ZURICH, Switzerland, July 16 (UPI) -- Global warming, a concern for its effects on the world's oceans, is also causing harm to the globe's freshwater lakes, researchers in Switzerland say.
Researchers from the University of Zurich said a study of Lake Zurich showed that because of global warming, there is insufficient water turnover in the lake following the winter and harmful algae known as Burgundy blood algae are increasingly thriving.
Many large lakes in Central Europe became heavily overfertilized in the 20th century through sewage, and as a result algal blooms form, reducing oxygen content in the water and threatening fish stocks, they said.
"The problem today is that mankind is changing two sensitive lake properties at the same time, namely the nutrient ratios and, with global warming, water temperature," Zurich researcher Thomas Posch said.
Historical data on Lake Zurich reveals the cyanobacteria Planktothrix rubescens, more commonly known as Burgundy blood algae, has developed increasingly denser blooms in the last 40 years.
The most important natural control of the cyanobacteria blooms occurs in the spring after the entire lake has cooled down during the winter, as intensive winds trigger the turnover of the surface and deep water.
But with global warming the winters are increasingly too warm and the lake water is not able to turn over fully as the temperature difference between the surface and depths poses a physical barrier, the researchers said.
"Unfortunately, we are currently experiencing a paradox," Posch said. "Even though we thought we had partly solved the nutrient problem, in some lakes global warming works against the clean-up measures.
"Therefore, we primarily need cold winters with strong winds again."