Oxygen-starved water a threat to corals

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  • Researchers spot up to 370 marine ‘dead zones’ with low oxygen levels that harm corals
  • Specialised equipment detects blenching and marine species mortality in Panama
  • Discharge of organic matter or waste water into coastal areas leads to lack of oxygen

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[SAN JOSÉ] A lack of oxygen in sea water harms coral reefs and other types of marine life linked with these ecosystems, according to research that analysed coral mortality on the Caribbean coast of Panama.
Tropical waters with extremely low levels of oxygen, called “dead zones”, are destroying coral reefs around the world. But their impact has hardly been studied in the tropics.
“I think [the effect of low oxygen levels] has been overlooked until now because oxygen measurements require specialised equipment, which is not available for all scientists or monitoring programmes”, said Andrew Alteri, marine scientist at the Smithsonian Tropical Research Institute and lead-author of the paper, which was published in this month’s issue (April 4) of the Proceedings of the National Academy of Sciences.

“This is a call to action about the need to incorporate oxygen measurements into the regular monitoring of reefs.”

Celeste Sanchez, Leibniz Centre for Tropical Marine Ecology in Germany

The finding adds a new threat to corals, in addition to acidification and global warming, that scientists need to take into account, according to a marine scientist.
In September 2010, scientists working on the coral reefs of Boca del Toro, in the Panama Caribbean, noted evidence of damage at a certain depth even as marine life looked unaffected on surface waters. Coral reefs were bleached and animals like crabs, urchins and sponges had died.
Using equipment they had available at the time, they were able to detect a low concentration of oxygen in the water — but two years later, they returned with equipment better suited to looking at oxygen levels. After eliminating other possible causes of the damage, such as high temperatures or salinity, they found low oxygen levels at a depth between 10 and 12 metres but normal levels near the surface.
According to Altieri, this lack of oxygen may be caused by discharge of organic matter, or by high levels of nutrients in the form of waste water discharged into coastal areas.
Based on statistical inference, the study estimates that there may be up to 370 “dead zones” in tropical waters globally which have not been described yet. And more than a half may be in areas with coral reefs.

In order to identify the impacts early, Altieri suggests that researchers should add oxygen concentration measurements to the battery of indicators — such as water temperature and salinity — that they normally use to monitor coral ecosystems.
But the equipment required to do this is expensive, he notes, and requires extra funding.
The research is a call to action about the need to incorporate oxygen measurements into the regular monitoring of reefs, according to Celeste Sanchez, PhD candidate from the Leibniz Centre for Tropical Marine Ecology in Germany, who is unconnected to the study. This is a good way to not just detect damage early, but to identify those areas more prone to be affected in the future, she points out.
This article was produced by SciDev.Net’s Latin America and Carribbean edition.