Coral Reefs in Palau Defy Ocean Acidification

Coral reefs have been the subject of much research given the ongoing threats they are dealing with related to climate change. Ocean acidification, for one, is wreaking havoc on these delicate ecosystems, but a remarkable new study says that coral reefs in Palau may be able to defy the odds.

As humans continue to release harmful fossil fuels into the air, the Earth's oceans are absorbing more and more atmospheric carbon dioxide (CO2). Consequently, in a process known as ocean acidification, this CO2 reacts with water molecules and lowers the ocean pH, making it more acidic.

This process also removes carbonate, an essential ingredient needed by corals and other organisms to build their skeletons and shells. As a result, these calcifiers are struggling in Earth's changing oceans, and the survival of coral reef ecosystems worldwide is thus in jeopardy.

However, such may not be the case for coral reefs in Palau, an archipelago located in the far western Pacific Ocean. The tropical, turquoise waters of Palau's Rock Islands are naturally more acidic due to a combination of biological activity and the long residence time of seawater in their maze of lagoons and inlets.

Seawater pH within the Rock Island lagoons is as low now as the open ocean is projected to reach as a result of ocean acidification near the end of this century. And if corals can survive there, then predictably they should be able to survive in the future despite climate change.

During the new study, published in the journal Science Advances, scientists at the Woods Hole Oceanographic Institution (WHOI) found that Palau corals were more or less unharmed by the region's low pH - at least in the way that you would expect. The only thing researches did notice was increased bio-erosion - the physical breakdown of coral skeletons by boring organisms such as mollusks and worms.

"Contrary to laboratory findings, it appears that the major effect of ocean acidification on Palau Rock Island corals is increased bio-erosion rather than direct effects on coral species," David Garrison, a program director in the National Science Foundation's Division of Ocean Sciences, which funded the research, said in a press release. (Scroll to read on...)

Previous research has shown coral respond negatively in a variety of ways to ocean acidification, including fewer varieties of corals, more algae growth, lower rates of calcium carbonate production (growth), and juvenile corals that have difficulty constructing skeletons. But that was not the case in Palau.

"Surprisingly, in Palau where the pH is lowest, we see a coral community that hosts more species and has greater coral cover than in the sites where pH is normal," added study co-author Anne Cohen.

"That's not to say the coral community is thriving because of the low pH," she continued, "rather it is thriving despite the low pH, and we need to understand how."

The WHOI team compared coral communities found on Palau's reefs with those in other reefs where pH is naturally low. They found that increased bio-erosion was the only common feature.

"Our study revealed increased bio-erosion to be the only consistent community response, as other signs of ecosystem health varied at different locations," lead paper author Hannah Barkley explained.

While corals all over the world are diminishing, including ones found in the Great Barrier Reef, those in Palau remain relatively unharmed. So what is the secret to their success? For now it is unclear, but researchers hope to find the answer in future studies.

"On the one hand, the results of this study are optimistic," Cohen said. "Even though many experiments and other studies of naturally low pH reefs show that ocean acidification negatively affects calcium carbonate production, as well as coral diversity and cover, we are not seeing that on Palau.

"That gives us hope that some coral reefs - even if it is a very small percentage - might be able to withstand future levels of ocean acidification."

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