The world's lakes are rapidly losing oxygen as the planet warms.

New research shows oxygen levels in the world’s temperate freshwater lakes are declining rapidly — faster than in the oceans.

Experts say the trend is driven largely by climate change, and could threaten freshwater biodiversity and drinking water quality.

The study published in the journal Nature found that oxygen levels in surveyed lakes across the temperate zone have declined 5.5 per cent at the surface and 18.6 per cent in deep waters since 1980. 

Meanwhile, in a large subset of mostly nutrient-polluted lakes, surface oxygen levels increased as water temperatures crossed a threshold favoring cyanobacteria, which can create toxins when they flourish in the form of harmful algal blooms.

Researchers analysed a combined total of over 45,000 oxygen and temperature profiles collected since 1941 from nearly 400 lakes around the world. Most of the long-term records were collected in the temperate zone, which spans 23 to 66 degrees north and south latitude. 

In addition to biodiversity, the concentration of dissolved oxygen in aquatic ecosystems influences greenhouse gas emissions, nutrient biogeochemistry, and ultimately, human health.

“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,” says researcher Stephen F Jane.

“We found that these disproportionally more biodiverse systems are changing rapidly, indicating the extent to which ongoing atmospheric changes have already impacted ecosystems,” he said.

Although widespread losses in dissolved oxygen across the studied lakes are linked to climate change, the path between warming climate and changing freshwater oxygen levels is driven by different mechanisms between surface and deep waters.

Deoxygenation of surface waters was mostly driven by the most direct path: physics. 

As surface water temperatures increased by 0.38 degrees Centigrade per decade, surface water dissolved oxygen concentrations declined by 0.11 milligrams per litre per decade.

“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 lead researcher Dr Kevin Rose.

The loss of oxygen in deeper waters, where water temperatures have remained largely stable, follows a more complex path most likely tied to increasing surface water temperatures and a longer warm period each year. 

Warming surface waters combined with stable deep-water temperatures means that the difference in density between these layers, known as “stratification,” is increasing. The stronger this stratification, the less likely mixing is to occur between layers. 

The result is that oxygen in deep waters is less likely to get replenished during the warm stratified season, as oxygenation usually comes from processes that occur near the water surface.

When oxygen levels decline, bacteria that thrive in environments without oxygen, such as those that produce the powerful greenhouse gas methane, begin to proliferate. 

This suggests the potential that lakes are releasing increased amounts of methane to the atmosphere as a result of oxygen loss. Additionally, sediments release more phosphorous under low oxygen conditions, adding nutrients to already stressed waters.