Surprising Resilience Against Acidification Observed in Palau Coral Reefs

Researchers working in the Western Pacific island nation of Palau have made some unexpected discoveries about coral reefs there that may lead to new insights on the organisms' resistance to ocean acidification.

Writing in the journal Geophysical Research Letters, the scientists report that the reefs around Palau are thriving amid shockingly high levels of acidification.

"We had no idea the level of acidification we would find. We're looking at reefs today that have levels that we expect for the open ocean in that region by the end of the century," said lead study author Kathryn Shamberger, a chemical oceanographer who was a postdoctoral scholar at Woods Hole Oceanographic Institution (WHOI) at the time of the research.

Increasing levels of of carbon dioxide in the atmosphere are contributing to a changing ocean chemistry. The ocean absorbs atmospheric CO2, which causes a chemical reaction that lowers the pH of seawater, making it more acidic. The acidification process also removed carbon ions from the water that are typically needed by reefs to support growth. In both laboratory tests and real-world scenarios, low pH seawater has been shown to cause negative effects on coral reefs including low species diversity, slowed growth rates, more algae growth and greater susceptibility to erosion.

In the study, Shamberger and her colleagues describe the processes that lead to the increased acidification documented in waters around Palau as well as the surprising find that the coral reefs in those waters are showing a remarkable resiliency to the acidification. Contrary to expectations set by previous studies of coral reefs and low pH seawater, the reefs around Palau show unexpected diversity and health.

The unusual finding may have implications for other coral reef systems in oceans around the world.

"When you move from a high pH reef to a low pH neighboring reef, there are big changes, and they are negative changes," said WHOI biogeochemist Anne Cohen, a co-author on the paper and lead principal investigator of the project. "However, in Palau where the water is most acidic, we see the opposite. We see a coral community that is more diverse, hosts more species, and has greater coral cover than in the non-acidic sites. Palau is the exception to the places scientists have studied."

The researchers found that at every reef around the island the low pH of the water was caused by shell-building done by organisms living in the water, a process called calcification, which removes carbonate ions from seawater. The shell-building organisms also add CO2 to the water as they breathe.

"Calcification and respiration are continually happening at these sites while the water sits there, and it allows the water to become more and more acidic. It's a little bit like being stuck in a room with a limited amount of oxygen - the longer you're in there without opening a window, you're using up oxygen and increasing CO2," Shamberger said.

Continuing her analogy, Shamberger said without fresh air coming through that window, it gets harder and harder for life to thrive. But in the case of the Palau coral reefs, the opposite was happening.

"What we found is that coral cover and coral diversity actually increase as you move from the outer reefs and into the Rock Islands, which is exactly the opposite of what we were expecting," she said.

Now, the researchers must determine whether Palao's reefs are generally adapted to a low pH, or if it's a "perfect storm" situation that creates environmental conditions that allows the reefs to thrive despite a low pH.

"If it's the latter, it means if you took those corals out of that specific environment and put them in another low pH environment that doesn't have the same combination of conditions, they wouldn't be able to survive," Cohen said. "But if they're genetically adapted to low pH, you could put them anywhere and they could survive.

"When we find a reef like Palau where the coral communities are thriving under low pH, that's an exception," Cohen continued. "It doesn't mean coral reefs around the globe are going to be OK under ocean acidification conditions. It does mean that there are some coral communities out there - and we've found one - that appear to have figured it out. But that doesn't mean all coral reef ecosystems are going to figure it out."