Suomen ympäristökeskus

Summary of the national blue-green algae monitoring June-August 2023: Varying blue-green algae summer both in lakes and at sea


During the summer, the blue-green algae situation varied due to changes in weather conditions. The blue-green algae blooms became more abundant with warm weather following Midsummer. The peak of the observations was reached in lakes and on the coast in mid-July. In the open sea, the blue-green algae season began at the end of June.

© Ilkka Lastumäki
© Ilkka Lastumäki

This summer, national blue-green algae monitoring included some 240 observation sites on inland waterways, of which about half did not have any blue-green algae blooms. A small amount of blue-green algae occurred at least once at 95 observation sites, abundant algae at 24 sites and very abundant algae at 8 observation sites. In coastal areas, a small amount of blue-green algae was observed at 44 of the approximately 100 observation sites, abundant blue-green algae at 18 sites and very abundant blue-green algae at 2 sites. In addition, over 2,300 observations have been made by citizens through the Järvi-meriwiki and service, of which 70% were blue-green algae observations.

The blue-green algae situation in lakes and the coastal areas of the Baltic Sea has varied during the past summer in accordance with the weather conditions. The cool early summer restrained the formation of blue-green algae blooms, and the situation was mainly typical of the time period. After Midsummer, as the weather and waters warmed, blue-green algae blooms became more abundant, and the peak was reached in mid-July. After mid-July, the situation remained mainly typical of the season. In early August, windy and unstable weather kept blue-green algae mixed in the water. Strong winds re-mixed nutrients stored in the sediment into the water, and heavy rainfall washed out nutrients from the catchment area for the use of blue-green algae, the surface blooms of which increased somewhat towards the end of August as the winds settled.

During the summer, blue-green algae blooms were found in nutrient-rich lakes throughout the country. In the coastal areas of the Baltic Sea, blue-green algae blooms were found especially in the southern and south-western coastal areas and in the Archipelago Sea.

During summer 2023, abundant blue-green algae were observed in nearly the entire open sea area of the Gulf of Finland. Compared to recent years, the blue-green algae season in the Gulf of Finland began a week earlier than usual. In the western Gulf of Finland, the abundance of blue-green algae was highest at the end of June. In the open sea area of the Gulf of Finland east of Helsinki, blue-green algae were found in large areas from June to August. Blue-green algae observations were also made in the Åland Sea from June onwards and still in the end of August. In the northern part of the main basin, the widest occurrences of blue-green algae were at the end of June. In the Bothnian Sea there was a large occurrence of blue-green algae from the beginning of July to the beginning of August.

Blue-green algae blooms risk estimate was realised

The risk estimate for blue-green algae blooms issued at the beginning of June was based on the nutrient situation in winter and spring. The availability of phosphorus-based nutrients was predicted to be high in a large sea area. In the main basin of the Baltic Sea and in the Gulf of Finland, the risk of blue-green algae blooms was assessed to be high, and blue-green algae blooms were also predicted for the Bothnian Sea. The blue-green algae bloom risk for this summer was largely realised in the areas indicated in the forecast.

In recent years, blue-green algae blooms in the Bothnian Sea have been more abundant than predicted by the winter nutrient situation. Blue-green algae observations in the Quark area have also increased in recent years. The oxygen and nutrient situation in the Bothnian Sea has changed, which may have affected blue-green algae species and their growth. In terms of changes in the Bothnian Sea ecosystem, further information is needed, which could be used to predict the risk of blue-green algae blooming.

The nutrient situation in the Baltic Sea main basin continues to favour the occurrence of blue-green algae blooms in a large part of the Baltic Sea. In particular, phosphorus keeps the risk of blooms high. However, based on the occurrence of blue-green algae blooms in individual years, no conclusions can be drawn on the development of the state of the Baltic Sea, as the blooms occurrence in risk areas is primarily determined by summer weather conditions.

A summary of the blue-green algae observations collected in the Järvi-meriwiki service during June-August and the blue-green algae surface bloom areas of the summer season, interpreted on the basis of satellite observations in open sea areas. The map shows the blue-green algae situation at its most abundant level at observation sites and in sea areas. Blue-green algae observations: Järvi-meriwiki, national blue-green algae monitoring and the Finnish Environment Institute (Syke) satellite observations. The colour scales of satellite observations and the Järvi-meriwiki observation classification correspond to each other. © Contains modified Copernicus and USGS Landsat Program data, Syke (2023).

SYKE monitors the occurrence of blue-green algae as part of monitoring the state of the environment

The Finnish Environment Institute SYKE monitors the occurrence of blue-green algae as part of the monitoring of the state of the environment and publishes a weekly overview of the blue-green algae situation in inland waters and in sea areas close to Finland. Monitoring of blue-green algae will continue until the end of September, but the weekly national reports end at the end of August.

