Baltic Sea increasingly murky - unique study examines data over a century

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A fresh study indicates that underwater visibility in the Baltic Sea has sharply declined in the past 100 years. Changes have been especially great since the late 1970s in the northern, eastern, and western part of the main Baltic Sea Proper, which lies south of the archipelago between Southern Finland and Sweden.
      In the Gulf of Finland the development has been more even, although the transparency of the water there has declined by 40% in the past century. The change has been most dramatic in the northern part of the Baltic Sea Proper, where visibility has declined by about half since the early part of the 20th century. In the eastern part of the basin, with shoreline on Estonia and Latvia, the decline has been 45%.
      In the southern and central part of the Gulf of Bothnia, and in the eastern part of the Baltic Sea Proper, the loss of visibility is just over one third.
      The decline in visibility is smallest in the northern part of the Gulf of Bothnia, where visibility has been reduced by just one quarter since the beginning of the last century.
     
The information is from a study coordinated by the Baltic Marine Environment Protection Commission, or Helsinki Commission (HELCOM). It was carried out mainly by the Finnish Institute of Marine Research.
      Researcher Maria Laamanen says that the study is unique in its scope. The information covers nearly the whole Baltic Sea region, with observations dating as far back as 1903. However, the chain of observations is not completely uninterrupted; it stops at the Second World War and resumes in the early 1970s.
     
The most likely cause of the decline in visibility is the increase in vegetative plankton, especially blue-green algae, in the Gulf of Finland, and the Baltic Sea Proper.
      They are caused by the increase in nutrients, and in the changes in the proportion of nitrogen and phosphorous, which results in eutrophication.
      Visibility varies in the different parts of the Baltic Sea. Measurements taken in the early 20th century showed that visibility in the Baltic was an average ten metres or more, with the exception of the Gulf of Finland, the northern end of the Gulf of Bothnia, and the western and southern parts of the Baltic Sea Proper.
      The water in the Gulf of Finland and the northern part of the Gulf of Bothnia are naturally more murky because of the many rivers carrying humus and other solid matter into the sea.
      The northern part of the Gulf of Bothnia was found to be the only one where visibility was not declining; waters in that area are not susceptible to blue-green algae blooms.
      Changes in the Kattegat and the Southern Baltic Sea were not as significant as in the more northerly areas. The Kattegat is an area where the salty waters of the North Sea and the brackish water of the Baltic tend to mix, which could explain the situation somewhat.


Links:
  The Finnish Institute of Marine Research
  Baltic Marine Environment Commission web site: Water transparency continues to decrease in all sub basins of the Baltic Sea

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