September brings summer high in blue-green algae

Inland waters recorded the highest number of blue-green algae observations of the summer in September, Yle reports, citing the Licensing and Supervisory Agency.

The agency said 55 observations were made at inland water monitoring locations this week, the highest number recorded throughout the summer.

It is rare for the peak in blue-green algae observations in inland waters to occur in September. According to the agency, this has happened only in 2025, 2013 and 2010 during the history of the monitoring programme.

Very abundant blue-green algae was observed in Saarijärvi lake in Sipoo and Onkivesi lake in Lapinlahti. Nearly 20 monitoring locations recorded abundant amounts.

Ten observations were made in coastal areas. The number fell from the previous week, but the situation was slightly worse than usual.