Global warming affecting lakes worldwide

Washington - Global warming, a major concern for the various water resources of the world, is also causing harm to the globe's freshwater lakes, a new study has warned.

Researchers from the University of Zurich say that their study on Lake Zurich has revealed that there is insufficient water turnover in the lake during the winter and harmful Burgundy blood algae are increasingly thriving.

Many large lakes in Central Europe became heavily overfertilised in the twentieth century through sewage. As a result, algal blooms developed and cyanobacteria especially began to appear en masse. Some of these organisms form toxins that can compromise the use of the lake water, and reduce the oxygen content in the lake affecting the fishes.

"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," said Thomas Posch, a limnologist from the University of Zurich.

The evaluation of this historical data on Lake Zurich has revealed that Burgundy blood algae have developed increasingly denser blooms in the last 40 years. Like many other cyanobacteria, Planktothrix contains toxins to protect itself from being eaten by small crabs.

The most important natural control of the cyanobacteria blooms occurs in the spring, once the entire lake has cooled down vastly during the winter. 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 stated.

"Unfortunately, we are currently experiencing a paradox. 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," Posch said.