Water transparency continues to decrease in all sub basins of the Baltic Sea

Helsinki, 10 December (HELCOM) - Summer time (June-September) water transparency has decreased in all sub basins of the Baltic Sea over the last one hundred years, according to a new assessment by the Helsinki Commission.

The decrease has been most pronounced in the Northern (about 50 %) and Eastern (45 %) Baltic Proper, and in the Gulf of Finland (40 %). In the Bothnian Sea, Western and Southern Baltic Proper the decrease has been in the order of 35 %. The primary cause for the decreased water transparency is most likely the increase in phytoplankton biomass, and especially in the Baltic Proper and the Gulf of Finland the increase of cyanobacterial blooms. The reduction in the Bothnian Bay and Kattegat areas was less, approximately 25 %.

Different areas of the Baltic Sea had different dynamics of water transparency. Depth observations from the first half of the 20th century were on average 10 m or more in all regions except in the Gulf of Finland, the Bothnian Bay and the Western and Southern Baltic Proper. In the Bothnian Bay and the Gulf of Finland natural turbidity caused by river run-off is higher than in the other regions causing the water to be less transparent even in the beginning of the 20th century.

Since the end of the 1970’s, the most dramatic reductions in transparency have been observed in the Northern, Western and Eastern Baltic Proper. In the Gulf of Finland, such dramatic reduction over the past 25 years has not been observed; instead, the trend has been linearly downward since measurements were started in 1905. Cyanobacteria blooms have increased especially in these regions and they contribute to decreased water transparency. Increased cyanobacteria blooms are an indication of increased nutrient concentrations, diminished nitrogen: phosphorus ratios and a sign of progressing eutrophication of the Baltic Sea.

In the Southern Baltic Proper and the Kattegat the changes have not been as dramatic as in the northern regions. Kattegat is a transitional region between the highly saline North Sea and the brackish water Baltic Sea, which may explain such variation.

Follow up:

http://helcom.navigo.fi/environment/indicators2004/secchi/en_GB/transparency/

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