Baltic Sea

From The Encyclopedia of Earth
Jump to: navigation, search
Oceans and seas (main)


March 30, 2010, 12:00 am
May 29, 2013, 12:45 pm
Content Cover Image

Baltic Sea coast in western Estonia. @ C.Michael Hogan

Seas of the World Seas-of-the-world-logo.gifThe Baltic Sea is situated between Scandinavia and mainland Europe, and connects to the North Sea through the Kattegat and Skagerrak. The sea is split into several smaller basins termed the Gulf of Bothnia, the Aland Sea (located in the southern part of the Gulf of Bothnia), the Gulf of Finland, the Gulf of Riga, Kattegat, and Skagerrak.

Countries that border the Baltic Sea to the north are Norway, Sweden and Finland. Russia, Estonia, Latvia and Lithuania border the east. The south is bordered by Poland, Germany, and Denmark.

400px-Baltic Sea map.png Figure 1. Map of Baltic Sea. Source: Norman Einstein

Physical Characteristics

The Baltic Sea is essentially a very large fjord covering an area of 415,266 km2, with topography varying orderly from shallow areas (about 17% is less than ten metres deep) to deep basins with depths up to 459 m (Landsort Deep). The average depth is 53 m and the average volume 21,547 km³. It is approximately 1500 km long and has an average width of about 230 km. Its formation is linked to sea level rise, most of which occurred in the early to mid Holocene.

A dilution basin type of Mediterranean Sea, the Baltic Sea is connected to and experiences limited, intermittent water exchange with the North Sea.

The annual fresh water river runoff into the Baltic Sea is about two percent of the total water volume of the Sea (greatest in May and least in January and February), draining an area nearly four times the size of the sea (1.7 million km2). A shallow and narrow connection with the North Sea means that water remains in the Baltic Sea for about 30 years. These two factors combine to make the Baltic Sea one of the largest brackish water bodies in the world, with a salinity that ranges from 1-2 parts per thousand in the far north and east to about 20 parts per thousand in the Kattegat near the North Sea. In contrast, fresh water has a salinity of of less than 0.5 parts per thousand, and an typical oceanic sea water more than 30 parts per thousand. A saline stratification layer (known as a halocline) exists at a depth of around 70 m.

The different regions of the Baltic Sea are summarized in Table 1 and illustrated in Figure 2.

Sub-Region

Surface area
(km2)

Volume (km3)

Salinity
(parts per thousand)

Maximum Depth
(m)

Average Depth
(m)

Baltic Proper

211,069

13,045

5-10

459

62.1

Gulf of Bothnia

115,516

6,389

0-7

230

60.2

Gulf of Finland

29,600

1,100

0-7

123

38

Gulf of Riga

16,330

424

6-10

>60

26

Danish Straits and Kattegat

42,408

802

8-32

109

18.9

Total

415,266

21,721

0-32

459

52.3

Table 1. Physical characteristics of different regions of the Baltic Sea. Source: Towards marine landscapes in the Baltic Sea (BALANCE interim report N. 10)

620px-Sub-division of Baltic Sea.jpg Figure 2. Sub-regions of the Baltic Sea. Source: Helsinki Commission

The circulation in the Baltic Sea is driven by three chief forces:

  • Wind stress;
  • Horizontal density differences due to freshwater inflow from rivers and saline water inflow via the Danish Straits; and
  • Sea level inclination from the Danish Straits towards the interior.

On average, sea level rises about 25 cm from the Danish Straits towards the Gulf of Bothnia due to the river runoff.

If not interrupted by wind–driven currents, a continuous inflow of saline water from the Skagerrak forms the deep water of the Baltic. It is estimated that 740 km3 of saline water enters the Baltic per year. The inflow enters through the Great Belt (65%), the Sound (25 percent) and the Little Belt (10 percent). After passing through these, the saline water passes over the shallow Darss Sill (18 m), crosses the Arkona basin, flows through Bornholm Strait into the Bornholm Basin, and finally flows through Stolpe Channel into the Gotland Basin.

Human settlement

Over 85 million people live in the catchment area of the Baltic Sea, 15 million within 10 kilometres of the coast.

Several islands are found within the Baltic Sea. Some major islands are are (alphabetically): Åland Islands (independent nation), Bornholm (Denmark), Gotland (Sweden), Öland (Sweden), Saaremaa and Hiiumaa (Estonia), and Zeeland and Funen (Denmark).

