A hurricane can destroy a coral reef. The fragile superstructure created by millions of marine microorganisms does not fare well when slammed by high winds and powerful waves. But coral reefs worldwide have bigger fish to fry, so to speak, most notably the bleaching brought on by warmer waters. In that case, it appears that hurricanes are actually a boon.
Marine biologist Derek Manzello of the University of Miami and the National Oceanic and Atmospheric Administration (NOAA) reviewed the water temperature records surrounding five coral reefs off the coast of Florida. In the summer of 2005 warm sea waters caused these reefs and others across the Caribbean to expel the algae that normally gives them their color and much of their food, leaving them white as a skeleton and often starving—a process called bleaching.
Come September of that year, hurricanes like Rita and Wilma churned the waters and actually cooled the sea in their paths by more than two degrees Celsius (nearly five degrees Fahrenheit). As a result, the Florida reefs recovered within two weeks, much faster than their counterparts in other areas of the Caribbean that did not experience the benefit of hurricane-induced cooling, such as those in the U.S. Virgin Islands, the researchers report in Proceedings of the National Academy of Sciences USA.
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"The wide swath of ocean that is cooled by hurricanes is much larger in area than the narrow swath where damage occurs on reefs," Manzello notes. "To those who are interested in or study coral reefs, hurricanes are almost somewhat of a blessing because they, in effect, provide respite to thermally stressed corals."
Hurricanes also cool reefs with their broad cloud cover and the effect seems to be strongest to the left of a given storm's track for reasons that remain a mystery. (This was not the case in another study in Okinawa that looked at the same effect, Manzello says.) It also remains unclear how the climate change that is warming the seas themselves will impact hurricane frequency and strength.
But bleaching is a problem that has been on the rise in the past 25 years—an El Niño occurring between 1997 and 1998 killed as much as 16 percent of reefs worldwide through warming and subsequent bleaching—and anything that counteracts its impact is to be welcomed, Manzello says. "Nothing can really be done about widespread bleaching events that can reach the global scale," he adds. "Which is why the consensus nowadays is to effectively manage every other potential stressor to reefs that is in our control."
What humans cannot help, hurricanes can. The tropical cyclones of 2005, despite their destructive impacts on patches of coral reef and U.S. cities, helped a broader swath of Florida reefs recover faster than other reefs in the Caribbean. "The next step is to keep studying reefs worldwide to determine the significance of natural feedbacks, like hurricane cooling," Manzello notes. "Because these may be our best hope for coral reef persistence over the next 100 years."
