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Summary of the summer 2005: Blue-green algae flourished as it was predicted

Press release 2005-09-14 at 12:00

In summer 2005 there were rather large mats of blue-green algae in the southern sea areas of Finland. The largest concentrations were found in the main basin of the Baltic Sea and the algae situation in those parts was one of the worst ever seen. An exceptional feature were the relatively large mats of blue-green algae in the southern part of the Gulf of Bothnia. By and large, the outcome corresponds fairly well to the algae prognosis published in May.

The algae conditions in lakes were not alarming. The warm weather in the beginning of July raised the algae problem to the surface but the situation was soon over. In late summer the amounts of blue-green algae were lower than usual.

Sea areas

The amount of blue-green algae in the Finnish sea areas began to increase in the end of June. In early July there were considerable amounts of blue-green algae in the whole Gulf of Finland and surface mats were also observed south of Åland Islands. In mid-July, the largest algal mats were formed in the Gulf of Finland east of Porvoo as well as southwest of the Åland Islands. Blue-green algae mass was also abundant in the Archipelago Sea where surface mats were mostly formed south of the line Nauvo-Korppoo. In the northern part of the Archipelago Sea the situation was not as alarming, even if some algae observations were made there as well.

In the third week of July, the surface mats in the Gulf of Finland were scattered by winds. Afterwards there were found only a few discontinuous mats, even if the amounts of blue-green algae in the water mass was considerable until the end of July. Between late July and mid-August, the toxic Nodularia algae formed continuous mats in the southern part of the Gulf of Bothnia. Large algal mats are uncommon in the Gulf of Bothnia and the occurrences of this summer must be regarded as unusually large in that part of the Baltic.

Taken all together, the abundance of blue-green algae this summer was about the same as in 2004, even if the peaks occurred at different times. Last year the amount of blue-green algae began to increase as late as in mid-July and the local surface mats were mostly found in early August. This year the peak blooms were seen already in early July in the main basin of the Baltic Sea and in mid-July in the Gulf of Finland.

Last year the rainy and windy weather prevented the formation of large mats, giving many people an idea that there were not so much algae last year. This year the warm and calm weather in early July allowed the formation of surface mat and the visual impression of the abundance of algal mats gave thus a more realistic idea about the seriousness of the algae problem in the Baltic Sea.


The national-scale algae monitoring started in the second week of June and blue-green algae could already be observed in three of the monitored lakes. However, the abundance of algae was low until early July when the warm period started. About one third of the observation sites reported about blue-green algae ? more than usual at this time of the year. However, only 20 % of the reported occurrences were abundant or very abundant. In most cases the observed amounts were low.

From the second week of July to mid-August the number of occurrences of blue-green algae as well as their abundances didn?t change much. Many of the most abundant occurrences were, as usual, found in either southwestern Finland or in Kainuu. Towards the end of August the amounts of blue-green algae decreased in the monitored lakes and the amounts were lower than usual. In some lakes, blue-green algae were still seen in early September and they may occasionally be seen even later in the autumn, depending on the weather.

Blue-green algae monitoring is over

The blue-green algae monitoring season is now over. The monitoring programme was carried out by the Finnish Environment Institute, the Finnish Institute of Marine Research, the regional environment centres and the Frontier Guard. The observers were environmental or health authorities, pilots of the Frontier Guard and voluntary citizens. The observations were made visually once a week at 263 sites in lakes and watercourses and at 64 sites in the sea. The algae conditions were also monitored by using satellite images.

The Toxicological information centre has this year received many reports about symptoms caused by the use of water contaminated with blue-green algae. A research project on the toxicity of blue-green algae, funded by the Academy of Finland, will continue at the Finnish Environment Institute until the end of 2005. The results of this project will be reported later.

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