Hurricanes Heal Reefs in Surprising Cases

Helen Scales
for National Geographic News
July 2, 2007
Hurricanes may actually provide a healing balm of sorts for dying coral reefs, a new study shows.

By mixing up cool deep layers of the ocean, a distant hurricane reduces sea-surface temperatures by several degrees—enough to help heat-stressed corals survive bleaching.

Bleaching occurs when sea temperatures warm, even slightly. This causes corals to eject their symbiotic, food-producing algae known as zooxanthellae (zoo-zan-thell-ay), leaving behind only the transparent coral tissue and bone white skeletons.

"It is well known that hurricanes can be catastrophic for reefs," said study co-author Derek Manzello, a marine biologist from the Cooperative Institute of Marine and Atmospheric Studies in Miami, Florida.

"[But] our results show that in [the study's Florida test area] hurricanes may actually have been beneficial," he said.

Critical Changes

The hurricanes also promote bleaching recovery on corals across a wide area.

The research team used temperature data from across Florida's reef tract, which arcs from just south of Miami to beyond Key West (Florida map) to show that winds whipped up by a hurricane can cool an 800 kilometer-wide (497 mile-wide) swath of water by an average of 1.5 degrees Celsius (2.7 degrees Fahrenheit) for ten days.

"Even such small temperature changes can be critical for the survival and recovery of bleached corals," Manzello said.

"Every extra day a coral is bleached increases the chances it might die."

Hurricane-induced cooling appears to have been important in aiding reef recovery in Florida during a mass Caribbean bleaching event in 2005. (Related: "Warming, Disease Causing Major Caribbean Reef Die-Off" [April 6, 2006].)

Florida waters were cooled by Hurricane Rita in September 2005 and then again in October 2005 by Hurricane Wilma.

"Our underwater surveys showed that bleached corals in Florida immediately responded to the cooler water," Manzello said. By November the reefs had almost completely recovered.

At the same time, elsewhere in the Caribbean, the U.S. Virgin Islands remained hurricane free, and the reefs underwent far more intense and persistent coral bleaching than their Florida counterparts. (See map of the Virgin Islands.)

U.S. Virgin Islands reefs only began to recover in January 2006, when sea temperatures finally dropped, Manzello said.

A Silver Lining?

"There is rigorous scientific debate on whether warmer oceans will result in increased storm frequency," Manzello noted.

In one sense, more storms would mean more direct damage to reefs. But "they could result in increased cooling pulses to temperature-stressed reefs," he said.

Nancy Knowlton, a marine biology professor at the Scripps Institution of Oceanography in La Jolla, California, said the study offers a different perspective on what is normally considered a disaster for reefs.

"This is a classic example of every cloud having a silver lining," Knowlton said.

"I'll take any good news for coral reefs."

Study co-author Manzello said that the study, which will appear in an upcoming issue of the journal Proceedings of the National Academy of Sciences, suggests hurricane cooling won't completely nullify the dire prognosis for coral reefs under climate change. (Related: "Global Warming Has Devastating Effect on Coral Reefs, Study Shows" [May 16, 2006].)

"Nonetheless, a well-timed hurricane has the potential to [lessen] the negative effects of increased temperatures."

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