for National Geographic Today
Scientists have found a way to rapidly assess the health of the world's
coral reefs without getting wet. The technique can distinguish living
from dead coral by detecting spectral differences in the light that the
Flying as low as 3,000 feet (1,000 meters), the scientists use a device called a spectrographic imager to measure the corals' color characteristics. Every coral has a distinct spectral "signature," and any changes in this may indicate a change in the health of the reef.
"We measure the colorhow much blue light, green light, yellow light, et cetera, is reflected by each coraland compare it to the known signature," said Peter Mumby of the University of Newcastle upon Tyne, United Kingdom, and author of a report on the technique in a recent issue of the journal Nature.
Mumby's study began in November 1998, when he set out to determine the effects of El Niño, which had harmed reefs around the world. The study focused on two reefs in the lagoon of Rangiroa Atoll in French Polynesia.
The scientists flew up and down the reef taking pictures of every square meter. They then took to the water, mapping every type of coraldead and alivewithin several study plots.
When the aerial and sea-based data were compared, the results were both good and bad. "The aerial data," Mumby said, "was accurate to within 3 percent of what was really there," which suggests that the aerial approach could be a useful method of reef surveillance. The bad news was what the data showed: Rangiroa Atoll suffered tremendous damage during El Niño in 1997-98.
The survey showed that about 25 percent of the colonies of slow-growing corals known as Poritessome of which may be hundreds of years oldwere dead. "Porites corals are considered the oak trees of the coral reefs, and are usually quite resistant to environmental stresses," said Mumby. "To see 25 percent destroyed was very disturbing."
The warm ocean waters created by El Niño also destroyed nearly all of the faster growing branched Pocillopora corals in the lagoon. Today, most of these corals have recovered, according to Mumby.
The advantage of remote sensing is that it enables scientists to assess an entire reef quickly. Images from reefs extending 92,500 square meters (110,630 square yards)composed of several thousand research plotswere acquired in only an hour. In comparison, it took the underwater team three days to survey ten such plots.
One of the most surprising findings in the study, Mumby said, was the significant impact that local weather patterns have on the health of coral. "Local weather can play a huge role in reef health, either enhancing or protecting it from environmental stresses," he said.
When the seawater covering the coral is already quite warm, he explained, additional energy from the sun becomes poisonous and causes bleaching.
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