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Global Warming Prevents Water Turnover in Lake Zurich

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Algal blooms of the cyanobacteria, Planktothrix rubescens, in Lake Zurich.
Credit: UZH

The University of Zurich reports that there is insufficient water turnover in Lake Zurich because of global warming. This phenomenon favors the growth and proliferation of the harmful Burgundy blood algae in the lake.

Water turnover usually occurs during the winter when the entire lake cools down. Intensive winds trigger the turnover or exchange of the surface and deep waters.

This process is beneficial to the lake ecosystem since it is the mechanism that transports fresh oxygen into the deep. It is also the natural method of controlling the population of the cyanobacteria, Planktothrix rubescens, which is commonly known as Burgundy blood algae.

During the turnover, the cyanobacteria are transported to the deep waters where they die from the rapid rise in pressure. The pressure can reach up to 13 bars at a depth of 130 meters.

Thomas Posch, a limnologist from the University of Zurich, reported that the natural cycle of water turnover in Lake Zurich had been disrupted. He analyzed over 40 years of data from the Zurich Water Supply, and the result of his study was published in the July 8 issue of Nature Climate Change.

According to Posch, global warming causes rising water temperatures at the surface of the lake. The current value is between 0.6 to 1.2 degrees Celsius above the 40-year temperature average. This results to a temperature difference between the surface and deep waters that acts as a physical barrier to a complete water turnover in the lake.

The consequences of this insufficient turnover are twofold: uncontrolled proliferation of the Burgundy blood algae, and oxygen deficit in deep waters.

Another factor that favors the growth of the cyanobacteria is the overfertilization of the lake water from sewage discharge. This alters the nutrient ratio; specifically it increases the nitrogen load of the water to favor massive algal blooms.

This poses serious health threats since cyanobacteria produce harmful toxins. The water must undergo intensive treatment and purification to be a source of drinking water.

It likewise poses an environmental threat to the local aquatic life. The danger is not only due to the presence of toxins, it is also due to possible oxygen depletion when the algal blooms die.

Efforts are under way to control the discharge of waste into the lake, but the problem of water turnover is still an issue. Posch said that an extremely cold and windy winter could help counter the effects of global warming.

Fortunately, the winter of 2011/2012 was an answered prayer and caused drastic lowering of temperatures that allowed a successful water turnover. For the time being, it reduced the Burgundy blood algae population in Lake Zurich.

[via phys.org]

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