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Summary of algal bloom monitoring June-August 2020: Varying cyanobacterial blooms in lakes during the summer, unusual cyanobacterial observations in northern parts of the Bothnian Sea and the Kvarken

Press release 2020-08-27 at 13:12
© Ilkka Lastumäki

The cyanobacterial situation over the past summer has been variable both in inland waters and sea areas. The number of cyanobacterial observations during June–August followed the weather conditions quite closely. In lake waters, coastal areas and the archipelago, the cyanobacterial summer began earlier than usual as a result of the warm early summer. In July, the cyanobacterial situation was calmer due to the cooler weather, but in mid-August, warm weather again enriched the cyanobacterial blooms.

Variable cyanobacterial situation in lakes

In lakes, surface waters warmed rapidly as a result of the warm spring and early summer, and cyanobacteria began to become abundant earlier than usual at many national algal monitoring sites. In June, surface waters were, in some places, several degrees warmer than the average, which accelerated the growth of cyanobacteria and the formation of blooms. In July, cool and rainy weather cooled the waters and curbed the formation of cyanobacterial blooms, and windy weather kept the cyanobacteria mixed with water. Very abundant cyanobacterial blooms were scarce. In July and early August, there were fewer cyanobacterial blooms than average in most parts of the country. The warm weather that returned in mid-August enriched the cyanobacteria again, especially in Central and Eastern Finland. Although the typical occurrence time of cyanobacterial blooms in lakes is from late June to August, if the weather is favourable, cyanobacterial blooms may still occur in September.

This summer, nationwide algal monitoring included about 240 observation sites. During the past summer, some cyanobacteria were found at 97 observation sites. Blue-green algae were abundant at 20 sites and very abundant at only four sites.

“Typically, the duration of abundant or very abundant cyanobacterial blooms was short — two weeks or less. In some lakes, minor cyanobacterial blooms may have lasted longer. In a few lakes, abundant cyanobacterial blooms have lasted for almost the entire observation period, which is typical of eutrophic lakes,” says Kristiina Vuorio, Senior Research Scientist from the Finnish Environment Institute.

Variable weather also reflected in cyanobacterial observations in sea areas

The number of cyanobacterial observations during the period June–August followed the weather conditions quite closely. Due to the warm early summer, cyanobacteria became abundant exceptionally early in late June in coastal and archipelago areas. During the cool and rainy beginning of July, the number of cyanobacterial observations was lower than the national average. Still, towards the end of July, the number of observations increased close to the 1998–2019 average. This summer, the number of cyanobacterial observations increased even after mid-August due to warm weather. However, the amount of cyanobacteria in the sea areas near Finland is usually highest at the end of July and the beginning of August.

"Based on the algae barometer of the Järvi-meriwiki online service, the number of cyanobacterial observations on the coast and in the archipelago was the highest in mid-August during the June–August observation period," says Sirpa Lehtinen, Senior Research Scientist from the Finnish Environment Institute.

Cyanobacterial observations from the coastal areas and the archipelago were made mainly at the observation sites in the Gulf of Finland, the Archipelago Sea, the Southern Bothnian Sea and the Kvarken. The ELY centres (Centre for Economic Development, Transport and the Environment) have provided more detailed information on the cyanobacterial situation and the changes in their own areas throughout the summer.

Based on satellite images and observations made by Finnish Border Guard pilots, cyanobacterial blooms were found in the entire Gulf of Finland, in the northern part of the main basin of the Baltic Sea, the southern part of the Archipelago Sea, the Sea of Åland, the Bothnian Sea, and the Kvarken. In windy weather, cyanobacteria remained mixed with water but during more windless periods cyanobacteria accumulated into surface blooms and cyanobacterial rafts.

SYKE and the Finnish Meteorological Institute jointly prepare weekly drift forecasts for the observed cyanobacterial surface occurrences. According to the drift forecasts, the cyanobacterial occurrences in the high seas remained mainly in the observed areas and did not drift on the Finnish coast, for example, to a considerable extent.

Cyanobacteria of the Aphanizomenon, Nodularia and Dolichospermum genera were present in the Bothnian Sea, the Gulf of Finland and off the island of Utö in the southern part of the Archipelago Sea. The species was analysed both microscopically from water samples and by monitoring the species images sent by the FINMARI optical device located at the Utö Atmospheric and Marine Research Station.

The cyanobacterial situation typically fades as the seawater cools and the light decreases, but surface occurrences of cyanobacteria can still be found later in the autumn. Some cyanobacteria species can form local occurrences under favourable conditions even in cold water. The phycocyanin dye pigment released from degrading cyanobacteria can dye water and beaches and, in winter, ice slices to turquoise or bluish and form a turquoise paint-like mass on beaches.