The typical time for blue-green algae blooms is from the turn of June-July until August. In late summer, cool nights usually lower the surface temperatures of the waters, which may curb the occurrence of blue-green algae blooms. As the amount of daylight decreases so does the growth of blue-green algae. However, even abundant blue-green algae blooms may still occur in September, and weak blooms may still occur in October-November. If sufficient nutrients are available for blue-green algae, the blooming may continue even under the ice cover. The phycocyanin colour pigment released from degrading blue-green algae cells can stain water and beaches, as well as crevasses to turquoise or blueish in the winter, and form turquoise paint-like mass on the beaches. Degrading blue-green algae mass may also cause an unpleasant smell.

National blue-green algae monitoring has been carried out since 1998. SYKE conducts the monitoring in cooperation with the Centres for Economic Development, Transport and the Environment (ELY Centres) and municipal environmental authorities. Finland’s Rotary is also participating in the blue-green algae monitoring for the fourth year. Citizens can participate in the monitoring by sending their observations to the Järvi-meriwiki through the Havaintolähetti application or via the blue-green algae map service on the website.

Information on the blue-green algae situation in Finland's nearby sea areas was obtained from satellite images, the Finnish Border Guard, the marine research vessel Aranda, measurements of blue-green algae species, biomass and pigment at Utö atmospheric and marine observation station, as well as from cruise and merchant vessels fitted with Alg@line equipment. The Finnish Meteorological Institute's marine services provided up-to-date information on the surface temperature of seawater. The drifting forecasts for blue-green algae mats were prepared weekly in cooperation between SYKE and the Finnish Meteorological Institute's marine services. This year, satellite observations were utilised not only in sea areas but also in 62 lake areas.

SYKE wants to thank all parties involved in blue-green algae observations for their important cooperation!

National algae reporting

Summer satellite observations can be found in the TARKKA service

Utö atmospheric and marine observation station and Alg@line monitoring



Inquiries about blue-green algae situationTelephone 1 pm to 3 pm

Lakes: Tel +358 50 5734 347 or +358 295 251 326

State of the Baltic Sea: Tel +358 50 5693 297 or +358 295 251 314

Satellite observations: Tel 358 50 4707 576 or +358 295 251 329, e-mail:



It is time to move beyond solving environmental problems one by one, to systemic sustainability transformations. The Finnish Environment Institute (Syke) contributes to building a sustainable society through research, information and services. The Finnish Environment Institute is a research institute with 700 experts and researchers located in Helsinki, Oulu, Jyväskylä and Joensuu.

Subscribe to releases from Suomen ympäristökeskus

Subscribe to all the latest releases from Suomen ympäristökeskus by registering your e-mail address below. You can unsubscribe at any time.

Latest releases from Suomen ympäristökeskus

Miljöskadliga stöd bedömdes – orsakandet av naturförlust bör identifieras i beskattningen18.9.2023 11:20:00 EEST | Tiedote

Genom att slopa statens ekonomiska stöd för verksamhet som förstör miljön kan man både förbättra miljöns och statsfinansernas tillstånd. Rapporten Miljöskadliga stöd i Finland. En översikt av stöd som påverkar klimatet och biologisk mångfald (på finska) som Finlands miljöcentral publicerat idag går igenom utvecklingen av bedömningsmetoderna för miljöskadliga stöd och bedömt de skadliga effekterna av stöden särskilt på klimatet och den biologiska mångfalden.

Ympäristölle haitallisia tukia arvioitiin – luontokadon aiheuttaminen olisi tunnistettava verotuksessa18.9.2023 11:00:00 EEST | Tiedote

Poistamalla valtion taloudellinen tuki ympäristöä pilaavalta toiminnalta voidaan kohentaa sekä ympäristön että valtiontalouden tilaa. Suomen ympäristökeskuksen tänään julkaisemassa raportissa Ympäristölle haitalliset tuet Suomessa. Katsaus ilmastolle ja luonnon monimuotoisuudelle haitallisiin tukiin on käyty läpi ympäristölle haitallisten tukien arviointimenetelmien kehitystä ja arvioitu tukien haitallisia vaikutuksia erityisesti ilmastolle ja luonnon monimuotoisuudelle.

Delta i en enkät som kartlägger betydelsefulla platser i Östersjön11.9.2023 09:11:47 EEST | Tiedote

Tillbringar du tid till havs med att njuta eller arbeta? Är öar, skär, stränder och vikar en del av ditt sinneslandskap eller din livsstil? Finlands miljöcentral (Syke) utreder betydelsefulla platser för medborgarna i havsområden och vid kusten i Finland. Med hjälp av en kartbaserad enkät får forskarna information om platser som är viktiga för människorna och som kan användas som stöd för havsplaneringen. Människors erfarenhetskunskap utnyttjas i arbetet med att identifiera havsområden som är värdefulla med tanke på ekosystemtjänsterna. Enkäten kan besvaras 11.9 –15.10.2023.

In our pressroom you can read all our latest releases, find our press contacts, images, documents and other relevant information about us.

Visit our pressroom