Geophenomena
Hurricanes a boon to bleached corals

Fierce hurricanes are usually considered destructive, rather than helpful, to anything in their path. By churning up ocean waters and temporarily cooling them, however, hurricanes may actually be giving some endangered corals a boost to recovery.

Coral reefs worldwide have been declining over the past three decades due to disease, pollution and warming sea-surface temperatures resulting from climate change, scientists say (see Geotimes, December 2003). One particular area of concern is that warmer ocean temperatures can cause widespread “bleaching” events, in which corals expel the symbiotic algae, or zooxanthellae, that normally take shelter within the coral polyps and, in return, photosynthesize sunlight into sugars, which feed the corals. When the zooxanthellae are expelled, their pigments no longer lend their color to the corals, and the corals appear whitened, or “bleached.” Prolonged bleaching can cause corals to die.

If the warming stress is short-lived, however, some corals can recover and regain their symbiotic algae. Although the ferocity of a hurricane can cause significant physical damage to a reef, hurricanes also stir ocean waters, bringing up cooler water from below to mix with warm surface waters. And that cooling effect, from hurricanes such as Dennis, Katrina and Rita in 2005, helped critically bleached corals in Florida to recover, according to research by Derek Manzello, an oceanographer with the University of Miami’s Rosenstiel School of Marine and Atmospheric Science in Miami, Fla., and his team.

Using sea-surface temperature and wind speed data from 1998 to 2006, the researchers assessed how ocean surface waters had cooled after the passage of hurricanes at five sites along the Florida Reef Tract in the Florida Keys National Marine Sanctuary. They also assessed coral bleaching at those sites from August 2005 through March 2006. For comparison, they assessed coral bleaching during the same period at reefs in the U.S. Virgin Islands, which did not experience any major storms in 2005.

Storms caused physical damages to the reefs within 100 kilometers of the storm center, but the storms had farther-ranging effects on sea temperatures, the researchers reported July 17 in the Proceedings of the National Academy of Sciences. In the wake of a hurricane that passed within 400 kilometers of the Florida Reef Tract sites, sea temperatures dropped by 0.3 to 3.2 degrees Celsius. If the storm’s path was more than 400 kilometers away, the temperatures dropped by 0.1 to 0.9 degrees Celsius. The cooling lasted from one to 40 days, with a mean around 11 days, for storms within 400 kilometers of the reef. Cooling resulting from more distant storms lasted from zero to six days, with a mean around two days. The cooling’s duration, the researchers found, was related to how much the temperature dropped, how far away the storm was and its wind speed.

Corals in both the Florida Reef Tract and the U.S. Virgin Islands were critically endangered in 2005 due to widespread bleaching across the Caribbean. In September 2005, bleaching in both locations was nearly identical. However, bleaching began to decline in Florida by October and most of the corals had recovered by November, while bleaching reached a peak at that time in the Virgin Islands, the team reported.

The study provides a good synthesis of information on a phenomenon scientists have been thinking about for awhile, says Mark Eakin, an oceanographer at NOAA in Silver Spring, Md., and the coordinator of NOAA’s Coral Reef Watch program. “If you look at hurricane tracks from 2005, you find that there’s a big triangular region in the northern Lesser Antilles where none of the hurricanes passed,” says Eakin, who was not involved in the study. “No one wanted the islands to be trashed, but during the bleaching [that year], there were people who were wishing that a milder storm might come along to cool things down.”

The balance between the mechanical damage that an intense storm can do to corals and its beneficial cooling effects is important, Eakin says. When hurricanes Wilma and Rita passed through the Florida Keys in September 2005, they were still low-intensity storms, not reaching their full destructive power until they crossed the Gulf of Mexico. By the time Hurricane Wilma passed through the Keys in October, however, there was no need for cooling and the storm was more intense, causing a great deal of mechanical damage to the reefs.

Bleaching in the Florida Keys in 2005 was still severe, but the storms may have saved those corals from mass mortality — unlike the high mortality in the Virgin Islands National Park, which lost half of its corals that year, Eakin says. “The timing of those two storms was critical,” coming right on the heels of a NOAA alert that heat stress in the Florida Keys had reached critical levels, he says. “Those two storms provided just the right amount of cooling at just the right time.”

Carolyn Gramling

Links:
"Unknown Future for Coral Reefs," Geotimes, December 2003

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