The volcanic island of Maug is really three volcanic peaks, rising up from a remote part of the Pacific Ocean. Underwater vents on the side of Maug’s caldera seep carbon dioxide into the ocean. The shallow water around the volcano is full of coral reefs — except near the underwater vents, where researchers have watched the coral die off, to be replaced by algae clinging to the rocks.


Scientists already knew that too much carbon dioxide in the water was bad news for coral, but this is the first sign of how vulnerable coral reefs are, and how drastically Earth’s oceans might change if they keep absorbing carbon dioxide at predicted rates.

NOAA researcher Ian Enochs and his colleagues studied coral populations and carbon dioxide levels in the sea surrounding Maug, and they’ve published their results in the journal Nature Climate Change.


“This is the first field evidence that increasing ocean acidification results in such a dramatic ecosystem change from coral to algae,” NOAA researcher Ian Enochs said in a statement. Coral polyps secrete calcium carbonate exoskeletons, an en masse, they form coral reefs. The problem is that when the surrounding water contains too much carbon dioxide, it reacts with the calcium carbonate so there’s less material available for coral to turn into skeletons. Carbon dioxide also makes water more acidic, and calcium carbonate tends to dissolve in acid.

As Maug’s vents release carbon dioxide into the ocean, the coral around the vents dies off. Algae, which is more tolerant of acidic water, moves in to take the coral’s place. Of course, algae doesn’t come anywhere close to filling coral’s vital role in the ocean’s ecosystem. “Healthy coral reefs provide food and shelter for abundant fisheries, support tourism and protect shorelines from storms. A shift from coral to algae-covered rocks is typically accompanied by a loss of species diversity and the benefits that reefs provide,” said Enochs.

It’s not just coral that are impacted. In other parts of the world, researchers are also finding shellfish with pitted shells as Earth’s oceans get more acidic, Rolling Stone reported last week.

And Maug’s weedy growth of algae where coral used to thrive could be a preview of what’s in store for Earth’s oceans. “If the predictions of the Intergovernmental Panel on Climate Change remain the same, by the end of the century, the impact of ocean acidification on coral reefs around the world will be comparable to what we see on the reefs near Maug’s carbon seeps today,” Enoch said in an interview last year.


[Nature Climate Change, NOAA]

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Top image: C. Edwards and M. Fox