Jump to content

Finnish Environment Institute | Suomen ympäristökeskus | Finlands miljöcentral

sykefi header valkoinen

Summary review on harmful algal bloom monitoring in Finland 2009: No major blooms of cyanobacteria this summer

Press release 2009-10-15 at 12:00

As usual, the occurrence of cyanobacteria in the summer 2009 was regulated by the weather conditions. Due to the absence of long continuous warm periods strong algal blooms were not formed. Compared with long-term averages, the algal bloom situation in lakes last summer was better than usual. In the sea areas the cyanobacterial bloom situation was also mostly calm.

The Baltic Sea areas

The summer 2009 was calm with respect to cyanobacteria. As the first cyanobacterial blooms typically occur in the southern sea areas of Finland around midsummer, this summer there were only very few mass occurrences of cyanobacteria in June and they were also regionally very limited.

Cyanobacteria increased towards the end of June, but more blooms began to occur at the beginning of July. Due to the windiness and cool waters in July, the blooms did not grow large and they were of short duration. For most of the time the cyanobacteria were mixed in the water column. Occasional blooms occurred in the Gulf of Finland, in the Archipelago Sea, in the Sea of Åland and at the coast of the Bothnian Sea.

At the beginning of August the weather became warmer and cyanobacterial surface blooms began to occur in the open sea areas of the Gulf of Finland and the Bothnian Sea and, to a lesser extent, in the Archipelago Sea. The weather was variable and the surface blooms were soon dispersed into the water column, but they were re-formed after the sunny and calm weather period. In early September there were still surface blooms at least in the eastern Gulf of Finland, in the archipelago west of Hanko and in the inner parts of the Archipelago Sea.

According to the blue-green algal prognosis published by the Finnish Environment Institute at the beginning of June, the bloom risk was moderate or considerable in the middle and western parts of the Gulf of Finland, in the Archipelago Sea, in the Sea of Åland and in the southern parts of the Bothnian Sea. Due to the variable weather conditions, the surface blooms did not grow large and they were of short duration in all these areas. However, in the eastern parts of the Gulf of Finland there were more blooms than what was expected.


The cyanobacteria situation in lakes in the summer 2009 was better than on average in the period of 1998-2009. Together with the years 2004 and 2008, the year 2009 belongs to the years with lowest numbers of cyanobacteria observations. As there were no long periods of warm weather, the growth of cyanobacteria and the formation of algal blooms were restricted. The average percentage of lakes in which cyanobacteria were observed was 13% which is a much lower figure than usual. The highest cyanobacterial occurrence was observed in early August when cyanobacteria were observed in 22% of the lakes. The observed quantities of algae were in most cases small. Abundant or very abundant algal blooms were observed in only 3% of the lake sites. The bloom forming cyanobacteria were typically potentially toxin-producing species belonging to the genera Anabaena, Microcystis, Woronichinia and Aphanizomenon.

Mass occurrences of cyanobacteria were still observed in September when the mixing of water had brought nutrients to the surface water. At this time of the year the numbers of grazing zooplankton are typically low. Cyanobacterial blooms late in the autumn are possible if the weather is warm and rainy.

Water temperatures in lakes during the summer 2009 were in most cases higher than the long-term averages. There were cool periods in June before midsummer and in early July. The surface waters began to cool down in mid-August, and at the end of the month the temperature was around the seasonal average. The weather in September has been warm and the water temperatures are again higher than the seasonal average.

Algal blooms are monitored at more than 300 permanent monitoring locations in the Baltic Sea and inland waters in all regions of Finland. These sites represent different types of waters in terms of their depth, size, water quality and nutrient levels. Observations from open marine waters are also taken automatically by commercial ships. Additional information for open marine waters are obtained by remote sensing and visual observations submitted by the pilots of the Finnish Border Guard.

The monitoring programme of harmful algal blooms provides a comprehensive overview of the state of algal blooms throughout Finland’s inland and coastal waters and the Baltic open sea area. The visual observations give information on the bloom situation at the observation site. Distribution of algae may vary between different parts of a lake, e.g. depending on the local water quality and the wind directions.

Citizen's algal phone

The citizen's algal phone received altogether 225 calls in June-August. Many questions dealt algal problems in lakes at which people spent their vacations. The callers were particularly concerned for the usability of water when there were only small amounts of algae in the water.

Algae information service is over for this season

Harmful algal bloom monitoring of the Finnish Environment Institute, the regional environment centres and the Finnish Border Guard is now over for this season. The monitoring has been carried out by environment and health authorities, border guard pilots and voluntary citizens.

Target group: