COURTESY UH SOEST
Chris Langdon, left, Mark Carmichael and Marlin Atkinson in research at Coconut Island found that coral growth can be slowed by increased carbon dioxide in the ocean.
Reef studies gauge global warming threats
Higher concentrations of carbon dioxide slow coral growth
Growth of reef-building corals and other marine organisms could be inhibited as the oceans become more acidic from increased carbon dioxide, University of Hawaii researchers have found.
Experiments with carbon dioxide on corals in a flume at the Hawaii Institute of Marine Biology at Coconut Island showed coral growth "can be slowed pretty dramatically" by decreasing carbonate ions, said Marlin Atkinson, UH oceanographer and specialist on coral reef biogeochemistry.
Carbon dioxide absorbed by the ocean is "causing measurable changes in sea-water chemistry," said Chris Langdon, associate director of the National Center for Caribbean Coral Reef Research.
The scientists studied effects of carbon dioxide on two coral species that build reefs in Kaneohe Bay. The results suggest potentially significant damage from naturally changing conditions in the ocean, they reported.
Atkinson, principal investigator for the project, said a chemical shift is occurring in sea water, with increasing carbon dioxide reducing calcium carbonate, an important mineral for coral growth.
The researchers simulated conditions of the world's oceans that they expect in 50 to 100 years. They also added different plant nutrients and looked at the results, Atkinson said.
If the process continues at its present rate, Langdon said, carbon dioxide levels and ocean acidity are expected to increase 200 percent to 300 percent in the next 50 to 100 years. "So it is important to learn how these changes might affect marine ecosystems," he said.
The study, reported in the September issue of the Journal of Geophysical Research-Oceans, showed skeletal growth of the coral decreased 50 percent while photosynthesis of algae within the coral increased.
The findings indicate a breakdown in what is usually a beneficial relationship between coral and algae, which cement corals together with calcium compounds, the researchers said.
Langdon said a similar breakdown has been reported when corals are exposed to elevated nutrient concentrations, possibly because of competition for carbon between the algae and coral.
"I don't think it's gloom and doom for coral reefs," said Atkinson, pointing out that many types of algae and microscopic animals create calcium carbonate and are abundant on the reefs.
The researchers tried as much as possible to account for other environmental factors known to affect coral growth, Atkinson said.
For instance, Langdon said, they conducted experiments in both summer and winter to explore possible interactions between elevated carbon dioxide and seasonal changes in temperature and light.
They also exposed corals to different amounts of nutrients that might occur in natural conditions.
"Everybody talks about nutrients coming off land and ruining reefs," said Atkinson, an expert on nutrient input on coral reefs. "I figure it's a lot of nonsense.
"Those are very small local impacts," he said. "The kind we're talking about are truly global. This is happening worldwide."
The impact of nutrients on reefs was the primary concern of coral reef scientists 10 years ago, but now it is carbon dioxide because of global warming and coral bleaching, he said.
Storm damage will probably be a major force on reefs in the future, with a larger impact globally than freshwater runoff or nutrients, Atkinson said.
"They (reefs) can respond and can handle storm damage, but when storms get so big and so frequent, they can't and they will be completely eroded away."
The researchers did not examine the effects of higher ocean temperatures associated with global warming, but they predict global ocean temperatures will increase by two to five degrees Celsius by the year 2100.
"Because many species of coral are already growing very near their thermal threshold, any warming will reduce their growth," Langdon said.
"The combined effects of global warming and ocean acidification on coral growth could be even worse than what we observed in our study," he added.