Finland is known as the land of a thousand lakes, but a good number of them suffer from green algal blooms that gobble up oxygen in the water as they die and decompose. The result is an oxygen-starved body of water that cannot support complex plant life or fish, which eventually die.
However a local lake management organisation managed to salvage one affected lake in southwest Finland by using a chemical clean-up method.
The use of chemicals to clean eutrophic lakes may become more common, the Finnish Environment Institute's (SYKE) Development Manager Seppo Hellsten told Yle's breakfast programme Aamu-TV on Monday morning.
The trouble starts when too much phosphorous builds up in the lake, Hellsten says, making it virtually impossible for the lake to maintain its chemical balance. That imbalance is a boon for algae, which then rapidly take over the body of water. When the algae die, microbes hog the lake's remaining oxygen as they break down the decomposing blooms, starving other lake flora and fauna of oxygen in the process.The phosphorous is usually swept into lakes and rivers from the land, where it is often used in treatment plants and fertilizers.
A couple of weeks ago in order to clean up the suffering, soupy and oxygen-depleted Littoinen Lake in southwestern Finland, workers sprayed several tanker truckloads of polyaluminum chloride onto it.
During the process, the chemical is applied to the lake's surface and as it slowly sinks to the bottom, it swiftly reduces the amount of phosphorous.
Chemicals clear water but kill fish
The cleaning worked; Littoinen Lake is now looks crystal clear and will remain chemically balanced for years to come. The cleaning process, however, isn't pain-free, as the treatment can kill fish living in the water.
Asko Sydänoja, an inspector with the southwest Finland Centre for Economic Development, Transport and the Environment (Ely-keskus) pointed that that the species that suffered in this case could also be a part of the eutrophication chain.
"A very small number of fish died, which was taken into consideration. The water cleared up but time will tell if it increased vegetation in the lake," Sydäonoja added.
He noted that the fish kill resulting from the chemical treatment affected bream, which died when the acidic water caused metals to precipitate and collect in their gills. He said that in terms of preventing further eutrophication the kill targeted the right species.
"Bream is a rapidly-reproducing fish and large numbers can be problematic for lakes, because they eat all the anmal plankton that regulate the plant plankton. This in turn contributes to algal blooms," Hellsten noted.
Hundreds of lakes ripe for chemical treamtent
Finland has used chemical treatmenrs to clean about 50 lakes since the 1970s, mostly ponds. At the moment, officials believe that there are roughly 1,500 Finnish lakes affected by eutrophication and that a few hundred would be good candidates for a chemical purge. However Hellsten stressed that the method is appropriate for situations when nothing else is likely to work.
According to the SYKE official chemicals are suitable for use in headwater lakes, or lakes that do not receive heavy inflows from other water sources. On top of that nutrient loads from sources such as agriculture must be minimal.
"If the nutrient load continues, the procedure will be pointless."
Perhaps a larger problem facing lakes in Finland is rivers, 40 percent of which would themselves require action to be restore ecological balance. According to Hellsten, the biggest issue for coastal rivers is eutrophication.
"Another problem is that rivers in Finland have many migration barriers such as dams, so the fish cannot move around freely," he pointed out.
"Finland can improve the state of its rivers by reducing nutrient loads emiited by agricultural activity and by promoting the free movement of animal life both up- and down-river," Hellsten concluded.
EDIT 29.9.2017 This story has been edited to reflect the fact that Liittoisten Osakaskunnat, not the Environment Agency, initiated the chemical treatment of the lake.