Scientists are getting a preview of the possible fate that might befall coral reefs should ocean acidification rise to expected levels, by observing coral growth discovered at submarine springs (known as ojos) found along the coast of Mexico's Yucatan Peninsula.
The ojos are naturally low in pH which provides similar conditions to those that are expected should the oceans absorb increasing amounts of carbon dioxide from the air. Surveys found patchy distributions of a few species of corals, and researchers who have been studying the ojos for the past three years have recently published their findings in the journal Coral Reefs.
"This study has some good news and some bad news for corals," said co-author Adina Paytan, a research professor in the Institute of Marine Sciences at UC Santa Cruz. "The good news is that some species of corals are able to calcify and grow at very low pH. The bad news is that these are not the ones that build the framework of the coral reefs. So if this is an indication of what will happen with future ocean acidification, the reefs will not be as we know them today."
The ojos occur along the eastern coast of the Yucatan Peninsula, and for thousands of years they have discharged brackish water into the surrounding seawater, which lowers the pH of the seawater and effects the equilibrium that seawater has with calcium carbonate; the concentration of carbonate ions is reduced in the seawater which makes it harder for corals to build their calcium carbonate skeletons.
The team monitored conditions at 10 ojos and found that as they got closer to the centre of the ojo, the number of coral species and colonies declined with only a few species of hard corals found in water with the lowest carbonate saturation levels. The hard corals found are rarely major reef builders, but the fact that they could form carbonate skeletons in low pH conditions should warrant further study, according to Paytan.
"We need to understand the mechanisms that allow these corals to calcify at these low-pH conditions. We should also make sure that the places where these species occur are protected," she said.
Although the low pH and low carbonate saturation near the ojos is consistent with the levels scientists expect to occur worldwide by the year 2100, other factors need to be considered; such as the high concentrations of nutrients that occur in the discharge water. These nutrient levels may help the corals compensate for the high energy needed for calcification under low pH conditions.
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