Lakes Harmed From Global Warming
July 17, 2012

Lakes Harmed From Global Warming

Brett Smith for - Your Universe Online

As average temperatures across the globe have ticked up, toxic blood-red algae are thriving in central European lakes–according to a new study out of the University of Zurich.

In a report published in Nature Climate Change, Swiss researchers assert that the warmest winters the country has seen in the past 40 years hampered the seasonal die-off of Burgundy blood-red algae, a photosynthetic bacterium that has bloomed en masse recently.

The microorganisms´ metabolism results in the accumulation of toxic waste that can compromise the quality of the lake water. In addition, the dying blooms consume vast quantities of oxygen, resulting in the reduction of oxygen content for the fish living that inhabit the same lake.

Scientists involved in the research also pointed to the levels of oxygen, phosphorous, and nitrogen in the lakes as the reason why the algae are flourishing. These levels were affected greatly by human activities, particularly the area´s civic sewage system, over the course of the last century. Clean-up efforts have reversed much of this damage, yet the toxic algae continue to thrive.

"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,” explains Thomas Posch, a limnologist from the University of Zurich, who based his study on 40 years´ worth of data.

The Zurich Water Supply, which systematically removes the organism and its toxins from the lake water, worked in collaboration with Posch to determine that the blood algae they see has developed increasingly denser blooms over the past 40 years.

The research team said the main reason for the algae´s aggressive takeover is the increasing temperatures throughout central Europe. A key negative pressure on the bacteria occurs in the spring, when the entire lake that has cooled during the winter begins to thaw. After the lake thaws, brisk winds trigger the cycling of the warmer surface water and cooler deep water. If the turnover of surface water is complete, many algae die off in the deep waters of the lake as they cannot withstand the higher pressure that exists there. Another positive effect of this turnover is the transportation of fresh oxygen to the deep waters where the lake´s fish typically reside.

However, because global warming causes warmer temperatures at the water surface and the central European winters were increasingly becoming too warm, the lakes have not been able to turn over fully as the temperature difference between the surface and depths posed a physical barrier. The results are larger oxygen deficits in the lake´s deeper waters and an inadequate decrease of the Burgundy blood-red algae blooms.

“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,” says Posch.

Despite the mild winters of recent years, the past winter´s lower temperatures and heavy storms allowed the lake water to completely cycle through, resulting in a reduction in algae.