Can Coral Reefs Delay the Damaging Effects of Ocean Acidification?

According to a paper published in the November issue of the journal Nature Climate Change, coral reefs may respond to ocean acidification in ways that will partially offset expected changes in seawater acidity taking place as the oceans take up human-produced carbon dioxide.

Andreas Andersson, a chemical oceanographer at Scripps Institution of Oceanography at UC San Diego, and lead author of the paper, said that most predictions of seawater acidification on coral reefs are based on observations from the open ocean. But the effects of increasing CO2 on coral reefs are very different than the changes in the open ocean, because the reef itself modifies the chemistry through various biogeochemical processes.

The study, based on observations of the Bermuda coral reef ecosystem, predicts that changes to this system in response to ocean acidification could offset human-induced, CO2-driven decreases in pH by 12 to 24 percent. Andersson and colleagues also predict that these reef responses will counteract a predicted decrease in the seawater aragonite saturation state, a measure of the availability of carbonate ions, by 15 to 31 percent.  This is an important parameter because corals need these ions to build their calcium carbonate (CaCO3) reefs.

“Other researchers have shown that different benthic communities can alter the chemistry on the reef, but we’re the first to show it on this scale, the whole ecosystem scale, over five years of observations,” Andersson said.

Atmospheric carbon dioxide has gone up by 42 percent and global average temperatures have increased by 0.8°C (1.4°F) since the Industrial Revolution. These changes have well-defined effects on the open ocean, increasing both the acidity and temperature of surface seawater. This decrease in ocean pH has left many scientists concerned about the detrimental effects it could have on coral reefs.

Increasing temperature and decreasing pH make it harder for corals to build calcium carbonate, and also cause calcium carbonate to dissolve more readily. The reef’s total ecosystem organic carbon production (photosynthesis minus organic matter consumed) will also be affected. All of these processes – calcification, dissolution, and ecosystem organic carbon production – affect seawater pH. By modeling how the balance between these processes will change in the future, Andersson and his coauthors discovered that the expected changes may actually increase the pH on the reef relative to the open ocean, thus partially offsetting the decrease in pH owing to uptake of CO2 from the atmosphere.

Many laboratory and field experiments have studied the effects of rising temperatures and ocean acidification on coral reef ecosystems. Although scientists aren’t sure exactly how much reef processes will change, they are confident that calcification will decrease and dissolution will increase as the ocean becomes more acidic. These changes to calcification and dissolution could be so drastic that eventually the coral reefs’ dissolution rate will catch up to the rate at which they build, resulting in stunted growth or deterioration.

“This is something that a lot of experiments and models have predicted will happen,” Andersson said. “This means the reef is dissolving as fast as it’s producing calcium carbonate, and this was the scenario in which we saw the greatest pH offset.”

A reef’s survival depends on putting down more calcium carbonate than is dissolving or it won’t be able to grow, so a reef in this state is not a healthy one, even if it’s able to maintain a more beneficial pH.  This outcome tempers the seemingly good news that corals can “fight” ocean acidification—these offsets will come at the cost of major changes to reef processes and ecosystem composition. The reefs may change from being dominated by calcifying corals to non-calcifying algae, a condition that may diminish their functional and biological diversity.

But there is some positive news in these results, Andersson says. Scientists believe some marine organisms may have “tipping points,” certain pH thresholds below which they aren’t able to survive. This reef feedback may buy them some more time.

“The take-home message [of these results] is that to understand the effect of ocean acidification on a coral reef we have to consider not just how seawater chemistry on the reef is changing owing to uptake of anthropogenic CO2 and how that affects the biogeochemical processes on the reef, but how these processes actually control the chemistry,” Andersson said.

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