Blue-green algae observations have decreased this week

Compared to the previous week, the number of blue-green algae observations has decreased in inland waters, along the coast, and in open sea areas. In inland waters, blue-green algae occur less than usual for this time of year, while the situation along the coast and in the open sea has remained at an average level. However, the situation varies by region, and individual, more abundant occurrences have been observed in different parts of the country.

Blue-green algae barometers for open sea areas:

The cyanobacteria situation in open sea areas can be monitored using algae barometers compiled from satellite data. In the barometer, observations from the current year are marked with black dots. The long-term variation range (from 2009 to 2024) is shown in light grey (90% of satellite observations).

Graph 1: Blue-green algae barometer of the eastern Gulf of Finland. The observation points represent the average of the blue-green algae observations in the area.  0=no blue-green algae, 1=possible blue-green algae, 2=likely blue-green algae and 3=evident blue-green algae. Syke dataset, contains modified Copernicus Sentinel & NASA data (2025)

Graph 2: Blue-green algae barometer of the Bothnian Sea. The observation points represent the average of the blue-green algae observations in the area.  0=no blue-green algae, 1=possible blue-green algae, 2=likely blue-green algae and 3=evident blue-green algae. Syke dataset, contains modified Copernicus Sentinel & NASA data (2025)

Map of the national blue-green algae monitoring observations on week 31

Syke observes the cyanobacteria occurrence as part of the monitoring of the state of the environment in Finland

The national cyanobacterial monitoring is carried out as part of the monitoring of the state of the environment in cooperation with the Centres for Economic Development, Transport and the Environment, municipal environmental and health authorities, and the Finnish Environment Institute (Syke). Finnish Rotary Clubs are also actively involved in nationwide cyanobacterial monitoring.

The cyanobacterial monitoring is based on the monitoring of cyanobacterial deposits in surface water. The intention is to provide an overview of the cyanobacterial situation in different water bodies. The monitoring includes about 400 permanent observation sites across the country on inland and coastal waters and in the archipelago.

Information on the cyanobacterial situation in the open sea areas is mainly obtained from satellite images, but also from the Finnish Border Guard, the marine research vessel Aranda, the optical device located at the Utö Atmospheric and Marine Research Station, as well as cruise and merchant ships (MS Finnmaid and MS Silja Serenade) equipped with Alg@line measuring equipment. The drift forecasts for cyanobacterial rafts in open sea areas are prepared in cooperation with the Finnish Meteorological Institute’s Maritime Services.

Syke reports on the national cyanobacterial situation on a weekly basis every Thursday from 19 June to 7 August 2024. The weekly algal reporting was launched in 1998.

Several compounds produced by cyanobacteria can cause health hazards

According to the Finnish Institute for Health and Welfare (THL), cyanobacterial occurrences can cause health hazards. Cyanobacteria produce a number of different compounds that can cause symptoms. Some cyanobacteria can produce liver or nerve toxins, but most of the symptoms experienced by swimmers may also be due to other compounds.

Small children and pets should particularly be kept out of water rich with cyanobacteria. Water with cyanobacteria should not be used in a sauna or as washing or irrigation water.

If you suspect a poisoning, seek medical advice, or take the pet to a veterinarian. If necessary, the Poison Information Centre will provide additional instructions.

The municipal health authorities monitor the cyanobacterial situation on beaches.

More information about blue-green algae and health:

This is how you identify cyanobacteria

A small amount of cyanobacteria in the water appears as green or yellowish particles. Narrow stripes of algae can drift to a beach. In calm weather, a substantial amount of cyanobacteria forms greenish or yellowish algal rafts and piles up in coastal water. In spring, yellowish pollen from coniferous trees may also be present in the water. Unlike cyanobacteria, pollen is found not only on the surface water but also, for example, on piers or yard furniture.

Cyanobacteria dissolve into tiny particles in the water if you touch the algal mass with a stick. If the algae become attached to the stick, they are something other than cyanobacteria. In a water vessel, cyanobacteria rise to the surface as tiny greenish particles within about an hour.

Read more: How do you recognize blue-green algae?

Report your cyanobacterial observations to the Järvi-meriwiki (Lake and sea wiki)

In Järvi-meriwiki, maintained by the Finnish Environment Institute, you can establish your own observation site and share cyanobacterial observations or make individual observations when moving around waterways. You can also report observations via the smartphone-friendly Havaintolähetti website.

The reported observations are shown on the national cyanobacterial observation map, and they support the national algal situation assessment. Observations about the absence of cyanobacteria are also important.

Järvi-meriwiki is an online service produced in collaboration with authorities and citizens. The service provides basic information on all lakes larger than one hectare as well as different areas of the Baltic Sea. Users can share, for example, photos and other observations on the service.

Municipalities and cities monitor the cyanobacterial situation on the beaches, so it is advisable to report rich cyanobacterial occurrences on beaches to the health authorities of the municipality in question.

Cyanobacterial observations also in the MarineFinland.fi and Waterinfo.fi services

The cyanobacterial maps presented on the websites MarineFinland.fi and Waterinfo.fi combine the observations reported to the Järvi-meriwiki and from the beaches of the City of Helsinki, as well as the observations based on satellite interpretations made by Syke during the last three days.

Summer 2025 algae forecast:

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

Satellite observations:

Tarkka, satellite observation service

Inquiries:

(Telephone 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: Eotuki@syke.fi

For matters related to regional blue-green algae situations, you can contact the ELY Centres

By phone: +358 295 020 900, Mon–Fri from 9 AM to 3 PM, local/mobile rates apply
By email: ympariston.asiakaspalvelu@ely-keskus.fi

Photo capture: A comparison of satellite observations from Friday, July 25, and Wednesday, July 30, over the eastern Gulf of Finland shows how blooms in open sea and nearshore areas have decreased during the early part of the week. © Contains modified Copernicus data and USGS Landsat data, Syke 25–30 July 2025