Archive of Blue-green algae bulletins

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Published 17.7.2025

Warm weather has led to more blue-green algae observations in different parts of the country

Warm and calm weather has increased the occurrence of blue-green algae in both inland waters and coastal and open sea areas. Rainfall and windy weather in the early summer have also created favourable conditions for blue-green algae blooms. While extensive observations have been made in different parts of the country, the blue-green algae situation continues to be at a level rather typical of midsummer.

Over the past week, the relatively calm and warm weather that has prevailed throughout Finland has increased surface water temperatures, which has promoted the growth of blue-green algae both in inland waters and on the coast. Rainfall in the early summer has carried nutrients into river and lake systems, and strong winds have mixed waters and raised nutrients from the bottom to the surface in shallow water systems. This enables the abundant growth of blue-green algae in midsummer.

Abundant blue-green algae have been observed at a total of seven inland water observation sites of the national blue-green algae monitoring in Pirkanmaa, Kanta-Häme, Päijät-Häme, Southwest Finland, Kainuu and Northern Ostrobothnia this week. Some blue-green algae have been observed at a total of 49 inland water observation sites, mostly in Southern and Central Finland, but observations have also been made in Lapland.

Although the number of blue-green algae observations has increased in coastal areas, it is at a level typical of midsummer.  Very abundant algae have been observed at three coastal observation sites of the national blue-green algae monitoring in Southwest Finland this week. Abundant algae have been observed at a total of four observation sites on the coasts of Uusimaa and Southwest Finland. In addition to these coastal areas, some algae have been observed at ten observation sites on the coasts of Kymenlaakso, Uusimaa and Ostrobothnia.

Blue-green algae have been observed extensively in the coastal and open sea areas of the Gulf of Finland, most of it between Hanko and the Gulf of Estonia. There are quite abundant blue-green algae especially between Hanko and Porkkalaniemi and in many sheltered bays, and these algae form rafts on the water’s surface. There are also abundant blue-green algae in the open sea areas south of the Archipelago Sea. Observations on the development of the blue-green algae situation have also been made in the Bothnian Sea.

In addition to official monitoring, valuable information on the blue-green algae situation is also gathered from the public. This week, many observations made by citizens were obtained through the Lake-seawiki and Vesi.fi, a total of 434 observations.