The cement that buttresses coral reefs, giving them the strength to withstand crashing waves and other onslaughts, may stop forming as oceans acidify under increased carbon dioxide in the atmosphere.
Researchers have already predicted that a more acidic ocean will make it more difficult for corals to build their calcium carbonate skeletons. The new finding suggests that the reef’s broader structure may also suffer because a lower pH reduces the formation of the reef’s cement binder. The binder is made from calcium carbonate that precipitates out of ocean water when it rushes through the pores of coral skeletons.
“Until now, we’ve mostly addressed acidification in terms of what it does to the living organism,” said study author Joan Kleypas of the National Center for Atmospheric Research in Boulder, Colo.
“Here we’re finding that the reef structure itself can certainly feel the effect of ocean acidification, even if the biology somehow finds a way to cope with acidification. This is mainly an inorganic process, so we’re looking at something that will happen regardless of what the biology does.”
The researchers made their findings by comparing places around the world where CO2 levels in the ocean vary naturally.
The eastern tropical Pacific Ocean, near the Galapagos Islands, has particularly high CO2 concentrations because it is an upwelling spot where deeper waters, enriched in CO2 by microbial degradation of organic matter down below, rise to the surface.
“The eastern Pacific is in essence a natural laboratory to study how coral reef ecosystems are structured and function under these acidic conditions,” said study author Derek Manzello of the University of Miami.
The researchers made measurements of the seawater and the amount of coral cement present in reefs near the Galapagos, and compared those with levels found near the Pacific coast of Panamá, and with those in the Bahamas.
Reefs in the Galapagos had the highest levels of carbon dioxide, which corresponded to water less saturated with carbonate ions — and only trace amounts of cement on the reefs.
“With less cement, reef structures appear to become more vulnerable to natural erosive processes,” Manzello said.
The Panama reefs had intermediate levels of CO2 and cement, while the reefs in the Bahamas showed the least CO2 and the most cement. The team published their findings today in the Proceedings of the National Academy of Sciences.
“The findings of this study suggest that any new reef development and growth will be severely limited in an acidified ocean,” Manzello added. “The mere existence of coral reefs as we know them is because corals are able to produce their skeletal materials at rates faster than they are ultimately eroded.”
Ocean acidification may tip this balance, he said.
The study does not conclusively prove the link between acidification and cementation, because the upwelling in the Galapagos also brings with it colder water and extra nutrients, which could contribute to the observed cementation differences. But Manzello said that CO2 levels have a larger effect on how saturated with carbonate ions the seawater is.
These findings provide “one more piece of the puzzle that helps us understand reef building in a high CO2 world,” said Ilsa Kuffner, a reef ecologist with the U.S. Geological Survey in St. Petersburg, Fla., who was not a part of the study.
“More and more evidence just keeps coming out that future coral reefs could be more susceptible to erosion, both mechanical and biological, because this cementation is not filling in those pores, and because the raw materials are not being built by corals and algae” she said.
Jessica Marshall, Discovery News, 29 July 2008. Article.
