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In the 1970s, Lake Vesijärvi was one of the most polluted lakes in Finland. Limnologist Juha Keto recalls the story of how the lake’s water quality was restored with decades of environmental conservation.

The photos in the article are part of Iida Hollmén’s work “Though the wind blows against

The Lake Vesijärvi lies between Lahti, Asikkala and Hollola. The lake has several basins, and it is rich in aquatic plants and fish. One of its shores is close to the city centre of Lahti. People go there year-round swim, from the warmest summer days to the iciest winter evenings.

But the lake was a different sight decades ago. Vesijärvi used to suffer badly from water contamination caused by industrial emissions and sewage.

“Not a single person would have gone for a swim in the lake in the 70s. People didn’t want to talk about the state of the lake,” says limnologist Juha Keto.

“The lake was filled with rubbish, algae and seaweed. You could have walked on the lake – that’s how bad it was. Fishermen would set their boats to sail from Lahti’s port eager to reach clearer waters and leave behind the stench of sulphur.”

A limnologist studies polluted lakes

As a young boy Keto loved to swim in Lake Päijänne, the second largest lake in Finland, which is connected to Lake Vesijärvi. In the 1940s, his relative got a house on the northern shore of Vesijärvi.

“The lake blew me away. I remember the water being so clear and warm. When I dived underwater, I’d see fish and water plants,” Keto recalls.

After his close relative passed away in the 50s Keto lost touch with Vesijärvi. The next time he saw the lake was a decade late, and he was blown away again. This time because of how polluted the lake had become.

“It wasn’t the same anymore,” says Keto. “Vesijärvi was a huge part of why I became a limnologist and studied lakes. When I started working for the water administration in the 60s, my first task was to study how Vesijärvi was affecting Päijänne. As I took water samples for work and for my master’s thesis, I realised how bad the water quality was. I began creating an action plan for what needed to be done to restore the lake.”


Restoration of the lake begins

The City of Lahti started its ambitious water conservation work in the 1970s, with the aim of stopping the visible eutrophication of Vesijärvi. Starting from 1976, wastewater that used to be released directly into the lake was treated at a wastewater treatment plant before being discharged.

“In 1975 the City of Lahti wanted to employ a full-time limnologist. I thought this is it – I’ll finally get to put my plan into action. But it wasn’t like that. I had support, but many still saw me as a strange character who was all worked up about restoring the lake. People instead discussed filling it, because they wanted to get rid of the issue as fast as possible,” Keto says.

The quality of the lake improved after wastewater discharge into the lake was stopped, but new problems arose. Massive cyanobacterial blooms occurred quickly afterwards, which severely damaged the water ecosystem. Addressing this problem required a new approach, called biomanipulation. This new method was pioneered at the time by Finnish biologist Ilkka Sammalkorpi.

“They had experimented with biomanipulation in Norway, and we thought it could work in Vesijärvi as well. We began trying out the method in the early eighties in Lahti,” says Keto.

“I discovered with my apprentice that the eutrophication of the lake was mainly caused by cyprinid. About 85% of the entire fish population consisted of roach,” Keto recalls. “Roach is a fish that is very adaptable and can tolerate organic pollution. It is often the last species of fish to disappear in polluted waters. And because they are bottom feeders, the organic material in the lake can’t settle on the bottom, but instead stays in the water.”

The Vesijärvi I project started in 1987. More than a million kilos of roach were removed from the lake and over a million young pike-perch were planted in the lake. As a result, blue algae blooms decreased significantly and the water began to appear clearer.

Improving water quality with selective fishing

In the early 1990s, the results of the Vesijärvi I project started to show – the water quality had improved and life below the surface had begun to flourish. As the condition of the lake improved, fishermen and local people returned to enjoy the lake and its many beaches.

“Back then Lahti was struggling through economic recession, but we were still able to launch and finish the first project. But I knew the lake’s ecosystem would experience a setback soon after. Ecosystems try to remain stable, and in this case the polluted state had been the normal state of the lake for years. After the lake was cleaned people also wanted to build more houses close to the shore, which would again cause even more pollution,” Keto says.

He was right. In the early 2000s the water quality worsened. Blue algae started to grow, and the oxygen levels of the lake plummeted. That is when the second conservation project was launched.

“This time we had a lot of volunteers. We organised many events for the lake, including over 580 lake evenings where I taught people about Vesijärvi. A lot of local people wanted to participate and take action for our lake.”

Aquatic life and water quality have been controlled for years now, and lake aeration and selective fishing as conservation methods are still in use. Nowadays also endangered eels are planted in the lake and their spawning migration is secured by moving mature eels to the Gulf of Finland.

“I want to thank everyone for stepping up for the lake. It’s a miracle, we’ve had so many people wanting to help. Someone has to have a plan, but nothing would have changed if I was doing this alone,” thanks Keto. “We saved our lake together.”


Conservation leads to research and innovation

Thanks to the Vesijärvi projects and related research, the Department of Environmental Ecology of the University of Helsinki was established in Lahti. The department has produced a lot of information on topics such as storm water management, algae, green roofs, soil and circular economy.

For example, in Lake Kymijärvi a unique restoration method has been put to use that removes the nutrients that have accumulated in the lake’s sediments for use as soil conditioners. The condition of the River Porvoonjoki has also been improved by ultraviolet disinfection.

Lahti has also built comprehensive systems for urban storm water management. Urban wetlands, infiltration basins and green roofs are utilised for storm water management together with traditional conveyance-based solutions.

New storm water solutions

Stormwater runoff from urbanised areas is a significant source of surface water pollution. To address this issue, Lahti set up a stormwater programme in 2011 that utilises urban wetlands, infiltration basins and green roofs. Lahti also actively participates in research and development projects. There are currently numerous pilot sites around Lahti where studies are being carried out in co-operation with universities and other stakeholders.

For example, a joint project called Stormwater Smart & Clean was carried out in 2018 among the Helsinki metropolitan cities (Helsinki, Vantaa, Espoo) and Lahti, funded by the Helsinki-Uusimaa Regional Council. The main purpose of the project was to build new nature-based stormwater management facilities and to start a long-term monitoring scheme.

As part of the project, a large-scale bio-infiltration pilot was constructed in the Hennala area in Lahti. The pilot project utilises new infiltration materials to purify transferred from Lahti’s city centre. In addition to the data gathered in pre-construction laboratory tests, future monitoring efforts will quantify the long-term performance and cost-effectiveness of the system

The Lake Vesijärvi Foundation

The Lake Vesijärvi Foundation is a unique Finnish initiative combining public and private resources to secure funding for research, maintenance and management efforts focusing on Lake Vesijärvi and its surrounding areas. The foundation also works to improve public awareness of Lake Vesijärvi and its condition and to promote all efforts to improve its water status.

The Lake Vesijärvi Foundation has a long-term plan for management efforts and a funding programme for their implementation. Funds are raised by companies and at various fundraising events. The foundation was established in 2007 by three municipalities and private sector bodies.