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Excellent summer for Finnish lakes: August blue-green algae level lowest in 20 years

Warm late-summer weather could still bring toxic algae to the surface though, experts warn.

Satelliittikuva näyttää sinilevätilanteen 24.7.2019. Vaaleanvihreät alueet meressä ovat veteen sekoittunutta sinilevää.
Satellite image showing the blue-green algae situation on 24 July, 2019. The pale green areas are blue-green algae mixed into the water of the Baltic Sea. Image: Original bild: © ESA Copernicus Sentinel-3 Data, Finlands miljöcentral (2019)

Toxic blue-green algae, technically known as cyanobacteria, has been unusually rare on Finnish inland waters this summer despite spells of hot weather that usually trigger the growth of surface algal blooms.

Since last week, the Finnish Environment Institute (SYKE) has detected fewer cyanobacterial blooms on Finnish lakes and sea districts. In maritime areas, cyanobacteria are mostly mixed into the water, but there are some surface blooms in southern and southwestern regions. Meanwhile the incidence of cyanobacteria in lakes has dropped significantly.

"Usually blue-green algae is at its heaviest in Finnish lakes in August, but this summer levels in August are lower than at any time in the 20 years of national monitoring," says Marko Järvinen, head of SYKE's algal bloom monitoring unit.

Most abundant in southern Finland

This week blue-green algae has been detected at 30 observation points around the country. At six of them the level was rated as abundant, with one classified as 'very abundant'.

That was at Haapajärvi, a lake between the eastern city of Lappeenranta and the Russian border. Most of the 'abundant' spots are in the south and southwest, between Sipoo, just east of Helsinki, and Rymättylä, near Naantali.

Järvinen attributes the paucity of blooms to unsettled weather. Meanwhile low levels of rain means that less nutrients have been washed in from lakes' watershed areas. Runoff from agriculture, including fertiliser and animal manure, is a key driver of cyanobacteria growth.

Although blue-green algae levels have declined, he says it's too early to declare the season over.

"Blue-green algae is now mixed into the water. If the weather warms up again, it will collect on the surface again," Järvinen predicts.

"Along the coast, the blue-green algae situation can vary significantly even between places close to each other and can change quickly," adds another SYKE researcher, Sirpa Lehtinen.

"Blue-green algae may form different types of toxins and agents that irritate the skin. Significant blue-green algae blooms should always be treated with caution," warns SYKE. Exposure to the bacteria can cause illness or even death in humans and animals such as dogs.

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