Summary of algal bloom monitoring June-August 2021: The hot weather boosted the growth of cyanobacteria in the early summer – still the amount of cyanobacterial blooms was below average since mid-July

Press release 2021-08-30 at 14:28
NC sinilevä
© Emilia Lehtinen

During the summer, the cyanobacterial situation has varied in both Finnish lakes and sea areas. The amount of cyanobacteria was exceptionally at its peak in the sea areas already in mid-July. Even in inland waters, the sunny and calm weather of early summer, as well as the warm waters, accelerated the growth of cyanobacteria, and as a result cyanobacteria began to bloom earlier than usual at many of the national algal monitoring sites. Nonetheless, fewer cyanobacterial blooms were observed at the national cyanobacterial monitoring sites in most parts of the country after mid-July. In the Gulf of Finland, the formation of cyanobacterial blooms was curbed by the lack of alternating periods of suitable phosphorus nutrient additions from the deep waters to the surface and subsequent longer windless periods and by the strong cooling of the seawater due to long-lasting upwelling as happens in very strong bloom tears.

In the sea areas, preparations were made for mass blooms of cyanobacteria – summer ended exceptionally well

In Finnish sea areas, the peak of cyanobacterial observations occurred exceptionally in mid-July, when it typically occurs in the turn of July–August. After mid-July, the number of cyanobacterial observations decreased significantly, and the cyanobacterial situation was even better than normally from mid-July to the end of August. This was mainly due to the fact that fewer cyanobacterial observations were made in the Gulf of Finland than usual. Very strong and extensive cyanobacteria surface rafts did not form in the Finnish offshore areas. Still, in the central parts of the Bothnian Sea, as in recent years, there was widespread cyanobacteria in the vicinity of the surface and occasionally on the surface.

“Nationwide, the amount of cyanobacteria in the sea areas near Finland peaked in mid-July, which is slightly earlier than usual. Thereafter, the amount of cyanobacterial blooms decreased based on the observations reported to the Järvi-meriwiki online service, satellite observations, and the species, biomass, and pigment results of cyanobacteria at the Utö Atmospheric and Marine Research Station. The most abundant cyanobacteria found in front of Utö were of the genera Aphanizomenon and Dolichospermum, but the amount of Nodularia spumigena was smaller,” says Senior Research Scientist Sirpa Lehtinen from SYKE.

The regional variation in the cyanobacterial situation was considerable. During the summer, the ELY Centres have provided more detailed information on the cyanobacterial situation in their own coastal and archipelago areas.

Warm weather accelerated the rise in sea surface temperature in early summer, and this promoted the growth of cyanobacteria. After Midsummer and at the beginning of July, cyanobacteria were extensively mixed with water in Finland's southwestern and southern offshore areas. Cyanobacteria were also found especially in sheltered bays in all coastal and archipelagic areas of Finland; in the Gulf of Finland, the Archipelago Sea, Åland and the Bothnian Sea as far as the Quark. Similarly, abundant cyanobacteria were reported off the coast of the Gulf of Bothnia, and according to the ELY Center for Northern Ostrobothnia, sea surface temperature at monitoring sites on the Gulf of Bothnia had risen to a record 22.2°C, which had contributed to the growth of cyanobacteria.

In mid-July, some cyanobacterial blooms were found close to the surface and occasionally on the surface, especially in the offshore areas of the Bothnian Sea, but also in the Archipelago Sea. Most of the cyanobacterial observations reported to the Järvi-meriwiki in mid-July came from the Archipelago Sea. However, in the central parts of the Archipelago Sea and in the Gulf of Finland, the amount of cyanobacteria was moderate even in mid-July, according to satellite images.

After mid-July, a large cyanobacterial occurrence that had been in the offshore areas of the Bothnian Sea for weeks mixed with the water. In coastal and archipelago areas, the cyanobacterial situation calmed down and the national cyanobacterial barometer remained below normal throughout the end of July. At the beginning of August, upwelling, where the deeper old water mass rose to the surface, was observed in almost all of the Finnish coastal areas. The upwelling further reduced the formation of cyanobacterial blooms, as the cyanobacteria-rich surface water gave way to the deeper water body and, in addition, the surface water temperature dropped sharply. The upwelling situation continued, so in the cool body of water, the cyanobacteria could not effectively utilise the phosphorus nutrient that had risen from the bottom. This affected the biomass of cyanobacteria, i.e., the amount of cyanobacteria in the water.

