Imaging identifies dead coral reefs

17 Jun 2002

Airborne imaging maps coral reef health in the South Pacific.

In the Austral summer of 1997 to 1998 El Niño/Southern Oscillation swept across the South Pacific, increasing ocean temperatures in its path. This so-called tropical Pacific heat engine had drastic effects on global weather and killed up to 99% of coral on some reefs.

Using airborne spectrographic imaging, Peter Mumby from the UK-based University of Newcastle upon Tyne and colleagues have surveyed the damage caused to two reefs off the French Polynesian Islands (Nature, 413, September).

"Until now if you wanted to measure the health of a coral reef you had to send divers down to record the amount of living and dead coral," Mumby told optics.org. "This is time consuming and expensive so the obvious alternative was to take similar measurements from an aircraft or satellite, which can cover much larger areas."

With this in mind, the researchers mounted a compact airborne spectrographic imager (CASI), which measures changes in light reflectance with wavelength, on an aircraft. At an altitude of 800m, the CASI obtained high-resolution multispectral images over 92 500 m² in just 1 hour.

According to Mumby the device measures light reflectance spanning visible and infrared spectra. "Satellite technology can only detect two or three bands, but this technique has a broad range and can be programmed to detect reflectance in specific bands," he said. "For studying reef health, the trick is to know which bands to look at."

The researchers have already used the imager to study other tropical coastal ecosystems, including seagrass beds and mangroves and now plan to investigate CASI's limitations. "We want to see how it works in deeper and cloudy waters," said Mumby.

Scientists at NASA have also started preliminary investigations into coral reef health detection via satellite-mounted sensors.