Coral-Reef Builders Vulnerable to Ocean Acidification


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Coral-Reef Builders Vulnerable to Ocean Acidification

By Ann Tihansky, Ilsa Kuffner, and Diane Noserale
March 2008

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Increasing carbon dioxide (CO₂) in the Earth's atmosphere has caused a corresponding increase in CO₂ in the world's oceans, leading to lower pH values (increased acidity) of seawater. A new report by the U.S. Geological Survey (USGS) and partners shows that as pH declines in the world's oceans, the effects on coral reefs could be more harmful than previously believed. This pH decline, commonly known as "ocean acidification," could severely limit the growth of coral-reef builders.

Above: Encrusting algae that developed on surfaces bathed in unaltered seawater from the adjacent reef (left) and those that developed in seawater with pH lowered to the level predicted for the year 2100 (right). [larger version]

To document the potential effects of ocean acidification predicted for the year 2100, scientists from the USGS and cooperating institutions have conducted a unique study looking at the growth of important reef-building plants called crustose coralline algae—a group of calcifying rhodophytes (red algae) that are widespread in the world's oceans.

"Scientists have already shown that coral growth may decrease as the ocean pH declines. This new evidence shows that other essential reef builders, the crustose coralline algae, may be even more sensitive than the corals," said Ilsa Kuffner, USGS scientist and lead author of the report published in the February 2008 issue of Nature Geoscience (v. 1, no. 2, URL http://www.nature.com/ngeo/journal/v1/n2/abs/ngeo100.html).

"The results of our study were visibly obvious and may provide a glimpse into the future. We saw a 92-percent decrease in the area covered by crustose coralline algae in the tanks with lower pH compared with tanks at the pH level of today's ocean. Non-calcifying fleshy algae increased by 52 percent," said Kuffner. "These findings suggest that at lower pH, the reef-building algae could be much less competitive on future coral reefs."

Kuffner and colleagues at the University of Hawai‘i conducted the 9-month study by rearing reef organisms from larvae in an outdoor, semicaptive environment. In the experiments, pH was lowered in half of the test tanks to simulate conditions predicted for the year 2100 by the Intergovernmental Panel on Climate Change (URL http://www.ipcc.ch/), on the basis of present trends in carbon emissions. Surfaces in control tanks bathed with unaltered seawater from the adjacent reef developed the characteristic pink crust made by the reef-building crustose coralline algae. In tanks where the seawater pH was lowered, recruitment and growth of crustose coralline algae were severely inhibited. Although questions remain regarding the influences of algae-eating animals and nutrient availability, it is possible that ocean acidification could hasten shifts away from dominance by stony corals and other reef builders to fleshy algae, as already observed on many reefs today.

Despite being fairly inconspicuous in nature, the crustose coralline algae are extremely important to coral reefs. They secrete skeletons of calcium carbonate, much like coral, and play key ecological roles that affect the health and sustainability of coral-reef ecosystems. Not only do the crust-forming algae build reef framework, produce sand, and help cement loose coral fragments into massive reef structures, but they also attract reef-building coral larvae by providing a place to settle. If these ecosystem services are left undone, coral reefs and associated systems and coastlines could be notably altered as the pH of the oceans slowly decreases.

The full reference for the new report is: Kuffner, I.B., Andersson, A.J., Jokiel, P.L., Rodgers, K.S., and Mackenzie, F.T., 2008, Decreased abundance of crustose coralline algae due to ocean acidification: Nature Geoscience, v. 1, no. 2, p. 114-117, doi: 10.1038/ngeo100 [URL http://www.nature.com/ngeo/journal/v1/n2/abs/ngeo100.html].

Additional information about ocean acidification is available in "Discovering the Effects of CO₂ Levels on Marine Life and Global Climate," Sound Waves, January/February 2007. The article "Molokai reef not exempt from deterioration" in the Molokai Times discusses implications of the new findings for coral reefs off the Hawaiian Island of Moloka‘i.

Discovering the Effects of CO₂Levels on Marine Life and Global Climate
January / February 2007