Coral Reef Oxygen Loss Revealed by Researchers in a New Study

Coral reef oxygen loss is a sign that the entire underwater world is experiencing ocean warming. A new study provides an unprecedented examination of oxygen loss in coral reefs. Researchers at UC San Diego’s Scripps Institution of Oceanography and a large team of national and international colleagues led the study and captured a state of hypoxia, or low oxygen levels, that was widespread on coral reefs at 32 different locations.

The decrease in oxygen content that occurs throughout the world’s oceans and coastal waters, known as oceanic deoxygenation, is well documented, but hypoxia that occurs in reefs is less explored. Loss of oxygen in the ocean is predicted to threaten marine ecosystems, although more research is needed to understand the biological impact on tropical corals and coral reefs.

Coral reef oxygen loss was found during the research, and predicted to worsen if ocean temperatures continue to warm due to climate change. They also show that projected ocean warming and deoxygenation will increase the intensity, duration, and rate of severe hypoxia on coral reefs by 2100.

Pezner and colleagues using autonomous sensor data to explore oxygen variability and hypoxic exposure at 32 diverse reef sites in 12 sites in waters off Japan, Hawaii, Panama, Palmyra, Taiwan, and elsewhere. SeapHOX sensors and other autonomous sensors are installed in coral reef habitats to measure salinity, pH, temperature and oxygen levels every 30 minutes.

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Historically in the 1950s, hypoxia had been defined by a very specific concentration limit in water of less than two milligrams of oxygen per liter, but researchers say that one universal threshold does not apply to all ecosystems, they found four different hypoxia thresholds namely, weak (5 mg/L), mild (4 mg/L), moderate (3 mg/L), and severe hypoxia (2 mg/L).

According to this threshold, it was found that more than 84% of the coral reefs in this study experienced weak to moderate hypoxia, and 13% experienced severe hypoxia.

Pezner said the results of their research were that oxygen was lowest in the morning at all locations and highest in the afternoon as a result of nighttime respiration and photosynthesis during the day. At night, there is no sunlight, no oxygen production, everything in the coral reef breathes in oxygen and exhales carbon dioxide resulting in an oxygen-poor environment. This process is considered normal, said Andersson, the study’s senior author.

The study involved a total of 22 authors representing 14 different research organizations and universities including UC San Diego and other universities. The National Science Foundation funded this research, and the National Science Foundation Graduate Research Fellowship and the Philanthropic Educational Organization (P.E.O.) support Pezner’s graduate studies

Source: UC San Diego