Led by researchers at the Scripps Institution of Oceanography, a new study has investigated low oxygen levels at 32 different sites and revealed that hypoxia is already pervasive on many reefs. The study, published March 16 in the journal Nature Climate Change, is the first to document oxygen conditions on coral reef ecosystems at this scale.
The overall decline of oxygen has been well documented, but hypoxia on coral reefs has so far been relatively underexplored. Although more research is needed to better understand the biological impacts on tropical corals and coral reefs, oxygen loss in the ocean is predicted to threaten marine ecosystems globally.
“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, now a postdoctoral fellow at the Smithsonian Marine Station in Florida. “We were surprised to find that a lot of coral reefs are already experiencing what we would define as hypoxia today under current conditions.”
Pezner and colleagues used autonomous sensor data to explore oxygen variability and hypoxia exposure at 32 diverse reef sites across 12 locations in waters off Japan, Hawaii, Panama, Palmyra, Taiwan, and elsewhere. As the researchers expected, oxygen was lowest in the early morning at all locations and highest in the mid-afternoon as a result of night-time respiration and daytime photosynthesis, respectively. During the day when primary producers on the reef have sunlight, they photosynthesize and produce oxygen, said Pezner. But at night, when there is no sunlight, there is no oxygen production and everything on the reef is respiring—breathing in oxygen and breathing out carbon dioxide—resulting in a less oxygenated environment, and sometimes a dip into hypoxia. This is a normal process, said Andersson, the study’s senior author, but as ocean temperature increases, the seawater can hold less oxygen while the biological demand for oxygen will increase, exacerbating this nighttime hypoxia.
The researchers found that as global temperatures continue to rise and marine heatwaves become more frequent and severe, low oxygen conditions on coral reefs are likely to become more common. Using projections adopted from climate models, the team calculated that by the year 2100, the total number of hypoxic observations on these reefs will increase under all warming scenarios, ranging from an increase of 13 to 42 percent under one scenario to 97 to 287 percent under a more extreme scenario relative to now.
“Baseline oxygen conditions varied widely among our reef habitats, suggesting that a singular definition of ‘hypoxia’ may not be reasonable for all environments,” said Pezner. “Determining which thresholds are relevant will be important moving forward in making predictions about how reefs might change under warming and oxygen loss.”
Image: Coral reefs at a study site off Taiping Island, South China Sea. Credit: Yi Bei Liang
Journal information: Nature Climate Change