Important cities along the Baltic include:

  • The Russian cities of St. Petersburg and Kaliningrad
  • Stockholm, capital of Sweden
  • Copenhagen, capital of Denmark
  • Oslo, capital of Norway
  • Helsinki, capital of Finland
  • Tallinn, capital of Estonia
  • Riga, capital of Latvia

Helsinki Convention

In 1974, the then seven Baltic coastal states, signed a treaty to address pollution on a comprehensive basis. This convention entered into force on 3 May 1980. Following the end of the Soviet Union and the establish of several new states bordering the Baltic Sea, a new treaty was signed in 1992 among 10 parties: Denmark, Estonia, European Community, Finland, Germany, Latvia, Lithuania, Poland, Russia and Sweden. The new convention entered into force on 17 January 2000. The governing body of the Convention is the Helsinki Commission - Baltic Marine Environment Protection Commission - also known as HELCOM.

The goal of the Convention is for "The Contracting Parties shall individually or jointly take all appropriate legislative, administrative or other relevant measures to prevent and eliminate pollution in order to promote the ecological restoration of the Baltic Sea Area and the preservation of its ecological balance." In addition to contracting parties agreed to: apply the precautionary principle, promote the use of Best Environmental Practice and Best Available Technology, apply the polluter-pays principle, ensure that measurements and calculations of emissions from point sources to water and air and of inputs from diffuse sources to water and air are carried out in a scientifically appropriate manner, and ensure that the implementation of the Convention does not cause transboundary pollution in areas outside the Baltic Sea Area.

Eutrophication

The Baltic Sea suffers from many consequences as a result of eutrophication, an excess of nutrients such as nitrogen, and phosphorus, which cause increased growth of algae and plants.

Because sinificant flows of water do not enter into or efflux out of the Baltic Sea (due to the low level of water exchange with the North Sea) or within the Baltic Sea (due to vertical stratification of water with differing levels of salinity), the Baltic Sea is sensitive to oxygen depletion when algae and aquatic plants make greater demands on dissolved oxygen.

Nutrients enter the Baltic Sea via deposition from the atmosphere, rivers flowing into the Sea, and direct discharge. Because of atmospheric deposition, nutrient sources extend beyond the catchment area of rivers feeding into the Baltic Sea.

A large number of human activities and natural flows are the ultimate sources of the nutrients. The Helsinki Commission notes the following:

  • atmospheric emissions of airborne nitrogen compounds emitted from traffic or combustion of fossil fuels (heat and power generation), and from animal manure and husbandry, etc.;
  • point sources including inputs from municipalities, industries and fish-farms both discharging into inland surface waters and directly into the Baltic sea;
  • diffuse sources, which mainly originate from agriculture, but also include nutrient losses from e.g. managed forestry and urban areas;
  • and natural background sources, mainly referring to natural erosion and leakage from unmanaged areas and the corresponding nutrient losses from e.g. agricultural and managed forested land that would occur irrespective of human activities.

It is estimated that in 2000, just over one million tonnes of nitrogen and over one-third of a million tonnes of phosphorus entered in the Baltic Sea. Three quarters of the nitrogen was waterborne, of which 60% came from agricultural activities, 28% from natural sources, and the rest from point sources. Nearly all phosphorus entering the Baltic Sea is waterborne, with half of it originating in agriculture forestry. Natural sources of phosphorus contributed about a quarter of the total. Thus, human activities increase the natural inflows of nitrogen and phosphorus over three-fold. As a result,

These increased concentrations will generally cause an increase in phytoplankton primary production and the growth of short-lived macroalgae. An increase in phytoplankton biomass will cause turbidity in the water and thus a decrease in light penetration through the water column which can ultimately reduce the colonization depth of macroalgae and seagrasses. In association with these changes, there may be changes in the dominance of various species groups. The increase in phytoplankton biomass will ultimately also increase the sedimentation of organic matter to the seabed. The decomposition of this sedimented organic matter may cause oxygen depletion in the sediments and bottom water, resulting in the loss of benthic animals and fish. (Eutrophication in the Baltic Sea - see Further Reading below)

Problems associated with eutrophication are not uniform in the Baltic Sea, but are most pronounced in

  • Baltic Proper, Eastern Gotland Basin;
  • Gulf of Gdansk;
  • Baltic Proper, northern parts;
  • Western Gotland Basin; and,
  • Kiel Bight and Mecklenburg Bight

The challenge of eutrophication is one being addressed by the Helsinki Convention and reductions in nutrients flows from most Baltic Sea countries have been achieved in decent years.

