NASA - National Aeronautics and Space Administration

Scientists will soon be able to learn more about the health of coral reefs thanks to instruments streaming thousands of feet and miles above them. A NASA researcher has found these so-called "canaries of the oceans" can be monitored by sensors on airplanes and satellites.

Coral reefs are essential in ocean productivity and coastline protection, often providing the first clues about marine ecosystem health. The world's reefs have shrunk to one-half to one-tenth of their original size due to climate and human impacts and respond immediately to environmental changes.

Getting a Closer Look

Liane Guild, a scientist at NASA Ames Research Center, and a group of researchers first use a handheld "spectroradiometer," a device that measures the amount of ultraviolet, visible and infrared light reflected from an object, similar to those aboard remote-sensing airplanes and satellites. By using the instrument at varying ocean depths, they are able to learn how light intensity decreases in columns of water throughout a coral reef.

Since water filters out infrared light, coral reef health can only be detected by using aircraft with just visible light sensors. But first, they "must look at coral close up, underwater, to see what spectral reflectance in the visible light range the sensor picks up from diseased, stressed, and healthy coral," says Guild.

In July and August 2002, Guild and four scuba divers took the first coral undersea readings near Andros Island, Bahamas, a University of Miami long-term research site. The divers, marine science experts, are from the universities of Miami, South Florida, and Puerto Rico.

New Instrumentation

In the future, researchers plan to fly "hyperspectral" instruments that contain several detectors to collect information in the visible light range. These instruments will provide the most useful information about coral reef community health from above the sea. Additional measurements taken underwater by scientists using sophisticated instruments will provide the necessary ground-truth data to calibrate airborne images.

The team and engineering students from the University of Arizona also are developing a specialized computer model to analyze coral reflected-light data. The model will allow scientists to better interpret the raw data gathered by aircraft or satellites.

The team's research emphasis is on Acropora palmata, or elkhorn coral, a major reef-building coral. It is prevalent in the study area, but is suffering from "white band disease." Elkhorn coral is on the verge of becoming an endangered species because it has severely declined in many areas of the Caribbean.

Above and below (click images for closer view): Checking Coral Health. Researchers using handheld "spectroradiometer" measure reflected light readings from Acropora palmata, or elkhorn coral to determine the reef health in the Bahamas. The collected data is compared to the reading taken during aerial surveys. CREDIT: NASA Ames Research Center.

Coral reef and marine scientists asked NASA to develop satellite sensor technology because of their expertise in instrument and mission development. The developed instrumentation will be the first remote sensing device dedicated to the monitoring of the world’s coral reef ecosystems and coastal zone ecosystems. In early 2002, NASA’s SeaWiFS satellite was critical in providing answers about a patch of "black water" spanning over 60 miles in diameter off southwestern Florida. The event, caused by harmful algal blooms, contributed to severe coral reef stress and death in the Florida Keys.


Above (click on image for closer view): Elkhorn Coral. Acropora palmata,
or elkhorn coral exhibiting white band disease. Acropora palmata is a
major reef-building coral prevalent in the Bahamas, but is suffering
from white band disease. Elkhorn coral has severely declined in
many areas of the Caribbean and may be on the verge of
becoming an endangered species.
CREDIT: NASA Ames Research Center

Above (click on image for closer view): Brain Coral with "white plague."
CREDIT: NASA Ames Research Center.

For more information on this research, please visit:

Clues to Coral Reef Health: Spectral Analysis and Radiative Transfer Modeling of Coral Reef Ecosystem Health

NASA Ames Research Center
Mike Bettwy
NASA Goddard Space Flight Center