For these reasons, the cyanobacterial situation in the Gulf of Finland, in particular, was very different from 2018. In 2018, the cyanobacterial situation in the Gulf of Finland developed particularly bad due to warm seawater and the alternating periods of suitable phosphorus nutrient additions to the surface and subsequent longer windless periods. Similar favourable conditions for the growth of cyanobacterial biomass and the formation of surface rafts did not exist in the Gulf of Finland this summer.

“For the offshore areas near Finland, wind conditions were one of the essential reasons why cyanobacteria did not accumulate as surface rafts during the past summer. Since we didn’t experience longer windless periods, the cyanobacteria mass accumulating in the surface layer remained mainly mixed with water in larger offshore areas in the Gulf of Finland, for example," says Senior Research Scientist Jenni Attila from SYKE.

Cyanobacteria were mainly found in the sea areas foreseen in the risk assessment

The occurrence of cyanobacterial blooms is primarily influenced by the availability of the phosphorus nutrient used by cyanobacteria, but favourable warm weather conditions are also needed for growth. This year, cyanobacteria sites were mainly foreseeable, but two phenomena require special attention. The expansion of cyanobacterial blooms in the Bothnian Sea to the far north and the abundant cyanobacterial occurrences observed on the coast of the Bothnian Bay are an exception to previous years. For the Bothnian Sea, the increased amount of phosphorus combined with warm surface water is the most likely explanation. The coast of the Bothnian Bay was also exceptionally warm. In the Gulf of Finland, the growth of cyanobacteria followed, as in some previous years, the pattern of rapid utilisation of nutrient reserves left after spring bloom in early summer. After that, the growth was reduced due to a lack of nutrient additions from the bottom layers and later due to cooling surface water.

“Summer 2018 was great for cyanobacterial blooms, also in terms of nutrient availability, but this summer the availability of nutrients was weaker than normal, especially in the Gulf of Finland,” says Harri Kuosa, Leading Researcher at SYKE.

The risk assessment of cyanobacterial blooms issued at the beginning of June is based solely on the nutrient situation in winter, in which case the weather during summer does not affect the assessment. With regard to the Bothnian Sea, the understanding of the growth potential of cyanobacteria needs to be clarified. However, it is worth remembering that in many sea areas, even the current average cyanobacterial blooms are much stronger than would be observed in their natural state.

Summary of cyanobacterial observations recorded in the Lake-Seawiki online service during June–August and of the summer surface algae areas interpreted on the basis of satellite observations. Cyanobacteria observations: Lake-Seawiki online service, national cyanobacterial monitoring and SYKE satellite observations. © Contains modified Copernicus data, SYKE (2021)

Fewer cyanobacterial blooms in lakes than during an average summer

Even in inland waters, the sunny, rainless and calm weather of spring and early summer, as well as the warm waters, accelerated the growth of cyanobacteria, and as a result cyanobacteria began to bloom earlier than usual at many of the national algal monitoring sites. The warm and sunny weather in spring and early summer warmed lake surface waters rapidly, and for this reason, the surface temperatures were above the long-term average in many places. Despite the warm weather, fewer than average cyanobacterial blooms were observed at the national algal monitoring sites in most parts of the country.

Low external nutrient loads reduced the cyanobacterial blooms, especially in the southernmost part of Finland. In areas with low rainfall, surface runoff into water bodies was low, which significantly reduced the nutrient load from the catchment area. On the other hand, in the rainy areas of Central Finland, Pirkanmaa and Etelä-Savo, for example, surface blooms of cyanobacteria occasionally occurred more than average. Since the turn of July–August, windy weather has kept any cyanobacterial occurrences mixed with water. In addition, lower night temperatures have lowered water surface temperatures, which has contributed to curbing the occurrence of cyanobacterial blooms in late summer.

