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.

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