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California's Fannette Island in at Lake Tahoe, which straddles the border of California and Nevada. Scientists reported last year that climate change has affected the water temperature and its clarity, per CBS News. Photo: George Rose/Getty Images

Oxygen levels in hundreds of freshwater lakes in the U.S. and around the world are plummeting — and climate change is largely to blame, according to a study published Wednesday.

Why it matters: Per a statement from study co-author Kevin Rose, a professor of biology at Rensselaer Polytechnic Institute: "All complex life depends on oxygen. ... when you start losing oxygen, you have the potential to lose species."

Driving the news: The study published in the journal Nature, analyzed temperature and dissolved oxygen — a measure of how much oxygen is in water — in almost 400 lakes across the temperate zone, mostly in the U.S. and Europe. Several lakes in New Zealand and one in Japan were also examined.

  • "Warmer surface water temperatures caused by climate change reduce oxygen levels due to lower solubility of oxygen in warmer water," said Rose in an email to Axios Wednesday night.

What they found: The researchers discovered that since 1980, oxygen levels have dropped 5.5% at the surface and 18.6% in deep waters. The fall is 2.75 to 9.3 times faster than the world's oceans.

  • Warming temperatures are leading to widespread losses in dissolved oxygen across the studied lakes, linked to climate change and human activity.
  • Some lakes saw increasing dissolved oxygen concentrations and warming temperatures, caused by pollution such as agricultural runoff.

Between the lines: "When there is no oxygen in deep waters, methane is produced, which is a greenhouse gas that is about 23 times as powerful as carbon dioxide," Rose noted to Axios.

  • A lack of oxygen in deeper waters "permits nutrients to come out of the nutrient-rich sediments, increasing algal growth, and potentially contributing to harmful algal blooms of toxic cyanobacteria," Rose added.

Our thought bubble, via Axios' Andrew Freedman: Just as global warming is fundamentally altering marine ecosystems, making ocean waters more acidic, increasing temperatures, and altering ocean currents, it's now clear that sweeping changes are happening in water bodies on land.

  • At risk are ecosystems long dependent on a particular temperature and oxygen structure within a lake. In other words, fish and other marine creatures are facing challenges wherever scientists look.

Of note: Researchers have previously documented dissolved oxygen losses in lakes over a sustained period, but "none" have surveyed so many lakes around the globe, noted Samuel Fey, a Reed College biology professor who studies lakes and wasn't involved in this study, to AP.

The bottom line: Rose told Axios that low oxygen can lead to fish kills, and cold-water fish are particularly sensitive.

  • "This is a particularly severe problem in regions where local populations depend on fish as a primary source of protein," he said.

Editor's note: This article has been updated with comment from Rose.

Go deeper

May 31, 2021 - World

Study: Climate change directly linked to over a third of global heat deaths

An elderly woman suffering from a heatstroke in a hospital in Karachi, Pakistan in 2015. Photo: Sabir Mazhar/Anadolu Agency/Getty Images

More than one-third of heat deaths each year can be directly linked to climate change, according to a study published Monday in the journal Nature Climate Change.

Why it matters: The findings require global actors to take "more ambitious mitigation and adaptation strategies to minimize the public health impacts of climate change," the study said.

Updated 33 mins ago - Sports

In photos: Tokyo Olympics day 6 highlights

An underwater view of swimmers including Team USA's Caeleb Dressel (R) in the final of the Olympic Tokyo Games men's 100m freestyle on July 29. Photo: François-Xavier Marit/AFP via Getty Images

There's been plenty of Olympics drama on day six of the Tokyo Games Thursday — notably China's women's swimming team beating the U.S. and Australia in the record-setting 4x200-meter freestyle relay.

The big picture: Katie Ledecky helped the U.S. win silver, which also beat the previous world record smashed by China's team. Team USA grabbed two more swimming gold medals, when Caeleb Dressel won the men's 100m freestyle and Bobby Finke triumphed in the first men's Olympic 800m freestyle.

Updated 1 hour ago - Sports

Olympics dashboard

Team USA's Simone Biles during the women's team final on day four of the Tokyo 2020 Olympic Games at Ariake Gymnastics Centre on Tuesday in Japan. Photo: Fred Lee/Getty Images

🤸🏾‍♀️: Simone Biles reacts to "love and support" after withdrawing from all-around gymnastics and team finals, citing her mental health

🏊‍♂️: Caeleb Dressel wins gold in men's 100m freestyle —Bobby Finke wins gold in first men's Olympic 800m freestyle

📷: In photos: Tokyo Olympics day 6 highlights

🗓: The Olympic events to watch today

🎾: "This one sucks more than the others," Naomi Osaka says on upset loss

🏃‍: Female Olympians push back against double standard in uniforms

Go deeper: Full Axios coverage - Medal tracker

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