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Palau International Coral Reef Center

Palau’s corals in an area with low pH.

Corals found thriving in acidified Palau seawater

by Zara Zhuang
Feb 6, 2014

Corals reefs in Palau are surviving, even thriving, as the surrounding Pacific Ocean becomes acidified, based on results published recently in the journal Geophysical Research Letters. But this finding does not mean corals — tiny invertebrates that grow in colonies — are safe from the effects of climate change everywhere.

Coral reefs in the Rock Islands of Palau, an island nation in the Pacific about 600 miles east of the Philippines, appeared to be healthier than those in nearby reefs with less acidified waters.

That is according to research done by Katie Shamberger and her colleagues from Woods Hole Oceanographic Institution in Massachusetts and the Palau International Coral Reef Center. The finding seemed to contradict the common expectation that more acidified seawater, resulting from climate change, impairs coral growth. But coral diversity is likely to suffer if only limited areas of coral can survive.

“It was a complete surprise,” said Shamberger, assistant professor of oceanography at Texas A&M University. “We expected reefs in the Rock Islands would be more acidic than others in Palau.

“The first surprise was that acidity was so high, and the second was that the reef was doing so well.”

The pH of seawater in the Rock Islands is 7.8 on average, while that of normal seawater outside the islands is 8.1, Shamberger said. A lower pH value denotes higher acidity, and because pH is measured on a logarithmic scale, a 0.1 drop in pH means a 26 percent rise in acidity.

The more acidified seawater is caused by three natural processes: the growth of reefs that removes carbonate ions, the breathing of organisms on the reef and the maze-like reef formation that retains water longer, so processes in the reef can change the chemistry of the water more dramatically.

The research team tracked the Rock Island reef during the wet season of September 2011 and the dry season of March to April 2012, collecting data throughout the day and across the tidal cycle.

In determining the health and diversity of the Rock Island reef, Shamberger and her colleagues measured the percent coral cover (the percentage of the ocean bottom covered with live corals), coral richness (the number of coral genera) and coral diversity (coral richness and how often the different genera appear).

This finding does not prove there is hope for corals everywhere suffering ocean acidification. “Palau’s coral communities probably had thousands of years to develop, while current ocean acidification is happening much more quickly,” Shamberger said. “Even if there is the potential for coral reefs to adapt to ocean acidification, they might not have time to do so.”

However, Tim Wootton, professor in the department of ecology and evolution at the University of Chicago, said Shamberger’s research might involve factors making interpretation less straightforward.

“The way it’s been pitched is that we see high coral in a low-pH area so corals are tolerating lower pH, or capable of that and even doing better,” Wootton said. “The main issue is that it takes ocean chemistry as a given and corals are responding to that.”

The scope and duration of the study might have affected the research outcomes. “I would have measured more environmental parameters,” he said. “I would have a longer term, more intensive study instead of a couple of weeks.”

The specific types of corals in the Rock Island could also influence the inferences made. “I’m not sure what kinds of corals are presented, and whether all the diverse corals are reef-building hard corals, soft corals or anemones,” Wootton said. “(The latter two) may contribute to the diversity of reefs but are not laying down new reef.”

Luisa Marcelino said, “(The results are) intriguing because they are unexpected.” Marcelino is a research assistant professor in the department of civil and environmental engineering at Northwestern University.

“Most publications show corals are affected negatively by acidity, but most are observing corals in controlled conditions,” she said.

Marcelino suggested there may be differences in the corals that could be due to the environments they are in, as ocean acidification is a global phenomenon but there are local stressors that corals face.

“There certainly could be a genetic component, or it could be the environmental combination of low pH, low carbonate ions, fewer stressors and low variability,” she said. “With fewer stressors maybe these corals can handle each one better. What remains to be seen is whether it’s a genetic trait or they are generally more strong.”

Whatever the real reason is, corals will need to up their game to survive in more acidic waters. According to the National Oceanic Atmospheric Administration, over the last 250 years the concentration of carbon dioxide in the atmosphere has jumped from 280 to more than 394 parts per million due to human use of fossil fuels and land conversion. And, with the ocean absorbing about a quarter of the carbon dioxide gas emitted, its pH is falling and becoming more acidic.

More acidic oceans would not be good news for corals. When the ocean absorbs carbon dioxide gas from the atmosphere, it forms carbonic acid. Hydrogen ions separate from the carbonic acid, lowering the ocean’s pH, and bind with carbonate ions in seawater, resulting in fewer carbonate ions available for corals to build their skeletons.

“I do not see the concentration of CO2 in the atmosphere to be dramatically reduced any time soon, and I expect a large majority of corals will not be likely to respond the same way (as the ones in Palau’s Rock Island reef),” Marcelino said.

“Even if some (corals) survive, it will be accompanied by dramatic changes in populations and ecosystems,” she said. “We don’t understand enough about ecosystem functions and losses of species, even if some do well.”