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

The nutrient situation in open seas continues to maintain a high risk of blue-green algae blooms in the open sea areas close to Finland. The risk of large blue-green algae blooms is high in the northern part of the main Baltic Sea basin, to the south of the Archipelago Sea and in central parts of the Bothnian Sea. The risk is significant in the Gulf of Finland and remains low in the Bothnian Bay. Large blooms may occur locally throughout the coastal area.

Phosphorus in surface water promotes mass occurrences of blue-green algae

Phosphorus bioavailable for phytoplankton remained in surface waters after the spring bloom in the open sea areas of the north, with the exception of the Bothnian Bay. This nutrient surplus provides a good springboard for blue-green algae growth.

Due to the weather conditions last winter, the water layers in the Gulf of Finland did not mix. This reduced the amount of phosphorus from the water layers close to the bottom that ended up in the surface water, and the phosphorus concentration close to the surface was lower in the Gulf of Finland than in the previous year.

“Despite the positive development in the surface water status, the risk of blue-green algae blooms in the Gulf of Finland was only reduced slightly, or to the level of significant risk. The reason for this is the generally high phosphorus level in the Gulf of Finland, which has persisted throughout the 2000s. This is why extensive blue-green algae blooms are possible in favourable weather conditions every summer,” says Jouni Lehtoranta, Senior Research Scientist at the Finnish Environment Institute.

The risk of blue-green algae blooms remains low in open sea areas of the Bothnian Bay

The risk of blue-green algae occurring in the Bothnian Sea has increased significantly over the past decade. In the Bothnian Bay, the scarcity of phosphorus still does not favour mass occurrences of blue-green algae in the open sea.

The blue-green algae species in the blooms occurring along the Bothnian Bay coast have differed from those in other coastal areas of Finland. The reason for this may be the extremely low phosphorus levels in the waters of the Bothnian Bay.

“Phosphorus is the limiting factor for algae growth in the Bothnian Bay, whereas in other open sea areas close to Finland, this factor is nitrogen. The dominant blue-green algae species in nitrogen-limited areas include Nodularia spumigena, Aphanizomenon flosaquae and Dolichospermum spp. These species may produce large blue-green algae blooms in the open sea. The dominant species on the phosphorus-limited Bothnian Bay coast are those in the genera Planktothrix and Phormidium, which are incapable of nitrogen fixation”, says Sirpa Lehtinen, Senior Research Scientist, who coordinates phytoplankton monitoring for marine resources management.

Blue-green algae blooms most commonly occur between Midsummer and late August. The cell division of blue-green algae accelerates when there is plenty of sunlight and the sea water is warm. Blue-green algae blooms occur after a few weeks of warm and sunny weather, especially if winds are slack during this period. Algal blooms mainly consist of blue-green algae cells that are in poor condition and no longer able to regulate their depth position in water. Blue-green algae blooms may also occur well into the autumn and even under the ice in winter.

Marine monitoring is also needed to predict blue-green algae risk. To model the blue-green algae risk, the Finnish Environment Institute’s open sea monitoring datasets and coastal monitoring data were used. Alg@line monitoring carried out by commercial ships as well as satellite images produced additional data. 

Finnish Environment Institute will not publish weekly blue-green algae bulletins this summer due to austerity measures

This summer, the Finnish Environment Institute will not publish weekly bulletins of the general blue-green algae situation. The reason for this is government funding cuts. Blue-green algae observations will be available in online services.

The algae situation in inland waterways is monitored by the Finnish Supervisory Agency, which communicates about it as necessary. Monitoring the blue-green algae situation and water quality in official bathing areas and providing information about them through their channels is up to the municipalities and cities.

Blue-green algae should be treated with caution

Blue-green algae can produce various toxins, including skin or eye irritants. Blue-green algae should always be treated with caution. In particular, young children and pets should be kept out of water containing blue-green algae. Water with blue-green algae must not be used in the sauna, or for washing or irrigation.

Seek medical attention if you suspect a poisoning, or take your pet to a vet. Contact the Poison Information Centre for further instructions if necessary.

Blue-green algae observations in online services maintained by the Finnish Environment Institute:

• Current blue-green algae situation in Finnish (jarviwiki.fi)
• Satellite images and blue-green algae observations from the open sea and the coasts (tarkka.syke.fi)