Carbon dioxide buildup harms coral reefs.

The accumulation of carbon dioxide gas in the atmosphere is stunting the growth of coral reefs and represents a significant new threat to reef health, reports an international team of researchers.

"We haven't really considered this before, and it appears that it is a potential problem," says biologist Joan A. Kleypas of the National Center for Atmospheric Research in Boulder, Colo.

Recent experiments conducted in French aquariums and in the Biosphere 2 facility in Arizona alerted Kleypas and her colleagues to the dangers of carbon dioxide. In these controlled situations, scientists found that adding extra carbon dioxide to water slowed the rate at which coral and reef-building algae secrete the mineral calcium carbonate--the skeleton of reefs.

"We have very limited information, but all the information is pointing in the same direction," says Jean-Pierre Gattuso of the National Center for Scientific Research in Villefranche-sur-mer, France, who conducted some of the experiments. Kleypas, Gattuso, and their colleagues calculate the potential effects of carbon dioxide on reefs in the April 2 SCIENCE.

The news will come as a surprise to many biologists because they have largely ignored how reef organisms respond to carbon dioxide changes, says co-author Robert W. Buddemeier, a geochemist at the University of Kansas in Lawrence.

As carbon dioxide pollution accumulates in the atmosphere, it seeps into the upper ocean and reacts with water. These reactions increase the concentration of bicarbonate ions [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and decrease the concentration of carbonate ions [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. Coral, clams, and many other organisms build their shells by bringing carbonate ions together with calcium to form three different calcium carbonates.

Coral and a group of organisms known as calcifying algae require a higher concentration of carbonate ion and hence are more vulnerable to the extra carbon dioxide than are clams, say the researchers.

Since the beginning of the Industrial Revolution, carbon dioxide amounts in the atmosphere have risen from 280 to 365 parts per million. This has caused a 6 to 11 percent decline in calcium carbonate formation by coral and calcifying algae, calculate the authors. This rate of growth, they project, will drop another 8 to 17 percent by the time carbon dioxide amounts double, expected midway through the next century.

The change will not kill coral, but it will slow its growth and weaken reefs. "The coral is likely to be more fragile, more susceptible to storm damage, erosion, and breakage," says Kleypas.

Stephen V. Smith of the University of Hawaii at Manoa says that "from the point of view of coral reef communities, this potentially is an extremely important consideration."

The experiments conducted thus far, however, have only examined a small number of species over short periods. Researchers need to test other species, he says, and also determine whether coral and calcifying algae eventually adapt to the extra carbon dioxide.

Coral reefs are already under siege from a vast assortment of threats, ranging from pollution to the recent problems of bleaching linked to excessive ocean temperatures (SN: 6/15/96, p. 379). The burden of additional carbon dioxide is adding another significant source of stress, says Kleypas.

Given all the problems, Buddemeier sees a bleak future for coral reefs: "A lot of the pretty reefs that people like to look at and fish in and that provide breakwaters are not going to be with us for a whole lot longer."