Coral reefs are increasingly exposed to low oxygen levels 

A recent study led by UC San Diego highlights the dangers faced by global coral reef systems due to ocean deoxygenation, which is the decline of oxygen in marine waters.

Although deoxygenation has been extensively researched, hypoxia in coral reefs is not well-studied. The current investigation is unprecedented because it explored hypoxia in 32 coral reef sites across Japan, Hawaii, Panama, Palmyra, and Taiwan.

“This study is unique because our lab worked with a number of collaborators to compile this global oxygen dataset especially focused on coral reefs – no one has really done that on a global scale before with this number of datasets,” said marine scientist Ariel Pezner, a postdoctoral fellow at the Smithsonian Marine Station in Florida.

A study of this scale is vital because deoxygenation is a serious threat to coral reefs and will become more relevant as oceans warm due to climate change. The results revealed that coral reef hypoxia is widespread.

The researchers discovered that 84 percent of the reefs in the study experienced “weak to moderate” hypoxia, and 13 percent experienced “severe” hypoxia.

“We were surprised to find that a lot of coral reefs are already experiencing what we would define as hypoxia today under current conditions,” added Pezner.

Due to the biological functions of the reefs, hypoxia is more severe at night, while many ocean creatures are respiring instead of producing oxygen.

“Imagine that you’re a person who is used to sea-level conditions, and then every night you have to go to sleep somewhere in the Rocky Mountains, where the air has less oxygen,” said biogeochemist Andreas Andersson.

“This is similar to what these corals are experiencing at nighttime and in the early morning when they experience hypoxia. And in the future, if the duration and intensity of these hypoxic events gets worse, then it might be like sleeping on Mount Everest every night.”

The researchers say that more studies are needed to determine baseline oxygen levels, track other hypoxia events, and to analyze how reefs are affected.

“Baseline oxygen conditions varied widely among our reef habitats, suggesting that a singular definition of ‘hypoxia’ may not be reasonable for all environments,” concluded Pezner. “Determining which thresholds are relevant will be important moving forward in making predictions about how reefs might change under warming and oxygen loss.”

The study is published in the journal Nature Climate Change.

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By Erin Moody, Earth.com Staff Writer

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