Sea surface temperatures were in maximum slightly over 20 degrees in open water areas in the Northern Baltic Sea and the Gulf of Finland, northern parts of the Baltic Sea were slightly cooler. Temperature increased in June but decreased in July. In August sea surface was warmer again.

Summary of blue-green algae observations on the Lake&Sea wiki during June-August and surface algal areas interpreted from satellite observations during the summer season. Blue-green algae observations: Lake&sea wiki, Nationwide algae monitoring. SYKE's remote sensing materials are licensed under the CC Name 4.0 International license. SYKE data, contains modified Copernicus data (2020).

Cyanobacterial observations not only in southern and southwestern sea areas but also in northern parts of the Bothnian Sea and the Kvarken

Like last summer, cyanobacterial blooms were again found throughout the Bothnian Sea. This summer, cyanobacterial blooms were observed in satellite images and, exceptionally, during the August observation trip of the research vessel Aranda in the Kvarken. In satellite images, cyanobacteria were also briefly observed in the northern high seas areas. Cyanobacteria have usually been observed annually in the coastal observation sites off the coast of Vaasa during the national cyanobacterial monitoring; however, this summer, based on the Järvi-meriwiki algae barometer, significantly more cyanobacterial observations were reported off the coast of South Ostrobothnia than in 1998–2019 on average. However, the algae barometer is only indicative as the number and location of observation sites have varied over the years.

The Gulf of Bothnia has generally been considered a low-cyanobacterial risk area as there are no deep-water oxygen deprivation problems in the Gulf of Bothnia and therefore the internal phosphorus load has been low. Cyanobacterial blooms in the Bothnian Sea and the Kvarken indicate a change in the state of these sea areas. Based on monitoring data collected since 1979, the cyanobacterial biomass of the Bothnian Sea has increased. Adequate intake of phosphorus nutrients promotes the growth of cyanobacteria. In addition, the rise in surface water temperature promotes the growth rates of cyanobacteria. The phosphorus content of surface water has increased in the Bothnian Sea throughout the 21st century, and both the surface water and the near-bottom water layers have also warmed.

The increase in hot weather explains the rise in surface water temperatures and the presence of cyanobacterial rafts, especially in calm weather. In contrast, there is no single explanation for the increase in phosphorus content that benefits cyanobacterial blooms. Phosphorus can enter the Bothnian Sea from four sources: with river waters, as a direct point source input, from oxygen-free bottom waters through internal loading, or with currents from nearby sea areas. According to nutrient load monitoring, the phosphorus load entering the Bothnian Sea via river waters has not increased in the 21st century. There are also no large oxygen-free areas in the Bothnian Sea from which phosphorus would be chemically released. The phosphorus supplement is likely to come largely from the main basin of the Baltic Sea, in particular, with deep water low in oxygen and phosphorus. It is estimated that this water flows, from time to time, into the basin west of Åland and from there over the underwater thresholds between Åland and Sweden to the Bothnian Sea.

Based on the winter and spring monitoring results, it was predicted that cyanobacterial blooms would be possible in all sea areas close to Finland, except for the Gulf of Bothnia and the northern part of the Bothnian Sea.

“According to the experience of recent years, the area where cyanobacteria occur in the Gulf of Bothnia extends further north than can be inferred from the nutrient situation in winter. The occurrence of cyanobacterial blooms is also influenced by the flow and mixing conditions in spring and early summer, which, in addition to the winter nutrient situation in the Gulf of Bothnia, obviously have a strong effect. The occurrence of cyanobacterial blooms was estimated to follow favourable weather conditions for cyanobacterial growth, which seems to be very true. The general nutrient level in the Baltic Sea is so high that it no longer limits the occurrence of cyanobacterial blooms,” says Harri Kuosa, Leading Researcher at the Finnish Environment Institute.

This week the cyanobacterial situation in lakes has been typical for the season, some cyanobacterial blooms found in the high sea areas

The surface blooms of cyanobacteria have decreased since last week. Some cyanobacterial blooms are still found at 30 national algal monitoring sites. Abundant surface blooms of cyanobacteria were observed at 10 observation sites, but very abundant surface blooms have not been observed for a few weeks. There have been no more observations of cyanobacterial blooms from the public this week.

“Cooler weather has slightly reduced cyanobacterial blooms. Currently, the cyanobacterial situation in lakes is typical for the season in most parts of the country. Slightly more than the average amount of cyanobacterial blooms were found mainly in Southwest Finland, Häme and North Karelia,” says Kristiina Vuorio, Senior Research Scientist from the Finnish Environment Institute.

Lake surface temperatures are close to the long-term average in most parts of the country. The surface water temperature is from 16 to 19 degrees in the southern and central parts of the country and from 10 to 15 degrees in the northernmost part of Finland.

