World - Majority of coral reefs examined suffer from low oxygen levels, study says
March 16 (UPI) -- A vast majority of 32 coral reefs examined around the world are suffering from low oxygen levels because of global warming, and the threat to marine ecosystems is expected to get worse, according to a study published Thursday.
The new research is documented in the journal Nature Climate Change, and its authors said this is the first time the current state of low oxygen, or hypoxia, was explored around the world.
The authors, led by researchers from the University of California-San Diego's Scripps Institute of Oceanography, found hypoxia pervasive on many coral reefs, demonstrating the overall decline in oxygen content across the world's oceans and coastal waterways.
A large team of national and international researchers joined the Scripps scientists in documenting the problem.
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"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," marine scientist Ariel Pezner, a post-doctoral fellow at the Smithsonian Marine Station in Florida, said in a news release.
"We were surprised to find that a lot of coral reefs are already experiencing what we would define as hypoxia today under current conditions."
Ocean deoxygenation is predicted to threaten marine ecosystems globally. However, current and future oxygen concentrations and the occurrence of hypoxic events on coral reefs remain underexplored. Here, using autonomous sensor data to explore oxygen variability and hypoxia exposure at 32 representative reef sites, we reveal that hypoxia is already pervasive on many reefs. Eighty-four percent of reefs experienced weak to moderate (≤153 µmol O2 kg−1 to ≤92 µmol O2 kg−1) hypoxia and 13% experienced severe (≤61 µmol O2 kg−1) hypoxia. Under different climate change scenarios based on four Shared Socioeconomic Pathways (SSPs), we show that projected ocean warming and deoxygenation will increase the duration, intensity and severity of hypoxia, with more than 94% and 31% of reefs experiencing weak to moderate and severe hypoxia, respectively, by 2100 under SSP5-8.5. This projected oxygen loss could have negative consequences for coral reef taxa due to the key role of oxygen in organism functioning and fitness.
Researchers used models of four different climate change scenarios to show that projected ocean warming and deoxygenation will substantially increase the duration, intensity and severity of hypoxia on coral reefs by 2100.
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Pezner and other participants used autonomous sensor data to explore oxygen variability and hypoxia exposure at the 32 diverse reef sites across 12 locations off the coasts of Japan, Hawaii, Panama, Palmyra and Taiwan, among others.
Hypoxia in the past has been defined by a very specific concentration cutoff of oxygen in the water, often less than 2 milligrams of oxygen per liter. That threshold was determined in the 1950s.
The researchers explored the possibility of four different hypoxia thresholds because of the different environments, including weak, mild, moderate and severe hypoxia.
Based on the new categories, they found that more than 84% of the reefs in this study experienced "weak to moderate" hypoxia and 13% experienced "severe" hypoxia at some point during the research.
The authors found that oxygen was lowest in the early morning at all locations and highest in the mid-afternoon as a result of nighttime respiration and daytime photosynthesis, respectively.