Global warming is reducing oxygen levels in freshwater lakes in the temperate belt. This threatens biodiversity and drinking water quality, an international team of scientists reports.
The researchers analyzed more than 4,500 temperature and oxygen profiles for nearly 400 lakes around the world, collected since 1941. Most of the long-term records were collected in a temperate climate zone spanning from 23 to 66 degrees north and south latitude.
Lakes were found to lose oxygen 2.75 to 9.3 times faster than oceans. Since 1980, oxygen levels in the temperate lakes surveyed have decreased by 5.5 percent at the surface and 18.6 percent near the bottom. This is due to an increase in freshwater temperature, which has increased by 0.38 degrees Celsius over a decade. Dissolved oxygen concentrations in surface waters have decreased by 0.11 milligrams per liter over the same time.
Although lakes make up only about three percent of the Earth’s land surface, they contain a disproportionately high concentration of the planet’s biodiversity. Rising temperatures also promote the proliferation of blue-green algae (cyanobacteria), which release toxins that are dangerous to the existence of other living organisms. In such lakes, there is little oxygen at depth due to difficult mixing between the different layers, while on the surface the level of dissolved oxygen increases, which is a sign of cyanobacteria blooms.
Reduced oxygen levels can also lead to the proliferation of anaerobic bacteria that release methane. As a result, the lakes will emit even more greenhouse gases, which could exacerbate global climate change.