Florida’s big chill may have hammered corals near shore

Early reports from rare cold bleaching offer hope offshore

Rough weather is slowing efforts to survey the extent of Florida’s first cold-weather coral bleaching event in 33 years. But reports so far find heavy damage in parts of reefs near shore.

LOST IN THE COLD This staghorn coral (Acropora cervicornis, transplanted as part of an ongoing restoration effort) was so stressed by Florida’s January cold spell that tissues lost their live-in algae and turned bleached white. The Nature Conservancy

LOCATION, LOCATION Corals close to shore along the Florida Keys, like this Montastraea faveolata, seem to be bleaching and dying in greater numbers than corals in offshore reefs, early surveys suggest. The Nature Conservancy

A cold snap in January stressed corals in the Florida Keys National Marine Sanctuary so much that many lost their colorful, live-in algal partners and turned bleached white, according to reports from the National Oceanic and Atmospheric Administration. Corals sometimes recover, but divers are already reporting dead patches from the event.

Cold bleachings in this region are very unusual, says Billy Causey, Southeast regional director of NOAA’s marine sanctuary office. “It’s a benchmark event for us.”

Mass coral bleaching has been mostly a warm-weather disaster in the region (1983, 1987, 1990, 1997 and 1998, plus some close calls later, Causey says). But he remembers substantial coral bleaching when water grew too cold several days in 1977.

This time the unseasonable cold lingered longer, from January 2 to January 13. Water temperatures close to shore dipped from their usual winter averages between 18° and 23° Celsius to a low of 11Ë. That was too cold for much of the marine life. Some sea turtles and manatees died, as did plenty of fish, including tarpon and barracuda.

Survey teams that the Florida Reef Resilience Program borrows from universities, government agencies and other sources are still assessing the extent of bleaching and deaths. A dead coral quickly gets a dingy caste from an algal film that grows over its remains.

Winds and chills may slow the teams’ work at least another week. But from what he’s heard so far, Causey says, the sanctuary’s offshore reefs, which lie on the ocean side of the Keys, seem to have been largely spared. The warmer water of the Florida Current may have buffered them, he says.

Volunteers in the Florida Keys’ BleachWatch network so far confirm minimal damage offshore, says marine biologist Cory Walter of Mote Marine Laboratory’s facility on Summerland Key. She, however, has surveyed some sites closer to shore with dead fish lying among bleached, dead coral. “It was actually pretty morbid,” she says.

The hard-hit zones include some of the coral nurseries where federal stimulus funds have been supporting efforts to restore reefs. Also, these nearshore and midchannel zones held what had been some of the healthiest stretches of corals, Causey says.

As for the frequency of cold bleaching, “it happens from time to time,” says coral biologist Ove Hoegh-Guldberg, who directs the Global Change Institute at the University of Queensland in St. Lucia, Australia. In 2003 a southern stretch of the Great Barrier Reef saw extensive bleaching when cold, dry winds blasted corals exposed at low tide.

Hoegh-Guldberg,points out that corals can cope with gradual, seasonal temperature swings of 12 degrees C or more. But at times, more sudden jolts of just a few degrees up or down can send corals into a tizzy.

Understanding how corals cope, or don’t, with these temperature disruptions is becoming an important challenge as climate change occurs, says Hoegh-Guldberg.

The chance to gather data may be the silver lining for the recent Florida bleaching. Researchers are rushing to study such a rare event, and “there’s going to a lot of science,” Causey says.

Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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