Hazardous Substances

While significant progress has been made in reducing the levels of pollutants (Water pollution) entering the Baltic Sea in recent decades, elevated levels of heavy metals such as mercury, cadmium, lead, copper and zinc, and persistent organic pollutants (POPs) such as dioxins exist. Many of these pollutants persist despite significant regulatory efforts (for example, persistent organic pollutants were banned in the 1980s) and are found in marine life such as fish like herring and salmon.

Tangent Terrestrial Ecoregions

Adjacent terrestrial ecoregions to the Baltic Sea are:

Scandinavian and Russian Taiga

Scandinavian and Russian taiga envelopes the northern coast of the Baltic Sea. Growing season in taiga areas are generally considered to be measured as the number of days for which average daily temperature exceeds five degrees Celsius. The longest growing season for the Scandinavian and Russian taiga occurs in the locales with marine influence from the North Sea and Baltic Sea: in coastal areas of Norway, Sweden and Finland the growing season of the closed boreal forest can reach as high as 145 to 180 days per annum. The shortest growing season of the ecoregion is found in continental Russia and at the far northern part of the ecoregion at the ecotone with tundra.

In the Scandinavian and Russian Taiga, soil nutrient levels are generally poor, but diversity of soil organisms can attain high levels, particularly in the southern reaches of the ecoregion. In these southern elements of the ecoregion, closed canopy boreal forest with some temperate deciduous tree species interspersed among the dominant conifers, including maple, elm and oak. This southern portion of the forest (especially coastal elements) exhibits the longest growing season of the ecoregion, and in some areas of Norway, Sweden, Finland and western Russia, this zone is often exploited for agriculture. Some of the elements of the ecoregion are rich in wetlands, as exemplified by the Femundsmarka National Park in Norway, which is replete with marshes and lakes.

Sarmatic Mixed Forests

Sarmatic mixed forests comprise an ecoregion distributed over a sizable portion of northern Europe and the Ural area of Russia; more specifically this forest type is found particularly in Scandinavia, the Baltics and the Ural area of Russia. Typically, Sarmatic mixed forests comprise a transition into boreal taiga at their northern limit and mixed broadleaf forests at their southern limit. In Sweden (Ecoregions of Sweden) these forests include certain areas of lowland to submontane hemiboreal and nemoral pine forests. The sarmatic mixed forests are comprised of a mixed conifer broadleaf plant association dominated by Norway Spruce (Picea abies) and Scots Pine (pinus sylvestris) with some broadleaf admixture, especially oak speciessuch as Quercus robur in the north.

The ecoregion is abundant in surface water resources, displaying an inventory of more than ten thousand lakes and 20,000 rivers and streams. On the European continent the Daugava River drains a large part of this ecoregion.


Baltic Mixed Forests

the Baltic mixed forests are present in the eastern portion of Denmark rimmed by the Baltic Sea and the Kattegat. These forests also occupy most of the island portions of Denmark, and also the eastern part of the Jutland peninsula. This ecoregion is within a temperate zone much farther north than comparable areas in the western part of the Eurasian continent or compared eastern North America. Comprised of both forests and wetlands, the Baltic mixed forests ecoregion displays many different habitats and, accordingly, a broad diversity of species. Migrating and breeding water-birds, shorebirds, and wading birds visit this habitat each year.

Baltic mixed forests have been exploited by man throughout the Holocene period; moreover, there are significant finds of sunken forests along the Baltic coastline, that reveal earlier forest composition, but that also suggest an early beginning of global warming and sea level rise, perhaps earlier than generally reported. Presently, the ecoregion is being threatened by ongoing habitat destruction and water pollution discharge, especially from agricultural runoff to rivers and wetlands.

Biodiversity

The Baltic Sea has limited biodiversity due to a number of factors illustrated in Figure 3.

BS-processes.jpg Figure 3. Specific features and processes which make the Baltic Sea sensitive and limit its biodiversity. Green boxes are natural characteristics, white boxes are human impacts, and yellow boxes are harmful effects). Source: Helsinki Commission

Three seal species breed in the Baltic Sea are the Grey seal, the Common seal, and the Baltic ringed seal

Further Reading

Citation

Peter Saundry, Tim Bransford and C. Michael Hogan (2013). Baltic Sea. Encyclopedia of Earth, National Council for Science and the Environment. Washington DC Retrieved from http://editors.eol.org/eoearth/wiki/_Baltic_Sea