Freshwater Lakes Are Losing Vital Surface Oxygen as Global Heating Continues: Study

Shasta Lake, California’s largest water reservoir, seen at 30% capacity and under smoky conditions due to the Dixie and other fires burning in the area near Redding, California on Aug. 4, 2021. George Rose / Getty Images

Freshwater ecosystems need adequate levels of oxygen to maintain healthy biological communities and sustain life. But the increasing intensity and frequency of short-term heat waves, along with long-term climate warming, are significantly reducing the levels of dissolved oxygen (DO) on the surface of lakes across the globe, according to a new study.

The researchers quantified the impacts of continuous global heating and intensified heat waves on surface DO levels, a press release from the Chinese Academy of Sciences said.

“The persistent decline in dissolved oxygen (DO) concentrations, observed across diverse aquatic ecosystems since the mid-20th century, has prompted substantial concern. Global observations demonstrate a widespread decline (2%) in DO concentrations across the open oceans, leading to the proliferation of ‘dead zones,’ areas characterized by very low DO,” the authors of the findings wrote.

The study was led by Professors Zhang Yunlin and Shi Kun at the Chinese Academy of Sciences’ Nanjing Institute of Geography and Limnology, in collaboration with scientists from Nanjing University and Bangor University in the United Kingdom.

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The research team analyzed surface DO variations of more than 15,000 lakes worldwide over the last two decades. They found that there had been a widespread decline in concentrations of surface DO, with 83 percent of lakes studied exhibiting significant deoxygenation.

“Adequate oxygen levels are critical for sustaining aerobic life and fostering robust biological communities. A decrease in DO concentrations results in substantial consequences, including reduced nitrogen fixation, increased emissions of N2O — a potent greenhouse gas, limitations on habitat suitability and productivity for oxygen-demanding organisms, as well as having adverse impacts on food security, livelihoods, and coastal economies,” the authors wrote in their findings.

The team also explored the roles of eutrophication — nutrient overgrowth in a body of water, such as those that lead to algae and plankton blooms — and climate warming in shaping the accumulation of surface DO.

“[E]levated lake water temperature can potentially influence DO concentrations by stimulating the growth of aquatic vegetation and phytoplankton, consequently enhancing both oxygen consumption and production rates,” the authors wrote.

The findings suggest that climate warming contributes to 55 percent of the world’s surface deoxygenation by reducing oxygen solubility. Increased eutrophication is responsible for roughly 10 percent of total surface oxygen loss globally.

“In lakes and oceans, surface warming also impedes vertical oxygen transport from the surface, where it is typically highest, into deeper waters by strengthening and prolonging thermal stratification. This disruption in vertical mixing can lead to a critical depletion of DO in bottom waters,” the authors wrote.

The researchers examined heat wave trends and quantitatively assessed how they impact surface DO levels. They found that heat waves have pronounced and rapid effects on the decline of surface DO, causing a 7.7 percent surface DO reduction compared with average temperature conditions.

“These findings underscore the profound impact of climate change on freshwater ecosystems, emphasizing the urgent need for mitigation and adaptation strategies to preserve lake ecosystems worldwide. The study provides crucial insights for policymakers and environmental managers working to combat the escalating threat of freshwater deoxygenation,” the press release said.

The study, “Climate warming and heatwaves accelerate global lake deoxygenation,” was published in the journal Science Advances.

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