Oxygen levels are rapidly declining in the world’s freshwater lakes. This trend, which is driven primarily by climate change, poses a threat to biodiversity and the quality of drinking water.
In lakes that were surveyed across the temperate zone, researchers found that oxygen levels have declined by 5.5 percent at the surface and by 18.6 percent in deep waters since 1980.
Across some of the more nutrient-polluted lakes, the experts found that surface oxygen levels increased as water temperatures crossed a threshold favoring cyanobacteria.
“All complex life depends on oxygen. It’s the support system for aquatic food webs. And when you start losing oxygen, you have the potential to lose species,” said study co-author Professor Kevin Rose of the Rensselaer Polytechnic Institute. “Lakes are losing oxygen 2.75 – 9.3 times faster than the oceans, a decline that will have impacts throughout the ecosystem.”
The researchers analyzed more than 45,000 dissolved oxygen levels and temperature profiles that were collected from nearly 400 lakes worldwide.
Study lead author Stephen F. Jane said the changes are concerning both for their potential impact on freshwater ecosystems and for what they suggest about environmental change in general.
“Lakes are indicators or ‘sentinels’ of environmental change and potential threats to the environment because they respond to signals from the surrounding landscape and atmosphere,” said Jane.
“We found that these disproportionally more biodiverse systems are changing rapidly, indicating the extent to which ongoing atmospheric changes have already impacted ecosystems.”
The link between global warming and changing freshwater oxygen levels is controlled by mechanisms between surface water and deep water.
The study shows that as surface water temperatures increased by .38 degrees Centigrade per decade, dissolved oxygen concentrations declined by .11 milligrams per liter.
“Oxygen saturation, or the amount of oxygen that water can hold, goes down as temperatures go up. That’s a known physical relationship and it explains most of the trend in surface oxygen that we see,” said Professor Rose.
At the same time, some lakes experienced increasing dissolved oxygen concentrations as the temperature rose. According to the researchers, these lakes tended to be more polluted with nutrient-rich runoff from agricultural and developed watersheds and have high chlorophyll concentrations.
“The fact that we’re seeing increasing dissolved oxygen in those types of lakes is potentially an indicator of widespread increases in algal blooms, some of which produce toxins and are harmful,” said Professor Rose. “Absent taxonomic data, however, we can’t say that definitively, but nothing else we’re aware of can explain this pattern.”
In deeper waters, temperatures have remained largely stable. The loss of oxygen in this part of the lake follows a more complex path that is most likely tied to increasing surface water temperatures and a longer warm period each year.
The combination of warming surface waters and stable deep-water temperatures means that the difference in density between these layers, known as “stratification,” is increasing.
“The increase in stratification makes the mixing or renewal of oxygen from the atmosphere to deep waters more difficult and less frequent, and deep-water dissolved oxygen drops as a result,” said Professor Rose.
Some bacteria thrive in environments without oxygen, such as those that produce methane. This indicates that lakes have the potential to release more methane as they lose oxygen.
“Ongoing research has shown that oxygen levels are declining rapidly in the world’s oceans. This study now proves that the problem is even more severe in fresh waters, threatening our drinking water supplies and the delicate balance that enables complex freshwater ecosystems to thrive,” said Dr. Curt Breneman.
“We hope this finding brings greater urgency to efforts to address the progressively detrimental effects of climate change.”
The study is published in the journal Nature.