Archive of Blue-green algae bulletins

Note: This is an archived report.
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Published 24.7.2025

The blue-green algae situation remains the same in many places, the number of observations in open sea areas has increased

Weather has remained warm and fairly windless, and the number of blue-green algae observations has remained almost unchanged on inland waters and on the coast. In open sea areas, the number of blue-green algae observations has increased in many places. The situation is typical of midsummer but may change rapidly as weather conditions continue to favour the growth of blue-green algae.

As weather has remained warm and fairly windless, the number of blue-green algae observations on inland waters and coastal areas has remained fairly similar compared to last week, and the situation is typical of midsummer.

Abundant algae have been observed at a total of seven inland water observation sites of the national blue-green algae monitoring in Uusimaa, Päijät-Häme, South Karelia, South Ostrobothnia, North Ostrobothnia, North Savo and Lapland. There have been observations of some blue-green algae at a total of 34 inland water observation sites in Southern, Central and Northern Finland.

Very abundant algae have been observed at one coastal observation site of the national blue-green algae monitoring in Southwest Finland this week. Abundant algae have been observed at eight observation sites in Kymenlaakso, Uusimaa, Southwest Finland, Central Ostrobothnia and North Ostrobothnia. Some algae have been observed at 15 observation sites on the coasts of Uusimaa, Southwest Finland and Central Ostrobothnia.

This week, the number of blue-green algae has increased especially in the eastern and central parts of the open sea areas of the Gulf of Finland and in the outer parts of the Archipelago Sea. Based on the most recent observations from Wednesday, surface blooms of blue-green algae are abundant or very abundant in all outer coastal waters of the Gulf of Finland and, in many places, also in the inner coastal areas. Abundant surface blooms have been observed especially in Inkoo, the Helsinki Metropolitan Area, Sipoo, Loviisa and Kotka as well as in the easternmost parts of the Gulf of Finland.

In the Archipelago Sea, surface blooms of blue-green algae occur especially in parts close to the Gulf of Finland. Blue-green algae were already observed at the beginning of the week in the Archipelago National Park and the western bay of Hanko. Since then, surface blooms have also been observed on the bay of Gullkrona, both as large algae masses and as smaller algae tracks. Elsewhere in the Archipelago Sea, blue-green algae occur in smaller quantities and mainly mixed with water. The number of blue-green algae in the Bothnian Sea is also increasing, as is typical of the season. However, no large surface masses have yet accumulated in the area.

The entire Baltic Sea is warm for the season, and the temperatures have varied between 20 °Cand 24 °C in the Gulf of Finland, being over 20 °C in other sea areas as well. Hot weather seems to continue for a few days in sea areas relevant to the occurrence of blue-green algae, and the winds remain weak. The blue-green algae situation may change rapidly locally if the weather conditions favour the growth of blue-green algae and the accumulation of deteriorated algae cells on the surface.

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 322 observations.