Lakes in temperate regions are facing a decline in oxygen levels that’s happening at a faster rate than the oxygen loss in the world’s oceans, according to a new study that measured levels over the last 76 years. This trend poses risk to the biodiversity around the lake, the humans who use the lakes’ water, and is also linked to a rise in greenhouse gas emissions.

Lakes are rich ecosystems, serving as habitats for several species (both aquatic and land-based wildlife, and also include birds). Research examining 393 lakes in temperate regions around the world from 1941-2017 found a widespread decline in oxygen, which is present as a dissolved gas in water, on both surface and deepwater regions. The findings, published in Nature, showed a 5.5% decline in dissolved oxygen and an 18.6% decline in dissolved oxygen in deep waters.

Dissolved oxygen in surface water dropped because of climate change — global heating has resulted in higher temperatures and resultantly warmer air. This phenomenon heats the top layer of the lake and makes it harder for gases to dissolve in water — causing oxygen to become scarce. Deep water dissolved oxygen levels, however, are much lower because water layers, classified by differences in temperature, can’t mix due to the surface water’s high temperature.

Oceans also experience similar deoxygenation due to the inability of several water layers to mix. The current study alarmingly shows the oxygen drop in lakes is happening at a faster rate than in oceans.

“All complex life depends on oxygen,” environmental biologist and study author Kevin Rose from the Rensselaer Polytechnic Institute, said in a statement. “It’s the support system for aquatic food webs. And when you start losing oxygen, you have the potential to lose species.”

Related on The Swaddle:

Hundreds of Fish Died in a Bengaluru Lake Due to Industrial Waste

The loss begins with a change in the lake’s chemical, physical, geological, and biological processes. This change will slowly alter the lake’s aquatic ecosystems: killing hypoxia-intolerant species (species that can’t survive without oxygen) like most fish and giving an advantage to hypoxia-tolerant species like microbes, jellyfish, and squid. Further, the changing chemistry of the lake might also produce a rise in populations of aquatic microbes that produce methane, which will increase the world’s greenhouse gas burden. Researchers have previously proved that half of the world’s methane emissions come from aquatic ecosystems — like flooded rice paddies, aquaculture ponds, lakes, and wetlands.

Lakes are also useful for human settlements, providing water for a variety of purposes ranging from drinking to fishing. “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,” aquatic ecologist Stephen Jane, from the Rensselaer Polytechnic Institute, said in a statement.

The wavering oxygen levels will go on to affect land animals, birds, and human reliance on the lake. More dead fish and more dead algae make the water undrinkable and the fish unconsumable, driving away animal and bird species, and further destroying human livelihoods.

“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 freshwaters, threatening our drinking water supplies and the delicate balance that enables complex freshwater ecosystems to thrive,” Curt Breneman, dean of the Rensselaer Polytechnic Institute School of Science, said in a statement.

“We hope this finding brings greater urgency to efforts to address the progressively detrimental effects of climate change.”