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”.