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As Earth's climate continues to warm, oxygen supplies in freshwater lakes across the planet are steadily dwindling.
According to a new study, published Wednesday in the journal Nature, surface level oxygen levels in major freshwater lakes have dropped an average of 5.5% over the last 40 years. In deep water, oxygen levels have dropped 18.6%.
"Lakes are losing oxygen 2.75 to 9.3 times faster than the oceans, a decline that will have impacts throughout the ecosystem." study co-author Kevin Rose said in a press release.
"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 Rose, a professor of freshwater ecology at Rensselaer Polytechnic Institute.
There are, however, exceptions to the trend.
In some lakes, rising temperatures and nutrient pollution caused by agricultural runoff have combined to fuel cyanobacteria blooms that can concentrate oxygen at the surface of freshwater lakes. However, these same blooms can leach toxins that harm local biodiversity.
For the study, scientists synthesized more than 45,000 oxygen and temperature profiles collected from 400 different lakes over the last 80 years.
Most of the measurements were collected from lakes within the temperature zone, the latitudinal bands stretching between 23 to 66 degrees, both north and south of Equator.
Lakes account for just 3 percent of the planet's land area, but they hold an outsized proportion of Earth's biodiversity.
Across the globe, freshwater lakes big and small have become increasingly plagued by not only hypoxia, but increased nutrient pollution, sedimentation and invasive species.
Compared to flora and fauna in other ecosystems, freshwater species have suffered considerable losses, with almost a third of freshwater species at risk of extinction.
"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 F. Jane, who recently earned a doctoral degree with Rose.
"We found that these disproportionally more biodiverse systems are changing rapidly, indicating the extent to which ongoing atmospheric changes have already impacted ecosystems," Jane said.
At the surface, the explanation for oxygen losses are rather simple. Warmer water can hold less oxygen -- it's just a matter of physics.
But the survey showed that in some lakes, surface level oxygen levels have spiked as cyanobacteria blooms proliferate.
"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," Rose said. "Absent taxonomic data, however, we can't say that definitively, but nothing else we're aware of can explain this pattern."
Oxygen levels have declined more drastically deep beneath the surface of Earth's major lakes. Scientists suspect a decrease in density differences between the top and bottoms of lakes -- yielding increased stratification -- is to blame.
Increased stratification means less mixing, and mixing is the only way for oxygen to make its way into deep water.
Changes in freshwater oxygen levels don't just influence biodiversity and ecology, but can also affect important greenhouse gas cycles.
"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 Curt Breneman, dean of the School of Science at RPI.
"We hope this finding brings greater urgency to efforts to address the progressively detrimental effects of climate change," Breneman said.
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