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As Ocean Warms, Coral Loses Anchor in Acidic Waters

Coral reefs can't find a strong purchase in the eastern tropical Pacific thanks to more acidic waters--a potential precursor of what the ocean will be like under global warming
eroded-reef



COURTESY OF T.B. SMITH

A new study confirms that coral reefs could become yet another casualty of climate change if something is not done to cool the warming globe. The reason: marine cements that bind together reefs can't form in waters full of dissolved carbon dioxide (CO2). Those off the west coast of Central America, particularly around the Galapagos Islands, are kept soft by the more acidic waters in that region—and may provide an early look at how coral reefs will fare in the rest of the world as atmospheric CO2 levels rise.

"The eastern Pacific is basically where the global ocean belches or burps. Thus, these waters have naturally occurring high CO2 and low pH" [an acidity/alkalinity scale; the lower the number the greater the acidity], says Derek Manzello, a biologist from the University of Miami and the National Oceanic and Atmospheric Administration's Atlantic Oceanographic and Meteorological Laboratory, lead author of the study appearing in Proceedings of the National Academy of Sciences USA. That makes the region "a natural laboratory to study how coral reef ecosystems are structured and function under these acidic conditions."

The sparse reefs off the coast of Central America face a host of tough conditions: cool waters that are rich in nutrients and also more acidic, courtesy of higher levels of dissolved CO2 from deep-sea microbes. Even Charles Darwin noted the absence of reefs in the area in his treatise on the subject.

So Manzello and his colleagues took samples from the reefs off the Galapagos and the west coast of Central America, along with those from other regions, such as the Bahamas where conditions are better for coral, and compared them. The analysis showed that acidic waters make it impossible for marine cement—limestone that precipitates out of the seawater as it flows against the coral reef—to form both between individual coral polyps as well as to anchor the entire reef ecosystem against the waves. "These results imply that coral reefs of the future may be eroded faster than they can grow," Manzello says.

These conditions mimic the ocean acidification that is already occurring as a result of elevated CO2 levels in the atmosphere from fossil fuel burning. "The pH of the global surface ocean has already decreased about 0.1 pH units since preindustrial times," Manzello notes. "This doesn't sound like much, but keep in mind that if a change of this magnitude occurred in the human blood stream, we would die."

The results are the first to analyze how a loss of marine cement might affect coral reefs in an acidified ocean. "Poor cementation will make reefs softer and therefore more vulnerable to erosion," says senior marine scientist Richard Aronson at the Dauphin Island Sea Lab in Alabama, who was not involved in the research. The geologic record offers potentially backing evidence as well, according to Aronson. There are some times in the geologic past when high acidity in the oceans goes along with poor reef-building, but in many cases it is not easy to establish a firm causal connection."

Short of controlling climate change, efforts to help the coral, such as providing reef structures, will not make up the difference either. "Artificial reefs are no match aesthetically, structurally or functionally to natural reef ecosystems," Manzello says. In other words, in a warmer world with more acidic oceans humans may not get the as much as $100,000 in economic benefits an individual reef can provide to a nearby community, according to one estimate, including that of protecting coasts against storms and supporting better fishing.

Adds Aronson: "We absolutely must control emissions of the principal greenhouse gas, carbon dioxide, if we are going to preserve life in the oceans in some approximation of the way it should be."

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