Baltic Sea
The Baltic Sea region
The Baltic Sea is a small semienclosed sea formed by the Ice Age. It is encircled by nine nationstates, but its catchment extends to a few more noncoastal countries. Over 85 million people live in the catchment area and the highest population density can be found in the southern part. There are hundreds of cities, eleven of which have populations exceeding 500,000, located in the catchment area. St. Petersburg, with a population of approximately 5 million, is the largest city in the region.
Hundreds of rivers discharge their waters into the Baltic Sea, which amounts to an estimated annual discharge of 14,200 cubic metres per second. The only connection to the North Sea and the Atlantic is through the Skagerrak and Kattegat, two narrow and shallow straits between Denmark, Sweden and Norway. Thus, the water remains in the Baltic for a long time and it is estimated that it takes 30-50 years for the total water mass to be exchanged.
The Baltic Sea is also shallow, with an average depth of about 54 metres. The deepest part, Landsort Deep, is 459 metres and is located near the south-eastern coast of Sweden.
Baltic Sea catchment area
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The Baltic Sea catchment area is over 1,600,000 km2, four times the area of the sea itself, which is 392,000 km2.
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Brackish water - stratified water
The Baltic Sea is the second largest body of brackish water in the world (after the Black Sea), and its salinity levels are on average only about a fifth of those in the open oceans. Near the mouths of major rivers, such as the Neva at St Petersburg, the water is almost fresh.
The water in the Baltic Sea is stratified. This means that the heavier saline water from the North Sea sinks to form a bottom layer while the fresher water diluted by rainfall and river water forms a layer on the surface. The zone where the salinity changes markedly is called the halocline and this zone prevents the mixing of the lower saline water layer and the upper fresh water layer. The bottom wa-ters, especially the deeps, suffer repeatedly from oxygen deficiency. The oxygen content of the bot-tom waters is improved only by heavy storms that mix the waters at a deep enough level and by salt-water pulses from the North Sea. Salt-water pulses may form when there are heavy storms of long duration from the west or south-west of Denmark and the water level in the Baltic is low. The saltier water from the North Sea is pushed through the Danish Straits and over the shallow sill that forms the mouth of the Baltic Sea. Water temperatures vary greatly over the year, averaging out at a chilly 10°C
Sensitive sea area
Due to the slow exchange of water, toxic and otherwise harmful substances remain in the Baltic for many years, exacerbating the problems already faced by the Baltic’s sensitive marine life. The cold winters and the long periods of ice cover also slow the physical, chemical and biological decomposition of harmful substances, including oil.
The International Maritime Organisation (IMO) designated the Baltic as a Particularly Sensitive Sea Area (PSSA) in 2005, making it the tenth such sea area in the world. Other PSSAs include the waters around the Canaries and the Galapagos Islands. PSSA status is granted to waters that are especially vulnerable to pollution and other risks related to shipping, due to their sensitive nature.
Flora and fauna
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© Sirpa Pellinen
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The Baltic Sea region includes some special flora and fauna. Both saline- and fresh-water species are found in the Baltic, as well as genuine brackish-water species and even some relict species. Although the species diversity is low, some organisms can be abundant. The food webs are simple compared to oceans and therefore more vulnerable to environmental changes. Especially in short food chains, changes at one end of the chain may easily spread through the entire chain (cascading effects) and may have unpredictable effects particularly on the top species. Because they are slow to breakdown, heavy metals and other harmful substances remain in the Baltic environment for many years and become concentrated (i.e. bioaccumulate) in the fauna. Because of the bioaccumulation of toxic compounds, White-tailed Eagles experienced breeding difficulties in the 1970s. It is also suspected that bioaccumulation has caused similar problems in seals and salmon. However, protective measures have resulted in successful reproduction particularly for the sea eagle.
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