Based on satellite images, there are still some cyanobacterial blooms in the northern part of the main basin of the Baltic Sea and in the Gulf of Finland, but the extensive cyanobacterial occurrence in the Bothnian Sea is beginning to disappear. Finnish Border Guard pilots also observed some cyanobacteria in the high sea areas of the Gulf of Finland at the beginning of the week. According to a drift forecast prepared in cooperation between SYKE and the Finnish Meteorological Institute, a small cyanobacterial occurrence in the northern part of the main basin of the Baltic Sea will initially drift slightly to the west, after which the occurrence will remain in place for the rest of the week.

Earlier this week, observations were received from 84 permanent observation sites in coastal areas and the archipelago, of which 59 were free of cyanobacteria, 19 reported some cyanobacterial blooms and 6 reported abundant cyanobacterial blooms. Algal bloom observations have been reported from observation sites in the Gulf of Finland, the Archipelago Sea, the southern part of the Bothnian Sea and the Kvarken.

"Based on the algae barometer, more cyanobacterial observations have been reported from the archipelago and coastal areas nationwide than on average in 1998–2019, although the number of observations has decreased from last week,” says Sirpa Lehtinen, Senior Research Scientist from the Finnish Environment Institute.

According to the Finnish Meteorological Institute's Maritime Services, the surface temperature of seawater is 15–20 degrees at all Finnish observation stations; the north being the coolest and the Archipelago Sea the warmest. The sea is fairly calm for the rest of the week, but there may be a slightly stronger north wind at the beginning of next week. Based on the models, seawater is currently upwelling on the Swedish side of the Gulf of Bothnia.


Algal barometer

The algal barometer of summer 2020 can be viewed at the algal archive of Lake-Seawiki.


The occurrence of cyanobacteria is monitored weekly during the summer

The Finnish Environment Institute SYKE has published a weekly overview of the cyanobacterial situation in inland waters and sea areas near Finland in June–August. Cyanobacterial monitoring will continue until the end of September, but the publication of weekly overviews will end. In favourable nutrient and weather conditions, cyanobacterial blooms are still possible in the autumn.

Nationwide cyanobacterial monitoring has been carried out since 1998. The monitoring is the result of cooperation between SYKE and the Centres for Economic Development, Transport and the Environment (ELY Centres) and municipal environmental authorities. Algal monitoring includes about 400 permanent observation sites around the country in inland waters and coastal areas and in the archipelago. This summer, Finnish Rotary Clubs have also been actively involved in cyanobacterial monitoring. As a consequence, the number of observation points on the southwestern and southern coasts of Finland increased substantially. Observations from coastal areas and the archipelago were obtained from about 100 observation points throughout the summer.

Information on the cyanobacterial situation in the high seas was gathered from, for example, satellite images, the Finnish Border Guard, research vessel Aranda, and cruise and merchant ships equipped with Alg@line measuring equipment. The Finnish Meteorological Institute provided up-to-date information on seawater surface temperatures, and the drift forecasts for cyanobacterial rafts were also prepared in cooperation with the Finnish Meteorological Institute.

Citizen observations also help in mapping the cyanobacterial situation

Citizens can participate in cyanobacterial observations by reporting their observations in the Järvi-meriwiki online service. In this online water website, citizens can also set up their own observation sites and monitor, among other things, the cyanobacterial situation, surface water temperature and visibility.

During the summer, a total of almost 1500 cyanobacterial observations were received in the Järvi-meriwiki, outside the official algal monitoring. Of these, blue-green algae had been detected in 814 observations, and 656 observations reported that the area was free of blue-green algae blooms. The highest number of citizen observations came during the week after Midsummer, when cyanobacterial observations were reported 379 times.

SYKE thanks all parties involved in algal monitoring for their important cooperation.

Cyanobacterial bloom risk analysis

Map images for media use

Information about blue-green algae situation 

Satellite observations from the summer can be found in the TARKKA online service

More information

(Telephone 1.00 - 3.00 pm)


  • Senior Research Scientist Kristiina Vuorio, Finnish Environment Institute SYKE, Tel. +358 295 251 757, firstname.lastname@ymparisto.fi

Sea areas  

Cyanobacterial bloom situation

  • Senior Research Scientist Sirpa Lehtinen, Finnish Environment Institute SYKE, Tel. +358 295 251 353, firstname.lastname@ymparisto.fi 

State of the Baltic Sea and cyanobacterial bloom risk analysis

  • Leading Researcher Harri Kuosa, Finnish Environment Insitute SYKE, Tel. +358 295 251 106, firstname.lastname@ymparisto.fi

Satellite observations

  • Senior Researcher Jenni Attila, Finnish Environment Institute SYKE, Tel. +358295 251 078, firstname.lastname@ymparisto.fi
  • eotuki.syke@ymparisto.fi


  • Communications Intern Iris Aarola, Finnish Environment Institute SYKE, Tel +358 295 252 189, firstname.lastname@ymparisto.fi 

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