Study says ocean acidification has already slowed coral growth

A diverse coral reef in the U.S. Virgin Islands. PHOTO BY CAROLINE ROGERS/USGS.
A coral reef in the U.S. Virgin Islands. Photo by Caroline Rogers/USGS.

‘If we don’t take action … coral reefs will not survive into the next century’

Staff Report

The most dangerous effects of global warming may still be decades away, but ocean scientists say that the buildup of carbon dioxide is already slowing down the growth of coral reefs. By simulating ocean acidification on a section of the Great Barrier Reef, the researchers showed that excess C02 in the atmosphere is affecting coral reefs.

“Our work provides the first strong evidence from experiments on a natural ecosystem that ocean acidification is already causing reefs to grow more slowly than they did 100 years ago,” said study lead author Rebecca Albright, a marine biologist in Carnegie’s Department of Global Ecology in Stanford, Calif. “Ocean acidification is already taking its toll on coral reef communities. This is no longer a fear for the future; it is the reality of today.”

The findings, published in Nature, compared today’s coral reef growth rates with growth rates in less acidic conditions that existed prior to the Industrial Revolution.

“The data analysis for the experiment was complicated by the natural variation of conditions in the reef,” said Rice University’s Kai Zhu, an expert in ecological statistics.”Statistically speaking, there was a great deal of noise in the data, and as scientists we needed to filter out the noise so that we could examine only the signal, the change in the growth rate that resulted from the change in alkalinity.”

Zhu designed a statistical model that was capable of quantifying the variation that occurred both naturally — in a portion of the reef that was measured as an experimental control — and as a result of the experiment. The data showed that the reef grew about 7 percent faster when seawater acidity approximated that of preindustrial conditions.

The carbon dioxide that is released into the atmosphere from fossil-fuel consumption is eventually absorbed by oceans and reacts with seawater to form an acid that is corrosive to coral reefs, shellfish and other marine life. This process is known as “ocean acidification.”

Numerous studies have shown large-scale declines in coral reefs over recent decades as calcification becomes increasingly difficult in more acidic water. Some studies project that reefs will begin dissolving within the century if acidification continues.

Previous studies by another team led by Caldeira found that rates of reef calcification were 40 percent lower in 2008 and 2009 than they were during the same season in 1975 and 1976. But it has been difficult to pinpoint exactly how much of the decline is due to acidification and how much is caused by warming, pollution and overfishing.

In the current study, the team manipulated the alkalinity of seawater flowing over a reef flat off Australia’s One Tree Island. They brought the reef’s pH closer to what it would have been in the preindustrial period based on estimates of atmospheric carbon dioxide from the era. They then measured the reef’s calcification in response to this pH increase. They found that calcification rates under these manipulated preindustrial conditions were about 7 percent higher than they are today.

Caldeira said some researchers have proposed increasing the alkalinity of ocean water around coral reefs through geoengineering to save shallow marine ecosystems. The results of the new study show that this idea could be effective, but he said it would likely be impractical to implement on all but the smallest scales.

“The only real, lasting way to protect coral reefs is to make deep cuts in our carbon dioxide emissions,” Caldeira said. “If we don’t take action on this issue very rapidly, coral reefs … and everything that depends on them, including both wildlife and local communities, will not survive into the next century.”

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