Low oxygen conditions on coral reefs could worsen by 287% by 2100: Study

NEW DELHI: Using climate models, scientists have predicted a significant increase in hypoxia, or low oxygen conditions, in coral reefs by 2100 under all warming scenarios. According to them, the increase ranges from 13 to 42 percent under one scenario, and from 97 to 287 percent under an extreme scenario relative to the present.
Scientists say hypoxia is becoming more common due to rising global temperatures and more frequent and intense heat waves at sea.
An international team of researchers led by the Scripps Institution of Oceanography of the University of California, San Diego (USA) determined the current state of hypoxia in 32 different regions of the globe and found that hypoxia is already widespread in many reefs.
Although ocean deoxygenation, which is the general reduction of oxygen in the world’s oceans and coastal waters, is well documented, hypoxia in coral reefs is relatively poorly studied.
Loss of ocean oxygen is predicted to threaten global marine ecosystems, although further research is needed to better understand the biological impact on tropical corals and coral reefs.
The research is claimed to provide an unprecedented study of the loss of oxygen in coral reefs around the globe as the ocean warms. It was published in the journal Nature Climate Change.
The authors found that hypoxia is already occurring in some reef habitats and will worsen if ocean temperatures continue to warm due to climate change.
They also used models of four different climate change scenarios to predict that by 2100, ocean warming and oxygen depletion will significantly increase the duration, intensity and severity of hypoxia on coral reefs.
The analysis was conducted by a marine scientist Ariel Pezner While a PhD student Scripps Oceanography.
Pezner and colleagues used autonomous sensor data to study oxygen variability and the effects of hypoxia at 32 different reef sites in 12 locations in waters off Japan, Hawaii, Panama, Palmyra, Taiwan and elsewhere. These sensors measured temperature, salinity, pH, and oxygen levels every 30 minutes.
Historically, hypoxia has been defined by the threshold of a very specific concentration of oxygen in water—less than two milligrams per liter (mg/L)—a threshold established in the 1950s.
The researchers noted that one universal threshold may not apply to all environments or all reefs or all ecosystems, so they explored the possibility of four different hypoxia thresholds: weak (5 mg/L), mild (4 mg/L), moderate (3 mg/L ), and severe hypoxia (2 mg/l).
Based on these thresholds, they found that more than 84 percent of the reefs in this study experienced “mild to moderate” hypoxia at some point during the data collection period, and 13 percent experienced “severe” hypoxia.
As the researchers expected, oxygen at all sites was lowest in the morning and highest during the day, as a result of nighttime respiration and daytime photosynthesis.
When the main producers on the reef have sunlight, they photosynthesize and produce oxygen, Pezner said.
But at night, when there is no sunlight, oxygen is not produced and everything in the reef breathes, it breathes in oxygen and emits carbon dioxide, resulting in an environment with less oxygen, sometimes hypoxia.
“This is a normal process, but as ocean temperatures rise, seawater can hold less oxygen, and the biological demand for oxygen increases, exacerbating nighttime hypoxia,” said the study’s senior author, a biogeochemist. Andreas AnderssonScripps Oceanography.
“Imagine being someone who’s used to sea-level conditions, and then every night you have to sleep somewhere in the Rocky Mountains, where the air is low in oxygen.
“This is similar to what happens when corals are exposed to hypoxia at night and in the morning,” Andersson said.
“And if the duration and intensity of these hypoxic events worsens in the future, it could be like sleeping on Mount Everest every night,” Andersson said.
Establishing baseline conditions through continuous and additional measurements of oxygen in coral reefs over different seasons and longer time scales is “imperative,” as a single definition of “hypoxia” may not be reasonable for all environments, the researchers said.

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