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, even in October–November. Under favourable conditions, mass occurrences of cyanobacteria can also occur under ice.

This summer, the nationwide algal monitoring in lakes included about 290 sites, of which cyanobacteria were observed at 136 sites. Minor cyanobacterial blooms were found at 108 sites, abundant blooms at 23 sites and very abundant blooms at only five sites.

“Typically, cyanobacterial blooms occur in eutrophic lakes where phosphate phosphorus is abundant. In eutrophic lakes, cyanobacterial blooms may occur throughout the summer. During the past summer, the duration of abundant or very abundant cyanobacterial blooms was short at most monitoring sites – two to three weeks or less. In some lakes, minor cyanobacterial blooms may have lasted longer,” says Senior Research Scientist Kristiina Vuorio from the Finnish Environment Institute.

Over the last monitoring week, rainy and windy weather has further reduced cyanobacterial blooms, which are now clearly below average. No very abundant cyanobacterial blooms have been observed. Abundant blooms are currently present at five sites and minor blooms at 27 sites. Similarly, since the end of the summer holiday season, observations made by citizens have clearly decreased. In 19–26.8, they have made altogether 24 observations, eight of which have been found to contain cyanobacteria.

Algal barometer

The algal barometer of summer 2021 can be viewed at the algal archive of Lake-Seawiki.

SYKE monitors the cyanobacteria occurrences as part of the monitoring of the state of the environment

The Finnish Environment Institute SYKE monitors the cyanobacteria occurrences as part of the monitoring of the state of the environment and has published a weekly overview of the cyanobacterial situation in inland waters and sea areas near Finland in June–August. Monitoring of the cyanobacterial situation will continue until the end of September, but the publication of weekly national overviews has now ended. In favourable nutrient and weather conditions, cyanobacterial blooms are still possible in the autumn.

National 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. Since 2020, Finnish Rotary Clubs have also been actively involved in cyanobacterial monitoring which has led to a significant increase in the number of observation points, especially on southwestern and southern coasts of Finland. National cyanobacterial monitoring includes over 400 permanent observation sites around the country in inland waters and coastal areas and in the archipelago. Individual citizens have been able to participate in the monitoring of cyanobacteria by sending their observations via the Havaintolähetti application to the Lake-Seawiki online service or through the cyanobacterial situation map service on the website.

Information on the cyanobacterial situation in the sea areas near Finland was gathered also from satellite images, the Finnish Border Guard, research vessel Aranda, the species, biomass, and pigment results of cyanobacteria at the Utö Atmospheric and Marine Research Station, and cruise and merchant ships equipped with Alg@line ferrybox 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. This year, satellite observations were utilised not only in sea areas but also in 43 lake areas.

At SYKE, weekly cyanobacterial overviews were published by experts together with university trainees. Communications on cyanobacterial situation was facilitated by fruitful collaboration between researchers and representatives of the media. SYKE thanks all parties involved in cyanobacterial monitoring for their important cooperation!

Map images for media use

Cyanobacterial bloom risk analysis

Information about blue-green algae situation 

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

Utö Atmospheric and Marine Research Station and Alg@line ferrybox monitoring

More information

(Telephone 1.00 - 3.00 pm)


  • Senior Research Scientist Kristiina Vuorio, Finnish Environment Institute SYKE, Tel. +358 295 251 757,

Sea areas  

Cyanobacterial bloom situation

  • Senior Research Scientist Sirpa Lehtinen, Finnish Environment Institute SYKE, Tel. +358 295 251 353, 

State of the Baltic Sea and cyanobacterial bloom risk analysis

  • Leading Researcher Harri Kuosa, Finnish Environment Insitute SYKE, Tel. +358 295 251 106,

Satellite observations

  • Senior Researcher Jenni Attila, Finnish Environment Institute SYKE, Tel. +358295 251 078,
  • Satellite observations:


  • Communications Intern Sofia Patama, Finnish Environment Institute SYKE, Tel +358 295 252 245,