The risk of extensive blue-green algae blooms remains significant in open sea areas

Extensive blue-green algae blooms are likely to occur in the open sea areas close to Finland during the summer as the nutrient situation in the Baltic Sea remains largely unchanged. The risk of the formation of rafts of blue-green algae is considerable in the northern part of the main basin of the Baltic Sea as well as in the Gulf of Finland, the Archipelago Sea and a large part of the Bothnian Sea. The risk remains low in the Bothnian Bay. 

The extent of algae rafts and the duration of the blue-green algae bloom are affected by the temperature and wind conditions of the summer. If the nutrient situation in the main basin of the Baltic Sea improved, this would significantly reduce mass occurrences of blue-green algae, as the status of the main basin is reflected in the situation of open sea areas in the north. Local variations in the blue-green algae situation may be great on the coast and in the archipelago and also take place faster than in the open sea. Local blue-green algae blooms have also occurred annually on the Bothnian Bay coast. 

Nutrient situation still creates favourable conditions for mass occurrence of blue-green algae 

The nutrient situation in the Baltic Sea continues to create favourable conditions for blue-green algae blooms in open sea areas close to Finland, with the exception of the Bothnian Bay. Last winter, the prevailing strong westerly winds promoted the mixing of water layers in the main basin of the Baltic Sea and in the Gulf of Finland. While the oxygen situation in the Gulf of Finland improved considerably, the mixing unfortunately brought plenty of phosphorus up to the surface layers. The phosphorus content in the Bothnian Sea has increased in the 2000s, and the risk of blue-green algae blooms appears to persist in this area. 

This year, too, the spring bloom of phytoplankton used up almost all available nitrogen in the surface layer, whereas plenty of phosphorus remained in the surface water. These conditions further provide good preconditions for blue-green algae blooms, as in sea areas the species that form extensive algae blooms are able to use nitrogen gas dissolved from air into water. This gives these blue-green algae species a competitive advantage over others. 

Large blue-green algae blooms usually begin to occur in the open sea area after Midsummer. Rafts of blue-green algae form in the open sea following a period of warm and sunny weather of a couple of weeks, especially if it includes a few days of slack winds. In summer, strong northerly winds may cause an upwelling of phosphorus-rich bottom water into the surface layer, which may contribute to blue-green algae growth. 

Monitoring the state of the sea also produces information for forecasting blue-green algae risk 

To assess the risk of occurrence of blue-green algae, monitoring data collected by the Finnish Environment Institute and the Swedish Meteorological and Hydrological Institute (SMHI) concerning the open seas as well as the ELY Centres' coastal monitoring data were used. In addition, data produced by Alg@line monitoring performed on merchant ships, satellite images and weather data of the EU’s Copernicus service were utilised. The forecast for blue-green algae raft formation was compiled using these datasets, combining forecasting methods based on statistical mathematics and machine learning. 

Blue-green algae reports start in Midsummer week 

This summer, the Finnish Environment Institute will report weekly on the general blue-green algae situation between 19 June and 7 August 2025. The report will be published on Thursdays at 13:00. However, the monitoring of the blue-green algae situation will continue from the beginning of June until the end of September. For blue-green algae observations, see the national map on Järvi-Meriwiki.

Blue-green algae can produce various toxins, including skin or eye irritants. Significant blue-green algae blooms should always be treated with caution. The blue-green algae situation in public bathing waters is monitored by municipalities and cities.