Summary of the national blue-green algae monitoring June-August 2024: This year, blue-green algae blooms were observed especially in open sea and coastal areas, whereas the blue-green algae situation in inland waters varied

This summer, the national blue-green algae monitoring included 372 observation sites, 270 of which were located in inland waters and 102 on the coast. No blue-green algae blooms were observed in about half of the sites in inland waters, whereas only one third of the observation sites in the coastal areas remained blue-green algae-free throughout the summer.  

The observation sites are visited once a week, to assess the blue-green algae situation on a four-step scale: no blue-green algae, some blue-green algae, abundant blue-green algae or very abundant blue-green algae. Abundant or very abundant blue-green algae situations occurred in 26% of inland water and 36% of sea area observation sites. 

In addition to the observation sites, the citizens made nearly 2,500 observations of the blue-green algae situation through the Järvi-meriwiki and waterinfo.fi service. Blue-green algae were detected in 70% of the observations made by citizens. 

Link to Järvi-meriwiki's blue-green algae barometers from Finland's inland waters, and arcipelagos and coastal areas

Summer 2024 observations in inland waters, coastal areas and open sea areas

In June, the blue-green algae situation in inland waters was calm, as typical in early summer. As a result of the warm weather that continued over Midsummer, the occurrence of blue-green algae was high after Midsummer and, momentarily, slightly more blue-green algae were observed than is typical this time of year. In July, the amount of blue-green algae varied slightly between weeks, but the situation remained calm and blue-green algae even appeared slightly less than typically at the turn of July and August. In July, single blue-green algae observations were also made in Lapland. The warm weather in August again increased blue-green algae blooms, which were slightly more common than is typical in late summer. The most abundant blue-green algae blooms were detected mainly in Southern and Central Finland.

As a result of the warm early summer and the early warming of seawater, observations of surface blooms on the coast of the Gulf of Finland increased earlier than typical in June. In late July, the amount of blue-green algae increased in Finland's southern and south-west coastal areas, and the number of blue-green algae observations was higher than average compared to the turn of July and August in 1998–2023. At the beginning of August, the amount of blue-green algae decreased slightly and was at a typical level of the season. Blue-green algae mainly occurred on the coasts of Uusimaa and Southwest Finland, but observations were made along the coast all the way to North Ostrobothnia.

The warm early summer also advanced blue-green algae observations in the open sea areas of the Gulf of Finland. In both the eastern and western Gulf of Finland, the peak of blue-green algae blooms was reached at the end of June. In the Bothnian Sea and the Sea of Åland, blue-green algae levels began to increase in mid-July. In these open sea areas, blue-green algae blooms had a peak at the end of July. The cooled and rainy weather at the beginning of August, as well as winds mixing the water mass, reduced blue-green algae blooms in open sea areas.  In the Bothnian Bay, blue-green algae were mainly observed in coastal areas. 

The blue-green algae situation in open sea areas can be followed using the Tarkka service’s blue-green algae barometers compiled from satellite data. In the barometer, the observations for the current year are marked as black dots. The grey background colour describes the range of observations from 2009 to 2023, which does not include abnormally abundant single observations. 

At the end of the summer, the open sea barometers are used to determine the duration, abundance and intensity of the blue-green algae blooms, which enables yearly comparison.  According to the overall estimate based on barometer data, more blue-green algae were observed this year in the eastern Gulf of Finland than average. The large number of blue-green algae in the east was probably explained by the nutrients transported by the bottom currents from the main basin of the Baltic Sea, and their mixing with surface water. Also,  the warm late June explains the blooms.  In the western Gulf of Finland and the Bothnian Sea, blue-green algae blooming was as abundant as typical. The barometers are calculated daily based on the cloudless observations of the Sentinel-3 satellite's OLCI instrument (available in the Tarkka service).

All blue-green algae barometers in six open sea areas can be found at the bottom of the front page of the Tarkka service.

The modelling-based risk assessment accurately predicted the occurrence of blue-green algae in open sea areas

The blue-green algae risk assessment for open sea areas in summer 2024 was prepared with AI-based modelling methods, using open sea monitoring data from the Finnish Environment Institute and the Swedish Meteorological and Hydrological Institute (SMHI) as input data, as well as coastal monitoring data from ELY Centres. Additional material was received from the Alg@line monitoring data of commercial vessels, as well as from satellite image and weather data. The prepared blue-green algae risk assessment was able to accurately predict the regional distribution and abundance of blue-green algae in the open sea areas surrounding Finland. The forecast was well able to distinguish areas where blue-green algae blooms were not observed and where there were abundant blooms. The realised blue-green algae abundance has small regional variations that the model forecast cannot capture.

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, between June and August, publishes a weekly overview of the blue-green algae situation in inland waters and in sea areas close to Finland. The blue-green algae monitoring continues until the end of September, and the weekly national situation report was published this year between 20 June and 8 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 well into the autumn. 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 shores, as well as ice fissures to turquoise or blueish in the winter, and form turquoise paint-like mass on the shores. 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 fifth 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 Vesi.fi 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 approximately 70 lake areas. 

Syke wants to thank all parties involved in blue-green algae observations for their important cooperation also this year! 

National algae reporting

• National algae monitoring (Järvi-meriwiki) 
• Itämeri.fi algae observations page 
• Waterinfo.fi 
• Havaintolähetti 
• Algae situation summary 
• Finnish meteorological institute marine weather and Baltic Sea

Satellite observations can be found in Syke's Tarkka service

• Syke's satellite observation service Tarkka

Utö atmospheric and marine observation station and Alg@line monitoring

• Utö species and pigment results
• Alg@line automatic observations from ships