Global warming fuels coral killer, study finds

Global warming worsens a disease that has almost wiped out Caribbean coral reefs, according to a new study by researchers at the Florida Institute of Technology.

In only 40 years, the iconic elkhorn and staghorn corals that have dominated Caribbean reefs for 3.5 million years have declined by more than 90%. The main culprit: a disease that causes dead, white bands across the coral. And ocean warming is playing a bigger role in the so-called "white-band" disease than previously thought, the researchers found.

"Up until this point, people didn't have any evidence between a warming temperature and this disease," said Carly Randall, a doctoral student at Florida Tech and lead author on the study.

Randall conducted the research with her adviser, FIT biologist Robert van Woesik. Their study — funded by a $257,000 grant from the National Science Foundation — is published in this month's issue of Nature Climate Change.

The branches of the two coral types they studied grow in the shapes of antlers. Elkhorn and staghorn corals provide foundations for coastal food webs, shoreline protection and erosion prevention. They also create habitat for important commercial and recreational marine life. The Florida Tech study suggests limiting the rate of ocean warming could help these two vital reef building blocks to recover.

"It's possible, if we reduce our emission of greenhouse gases," van Woesik said.

But white-band disease is likely to worsen, the researchers said, until the corals get longer cooling-off periods after hot summers.

Both elkhorn and staghorn coral are listed as threatened under the Endangered Species Act. They are especially of concern because they form the foundations of reefs that support economically important fish and other marine species.

Florida's coral reefs produce an estimated annual economic value of $324 million, according to NOAA Fisheries.

The northern limit for elkhorn and staghorn corals in Florida is Biscayne Bay. But elkhorn and staghorn coral reefs are vital habitat for grouper, snapper, grunt, lobster, conch, sea urchins and a variety of other marine life.

White-band disease has already devastated Florida corals, especially in the Florida Keys.

The Florida Tech researchers compared sea temperature data with white-band disease records from 473 coral sites in the Caribbean and the Keys.

They found white-band disease is more common where waters have warmed most rapidly and stayed unusually warm during winter.

"As the rate of rise increased, we saw more white-band disease," Randall said. "It could be that the warm water is making the pathogen more virulent."

Biologists aren't sure the disease is caused by a pathogen. But warmer waters may be making marine microbes more infectious and/or weakening the coral's innate immunity.

Scientists also don't know whether threatened corals can survive another major stress beyond the ocean warming and runoff pollution they already endure.

But knowing why, when and where coral disease outbreaks happen can help focus conservation efforts.

"I think there's some hope," van Woesik said. "We are a step closer to predicting where diseases are occurring, because now we know why they are occurring."