At PNG Reef W. Natural CO2 Leak, Scientist Find Young Corals, Branching Corals & Soft Corals Die Off From Acidification

At a natural underwater laboratory off the coast of Papua New Guinea, researchers examined what happens to a diverse reef ecosystem as it experiences gradually increasing levels of ocean acidification. They found that as the pH decreased, complex branching corals, soft corals, and young corals died off. In their place grew hardy boulder corals and non-calcium-based algae.

One thing the team didn’t find: a specific tipping point at which corals began to die off. “That was something we really hoped to be able to detect from the data,” said Sam Noonan, a coral reef ecologist at the Australian Institute of Marine Science (AIMS) in Townsville and lead researcher on a new study reporting the work. “Do you have this increase in acidification and everything seems fine, and then species start falling off a cliff? But that was not the case at all. With every little increase, we saw a smooth decline.” These observations, which took place near a volcanic seep that leaks carbon dioxide (CO2) into the ocean from the seafloor, provide a preview of how reefs around the world could respond as the ocean absorbs increasing quantities of atmospheric CO2.

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To overcome those limitations, Noonan and his AIMS colleagues traveled to Milne Bay on the southeastern coast of Papua New Guinea, which is home to a diverse and thriving coral reef ecosystem. It’s also home to a volcanic seep that releases nearly pure CO2 gas from vents in the seafloor. A reef like this one is “a natural laboratory that allows us to understand how real coral reefs respond to acidification,” Enochs said. Enochs was not involved with the new research.

The scientists spent more than a decade measuring the ambient properties of the seawater throughout the reef and documenting, via a proxy called aragonite saturation, how acidity changes on the basis of proximity to a seep. Aragonite saturation levels across the seep match values predicted to occur by 2100 under a wide range of carbon emission scenarios.

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https://eos.org/articles/coral-diversity-drops-as-ocean-acidifies