Early-warning system may help coral reefs recover from bleaching
By forecasting coral colonies that are able to tolerate high sea temperatures, satellite data can help researchers mitigate other stressors.
The Internet alerts began in July, warning Caribbean coral-reef managers to start looking for signs of coral bleaching in the next 2–4 weeks. By early fall, sustained sea surface temperatures of 1 °C above the normal maximum for that time of year had generated the most extensive bleaching event ever recorded in the Caribbean, says Al Strong, a physical oceanographer with the U.S. National Oceanic and Atmospheric Administration (NOAA).
NOAA’s advance warnings of coral bleaching events are helping reef managers to launch monitoring efforts as well as take action to lessen other stresses on reefs during times of heat shock. The alerts are also opening a window on how to predict damage and enhance recovery of the corals.
The goal is to mitigate mass bleaching events, which are predicted to increase with rising global temperatures. Experts point to mass bleaching as one of the reasons why half the world’s coral reefs are already dead or declining.
“All the notice that the alerts have given us really allowed us to mobilize a team of scientists to do surveys at sites throughout the Caribbean,” says Drew Harvell, an ecologist at Cornell University. When sea surface temperatures rise above tolerance thresholds, the stress causes corals to reject the symbiotic algae that give them their color and food. The corals appear bleached and may die if deprived of algae for more than a week, Strong explains.
Scientists reported that this year’s bleaching affected 75–80% of the coral reefs off the U.S. Virgin Island of St. Croix, 85–95% in areas around Puerto Rico, and 70% of those near Grenada.
“We get maps from NOAA’s Coral Reef Watch Satellite Bleaching Alert system that show elevated sea surface temperature and maps that show how long those temperatures have been occurring,” says Brian Keller, science coordinator for the Florida Keys National Marine Sanctuary. The maps use data from infrared-sensing satellites to provide alert messages for 24 coral-reef ecosystems worldwide, Strong says. The Caribbean also has more than two dozen high-tech buoys that broadcast information about water and air temperature and wind speed to satellites, says Eric Stabenau, an oceanographer with NOAA.
Widespread bleaching sets in after a sea temperature rise of 1 °C above the average maximum for that month persists for 4 weeks, and after 8 weeks many corals are likely to die or have difficulty recovering, Strong says. Such high temperatures persisted for a record-setting 15–16 weeks in parts of the Caribbean this year, he notes. NOAA can give advance warning of bleaching events because of the lag time for corals to respond to the onset of high temperatures, he adds.
When bleaching warnings come in, Keller and his colleagues alert dive shops and tour boat operators to stay off the reef. Divers and boats stress the corals through accidental breakage and by stirring up smothering sediments, Keller explains. Other activities, such as applications of pesticides and fertilizers, which scientists suspect further stress reefs, can also be postponed, Strong adds. Moreover, the warning system, by carefully timing reef surveys, helps managers identify oases of live coral that can serve as reservoirs for recolonizing nearby damaged areas, Keller says.
The satellite data, in combination with water mixing models, are beginning to reveal where scientists should search for bleach-resistant oases, Strong says. Preliminary evidence from the Pacific island of Palau suggests that reef areas that experience the greatest amount of mixing, and therefore the biggest swings in temperature, are better adapted and more resilient to bleaching than corals in calm, warm lagoons.
“The Caribbean’s bleaching season is ending, and we’re beginning to see some recovery in the Bahamas and the Florida Keys,” Stabenau says. By addressing other stressors, such as pollution and loss of seagrass beds and grazing fish important to coral health, scientists can offer hope for mitigating the impacts of high temperatures, he says.
