Global warming also affects lakes. Based on the example of Lake Zurich, researchers from the University of Zurich demonstrate that there is insufficient water turnover in the lake during the winter and harmful Burgundy blood algae are increasingly thriving. The warmer temperatures are thus compromising the successful lake clean-ups of recent decades.
Many large lakes in Central Europe became heavily overfertilized in the twentieth century through sewage. As a result, algal blooms developed and cyanobacteria (photosynthetic bacteria) especially began to appear en masse. Some of these organisms form toxins that can compromise the use of the lake water. Dying algal blooms consume a lot of oxygen, thereby reducing the oxygen content in the lake with negative consequences for the fish stocks.
The problem with overfertilization was not merely the absolute amount of oxygen and phosphorus, the two most important nutrients for algae. Mankind has also changed the ratio between the two nutrients: The phosphorus load in lakes has been reduced vastly in recent decades, yet pollution with nitrogen compounds has not decreased on the same scale. The current ratio between the nutrients can thus trigger a mass appearance of certain cyanobacteria, even in lakes that have been deemed “restored”.
“…became heavily overfertilized in the twentieth century through sewage. ”
This is not a consequence of global warming; this is a consequence of the prosperity that technology has brought, allowing populations to triple in the last 10o years or so:
en.wikipedia.org/wiki/Canton_of_Zurich#Historical_population
Actually, it has a lot to do with rising lake temperatures, just the editor decided not to include that part in the blurb. Another paragraph in the larger article explains:
But why does Planktothrix increasingly thrive? 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. If the turnover is complete, many cyanobacteria die off in the deep waters of Lake Zurich as they cannot withstand the high pressure, which is still 13 bars at depths of 130 meters. Another positive effect of this turnover is the transportation of fresh oxygen to the deep. However, the situation in Lake Zurich has also changed drastically in the last four decades. Global warming causes rising temperatures at the water surface. The current values are between 0.6 and 1.2 degrees Celsius above the 40-year average. The winters were increasingly too warm and the lake water was not able to turn over fully as the temperature difference between the surface and depths posed a physical barrier. The consequences are larger oxygen deficits for a longer period in the lake’s deep water and an insufficient reduction of the Burgundy blood algae blooms.
(from the original article: http://phys.org/news/2012-07-global-lakes.html#jCp)
Blowing up hydro-electric and flood control dams doesn’t help lakes a whole lot either.