By Gary Thomas
According to Thomas Posch, a limnologist at the University of Zurich, changes in nutrient ratios and water temperature, the two key lake properties, are a major issue that triggers the growth of adverse Burgundy blood algae and affects natural clean-ups of lakes in recent decades.
The cyanobacteria Planktothrix rubescens (Burgundy blood algae) in Lake Zurich. The threads are only 0.005 by two millimeters in size, but primarily form a mass presence at a water depth of 12 to 15 meters.
The findings are based on a study of a 40-year data on Lake Zurich by Posch in partnership with Zurich Water Supply. The study has been reported in the journal, Nature Climate Change. The data demonstrates that the Burgundy blood algae or cyanobacteria Planktothrix rubescens, which release toxin for protection, have grown heavily in the past four decades.
Mankind has reduced the load of phosphorus in recent decades, however are not able to reduce the load of nitrogen compounds on the same level, causing changes in nutrient ratios. This activates the growth of Burgundy blood algae.
The key natural control of the Burgundy blood algae happens in the spring after the cooling down of the entire lake during the winter. Powerful winds activate the turnover of the surface water and deep water, causing the death of majority of cyanobacteria in the deep water because of high pressure of 13 bars at 130-m depth. This turnover also results in fresh oxygen transportation into the deep. On the other hand in the past 40 years, the scenario in Lake Zurich has also altered significantly. Global warming has increased surface temperatures of lakes, causing a 0.6-1.2° C increase from the 40-year average.
The winters have become increasingly warmer, while the temperature difference between surface water and deep water creates a physical barrier, affecting the turnover o the lake water. These changes will affect the natural control of the Burgundy blood algae blooms and will result in oxygen deficit in deep water, which in turn will have negative impact on fish stocks.
Although the nutrient problem is believed to be partly solved, global warming affects clean-up in some lakes. Hence, cold winters and powerful winds are necessary, concluded Posh.