Algae bloom leaves Kolvosenjärvi facing decades of recovery

Kolvosenjärvi, a lake between Savonlinna and Enonkoski in South Savo, could take decades to recover after a widespread cyanobacteria bloom last summer, Yle reports. A water expert says the lake’s nutrient load has exceeded its capacity to cope.

The relatively small, shallow lake was naturally clear, but in late July it was covered by a bloom involving four species of cyanobacteria. At the worst point, visibility in the water was only about 20 centimetres, compared with the usual couple of metres, said Pekka Sojakka, a water expert at the Lupa- ja valvontavirasto (LVV), Finland’s Permit and Supervisory Agency.

More than 50 residents and people with holiday homes by the lake attended a public meeting in Savonlinna on Tuesday, with dozens more following online. Sojakka urged them to organise for water protection, saying that only a registered group, such as an association or water-shareholders’ association, can receive grants for restoration work.

Possible measures include fish removal, restoring catchment areas, and building filtration fields and wetlands. Sojakka warned there were no quick fixes and that sustained work might show results only over decades. Nutrients are also released from the lake bed.

Hannu Sihvonen, who has lived on the lake’s shore for 78 years, said it was still clear about 40 years ago and gradually darkened. He noticed a clear change more than a decade ago, and said the water did not clear even during the winter two years ago.

Sihvonen said he had built a half-hectare wetland on his land after stopping farming. Wetlands help prevent nutrients from flowing into waterways.

The lake’s deterioration has been driven by several factors, including agriculture and forestry, which can make it easier for nutrients to enter the water. Sojakka said forestry activity around the lake has increased, including final fellings. He also pointed to unusual weather, low water levels that reduce water flow, and heavy rain that can wash nutrients into the lake.

Sojakka said around ten lakes in South Savo have become heavily eutrophied for similar reasons. He described the shift in approach to Kolvosenjärvi as moving “from healthcare to medical treatment.”