Suomen ympäristökeskus

In sea areas, the risk of extensive blue-green algae blooms remains significant


Blue-green algae risk is significant in the Gulf of Finland, the Archipelago Sea and also in the southern and central parts of the Bothnian Sea.

Map showing summer 2024 risk levels of blue-green algae blooms in the Baltic Sea, with risk ranging from low (blue) to high (red). White areas lack data.
Algae bloom risk 2024. The white area has not been evaluated because no winter nutrient data are available for the area. Finnish Environment Institute

As in previous years, the risk of extensive blue-green algae blooms in open sea areas close to Finland continues to be high, as the seawater's nutrient situation is largely unchanged. This summer, 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, in the Gulf of Finland, the Archipelago Sea and in a large part of the Bothnian Sea. The risk remains low in the Bothnian Bay. The extent of algae rafts is affected by the temperature and wind conditions of the summer.

The nutrient situation, weather conditions and the annual cycle of phytoplankton create the preconditions for the blue-green algae blooms

The Baltic Sea main basin’s deeps still contain plenty of phosphorus. Different water layers get mixed during winter, which brings deep water nutrients to the surface layer. With deep currents, nutrients are often transferred from the main basin to the eastern Gulf of Finland, where nutrient-rich deep water is mixed with surface water during the winter. After winter, as the light increases, growth of phytoplankton algae starts. This is known as the spring bloom of phytoplankton, and is mainly made up of diatoms and dinoflagellates.

Different cyanobacteria in the microscope image. The ability to bind nitrogen gives blue-green algae species Aphanizomenon, Dolichospermum and Nodularia a competitive advantage compared to other phytoplankton species. Sirpa Lehtinen / Finnish Environment Institute

During spring bloom, the amount of phytoplankton in the water is many times higher than during blue-green algae blooms in summer. Water is often discoloured to brownish and fish nets can get slimy, but the spring bloom of phytoplankton does not form algae rafts, as with blue-green algae, nor does it cause any health hazard. During the 2000s, the spring bloom season has started a couple of weeks earlier than before. The spring bloom ends when the nitrogen limiting the growth of diatoms and dinoflagellates is almost used up. Nowadays, after nitrogen is used up, phosphorus nutrients remain as a surplus in surface water in offshore areas close to Finland, with the exception of the Bothnian Bay. Nitrogen scarcity favours the mass formations of blue-green algae during the summer, as Aphanizomenon, Dolichospermum and Nodularia blue-green algae are able to utilise nitrogen gas dissolved from air into water, which, in practice, is almost unlimitedly available. In other words, the ability to bind nitrogen gives blue-green algae species a competitive advantage compared to other phytoplankton species.

In summer, warm water accelerates the growth of blue-green algae. Even abundant blue-green algae do not necessarily attract attention if it is mixed in the water column. The formation of large  blue-green algae rafts, which can also be seen from satellites, will require favourable growth conditions for a couple of weeks: warm and sunny weather, as well as sufficient phosphorus, and at least a few days of calm wind conditions. Algae rafts consist mainly of old blue-green algae cells that are no longer able to regulate their depth position in water. At the end of the bloom, most blue-green algae cells break down in water and only some settle down to the bottom. Blue-green algae species winter as resting-state cysts at the bottom of the sea or as vegetative cells in the water column.

In other words, the risk of the mass formation of blue-green algae in the open sea is affected by the weather conditions in the winter, the nutrient situation in the spring, the intensity and species of spring bloom, and the nutrients bound with them, as well as the weather conditions in the summer. Upwelling of phosphorus-rich water into the surface layer during the summer can also favour the growth of blue-green algae.

Blue-green algae risk in the Bothnian Sea has increased significantly

Over the past ten years, the regional risk of blue-green algae in open sea areas has remained largely unchanged, but the risk has increased significantly in the Bothnian Sea. The occurrence of blue-green algae is mainly affected by summer weather conditions. The local blue-green algae situation on the coast and in the archipelago varies faster than in the open sea. An improvement in the state of the Baltic Sea main basin would significantly reduce the mass occurrence of blue-green algae in large areas.

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

Data from a variety of sources are used to assess the risk of occurrence of blue-green algae. This forecast used data from extensive monitoring cruises conducted by the Finnish Environment Institute and the Swedish Meteorological and Hydrological Institute (SMHI), and from the coastal monitoring of ELY Centres. Additionally, information was used from satellite observations, weather data and from Alg@line monitoring utilising automated sampling on merchant ships. The forecast for blue-green algae has been prepared using AI-based modelling methods.

Weekly cyanobacterial reviews start in Midsummer week

This summer, the Finnish Environment Institute will report weekly on the general blue-green algae situation, between 20 June and 8 August 2024. The report will be published on Thursdays at 13:00. Monitoring of the blue-green algae situation has started in the beginning of June and will continue until the end of September.

Blue-green algae can form various toxins, including skin or eye irritants. Blue-green algae blooms should always be treated with caution. Municipalities and cities monitor the algae situation of bathing waters.



Risk evaluation of  blue-green algae blooms in Finnish marine areas: 
Senior Research Scientist Jouni Lehtoranta, Finnish Environment Institute Syke,, tel. +358 295 251 363 

Communications Specialist Eija Järvinen,
Finnish Environment Institute Syke,, tel. +358 295 251 242

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