The late spring and early summer heatwave has led to Finland's lake waters becoming significantly warmer than usual.
The median temperature of lakes across the country was 20 degrees Celsius at the end of May, which is 7 degrees warmer than the long-term average, according to data provided by waterinfo.fi.
The water has been particularly warm in Näsijärvi lake, where the mercury climbed to 24 degrees Celsius over the past week. This reading also represents the highest median deviation recorded in Finland by the end of May, as Näsijärvi's water was 11 degrees warmer than usual.
Sini Heikkilä, a water management expert at the Pirkanmaa ELY Centre, told Yle that the recent heatwave has had a significant impact on the nation's lakes.
"It is directly linked to how warm our air is. It has been quite calm, with very little wind, so that has been another factor," Heikkilä said.
While many people have been enjoying the warmer-than-usual water temperatures, there are also fears about the level of cyanobacteria — or blue-green algae — in Finnish lakes and the health problems that arise from it.
"The amount of blue-green algae will certainly increase towards the end of summer, but this will be affected not only by temperature but also by factors such as nutrients, rainfall and winds," Heikkilä noted.
You can monitor the blue-green algae situation across Finland on this map or get location-specific information from your municipality’s website.
The impact of climate change
Lake water temperatures tend to vary depending on the prevailing weather conditions, making it difficult to predict how warm the water will be in future summers.
However, climate change is warming the whole planet, and water temperatures will rise too.
Heikkilä told Yle that this is likely to mean Finland's lakes will be 1-2 degrees warmer, on average, by 2050.
"These forecasts for average air temperatures are probably not going to decrease, so we will continue to have warmer water," she said.
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