Lakes in New Zealand and around the world are rapidly losing oxygen, scientists say, putting biodiversity and access to fresh drinking water at risk.
Since 1980, oxygen levels deep in 400 different lakes in temperate regions around the world - including Taupō, Tarawera and Brunner - have dropped significantly, according to new research published in Nature.
At the surface, they have on average 5.6 percent less than 40 years ago. In deep water, there's 18.6 percent less oxygen.
And in the few lakes where oxygen concentrations have risen, it's been accompanied by blooms of toxin-releasing cyanobacteria.
"All complex life depends on oxygen. It's the support system for aquatic food webs," said Kevin Rose, freshwater ecologist at the Rensselaer Polytechnic Institute in New York.
And when you start losing oxygen, you have the potential to lose species. Lakes are losing oxygen 2.75-9.3 times faster than the oceans, a decline that will have impacts throughout the ecosystem."
While lakes only cover about 3 percent of the surface of the Earth, they "contain a disproportionate concentration of the planet's biodiversity".
"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 lead author Stephen Jane, also of Rensselaer.
"We found that these disproportionally more biodiverse systems are changing rapidly, indicating the extent to which ongoing atmospheric changes have already impacted ecosystems."
Warm water can't hold as much oxygen, which explains the declining concentration at the surface.
But deeper down, where oxygen losses have been far more extreme, temperatures have remained stable. The scientists say that's because the temperature difference between surface and deep water has increased, resulting in "stratification" and less mixing between the two - so oxygen at the bottom isn't replenished.
"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 Dr Rose.
Less oxygen means less life that relies on it to thrive - but an increase in bacteria that don't need it, resulting in more methane emissions, which further drive climate change and warming, the study says.
"Ongoing research has shown that oxygen levels are declining rapidly in the world’s oceans," said Curt Breneman, dean of the Rensselaer School of Science.
"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."