Global warming: Lake ecosystems feeling the heat - Summit County Citizens Voice

Increasing bacteria growth adds to treatment challenges

Switerland’s Lake Zurich is seeing increased levels of cyanobacteria, in part because of global warming. Photo via Wikipedia and the Creative Commons.

By Summit Voice

SUMMIT COUNTY â€" Steadily climbing temperatures pose a significant threat to lake ecosystems, according to researchers with the University of Zurich.

Warming has led to increased stratification and inadequate turnover in Lake Zurich during the winter, causing persistent blooms of harmful cyanobacteria. In the case of Lake Zurich, the changes are threatening to undo recent of lake cleanup efforts, the scientists said.

Like many other large lakes in Europe, Lake Zurich was polluted by sewage during the 20th century. The nitrogen-rich waste resulted in massive algal blooms that started to kill the lake ecosystem by eutrophication.

“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. In collaboration with Zurich Water Supply, he analyzed 40 years’ worth of data in a study that has just been published in Nature Climate Change.

The study shows that a certain strain of  cyanobacteria (Planktothrix rubescens), more commonly known as Burgundy blood algae, has 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.

Burgundy blood algae were first described in Lake Zurich in 1899 and are a well-known phenomenon for Zurich Water Supply. Consequently, the lake water is treated extensively to remove the bacteria and its toxin.

In addition to the mix of nitrogen and phosphorus, global warming plays a key role in the algal blooms.

The researchers explained that the most important natural control mechanism for the cyanobacteria is the annual turnover of lake waters that occurs in many lakes each spring. After the lake cools off in the winter, spring winds help drive the cold water down into the deep.

If the turnover is complete, many cyanobacteria die off in the pressure of deeperwater. The turnover also helps push oxygen into the depths. But at Lake Zurich, the dynamics have changed drastically in the last 40 years, with surface temperatures anywhere between .6 and 1.2 degrees Celsius above the 40-year average.

With warmer winter temperatures, the lake hasn’t been turning over completely, with the increasing gradient between surface temperatures and deep-water temperatures creating a physical barrier.

That results in a growing oxygen deficit in the deeper waters and enables the cyanobacteria to persist in the surface layers.

“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,” said Posch.

As far as the researchers are concerned, the winter of 2011/12 was just what the doctor ordered: The low temperatures and heavy storms allowed the lake to turn over completely and ultimately resulted in a reduction in Planktothrix.

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Filed under: climate and weather, Environment, global warming, water Tagged: | Algal bloom, climate change, Cyanobacteria, global warming, Lake Zurich, University of Zurich