grounds. In contrast, the changes in the northern Baltic Sea are mainly seasonal. The length of the ice season has decreased in the range of fourteen to fourty-four days during the last century. Precipitation is predicted to increase in the future, which may lower salinity levels and increase riverine flow of nutrients that may, in turn, together with rising water temperatures, aggravate eutrophication and anoxic conditions.

International management of the Baltic Sea, in general, aims to improve its health and support the ecosystem services the sea can provide to its people in the future. A principal goal for recovery is to reduce the nutrient input sources. Coordinated actions will likely be costly in the near term, but delaying such actions will likely increase overall costs.

Bibliography

Almroth-Rosell, Elin, et al. "A Regime Shift Toward a More Anoxic Environment in a Eutrophic Sea in Northern Europe." Frontiers in Marine Science, vol. 8, 24 Dec. 2021, doi:10.3389/fmars.2021.799936.

Beaugrand, Gregory, et al. "Causes and Projections of Abrupt Climate-Driven Ecosystem Shifts in the North Atlantic." Ecology Letters, vol. 11, no. 11, 2008, pp. 1157-68.

Heck, Kenneth L., Jr., and John F. Valentine. "The Primacy of Top-Down Effects in Shallow Benthic Ecosystems." Estuaries and Coasts, vol. 30, no. 3, 2007, pp. 371-81.

Helsinki Commission (HELCOM). Ecosystem Health of the Baltic Sea 2003–07: HELCOM Initial Holistic Assessment. Helsinki, Finland, 2010. https://helcom.fi/wp-content/uploads/2019/08/BSEP122.pdf. Accessed 14 Nov. 2024.

HELCOM. Baltic Sea Environment Proceedings: Climate Change in the Baltic Sea: 2024 Fact Sheet. 2024, https://helcom.fi/wp-content/uploads/2024/10/Baltic-Sea-Climate-Change-Fact-Sheet‗2024.pdf. Accessed 14 Nov. 2024.

Leppäranta, Matti, and Kai Myrberg. Physical Oceanography of the Baltic Sea. Springer, 2009.

MacKenzie, Brian R., and Ransom A. Myers. "The Development of the Northern European Fishery for North Atlantic Bluefin Tuna (Thunnus thynnus) During 1900–1950." Fisheries Research, vol. 87, no. 1, 2007, pp. 229-39.

Paaver, Toomas. "Historic and Recent Records of Native and Exotic Sturgeon Species in Estonia." Journal of Applied Ichthyology, vol. 15, no. 4-5, 1999, pp. 129-32.

Powilleit, Martin, et al. "Impacts of Experimental Dredged Material Disposal on a Shallow, Sublittoral Macrofauna Community in Mecklenburg Bay (Western Baltic Sea)." Marine Pollution Bulletin, vol. 52, no. 4, 2006, pp. 386-96.

Rönkkönen, Soili, et al. "Long-Term Changes in Baltic Herring (Clupea harengus membras) Growth in the Gulf of Finland." Canadian Journal of Fisheries and Aquatic Sciences, vol. 61, no. 2, 2004, pp. 219-29.