Worldwide Coral Health Monitoring From Space Now Possible

SYDNEY, Australia, October 4, 2005 (ENS) - Australian researchers have found that a sensor orbiting aboard a European satellite can detect coral bleaching down to ten meters (39 feet) underwater. The European Space Agency's environmental monitoring satellite Envisat carries the sensor that could potentially monitor coral reefs worldwide on a twice weekly basis.

Coral bleaching happens when symbiotic algae living in symbiosis with living coral polyps are expelled. The algae lend corals their distinctive colors, and when the algae gone, the whitening coral may die.

Coral death is linked with impacts on the reef ecosystem, and associated fisheries, regional tourism and coastal protection.

Coral bleaching is linked to sea temperatures above normal summer maxima and to solar radiation. Bleaching may take place on localized and mass scales – there were extensive bleaching events in 1998 and in 2002 likely linked to the El Niño warming pattern in the Eastern Tropical Pacific Ocean.

"An increase in frequency of coral bleaching may be one of the first tangible environmental effects of global warming," says Dr. Arnold Dekker of Australia's Commonwealth Scientific and Industrial Research Organisation’s (CSIRO) Wealth from Oceans Flagship program.


Dr. Arnold Dekker is team leader of the Environmental Remote Sensing research team at CSIRO. (Photo courtesy CSIRO)
"The concern is that coral reefs might pass a critical bleaching threshold beyond which they are unable to regenerate," says Dekker.

Observation by boat or airplane is the current method of detecting bleaching, but many reefs are either inaccessible or too large for an event that happens within two weeks, said Dekker.

The Great Barrier Reef has an area of 350,000 square kilometres, far too extensive for twice weekly monitoring.

A bleaching event normally lasts a few weeks. The coral will then either recover or die. If it dies, it will be overgrown by colonizing turf algae, similar in appearance to many live coral, making it difficult to distinguish from healthy coral.

At a workshop last week in Frascati, Italy, the CSIRO team presented initial results using Envisat's Medium Resolution Imaging Spectrometer (MERIS). Dekker conducted the research with Magnus Wettle and Vittorio Brando.

MERIS acquires images in 15 different spectral bands at 300 m resolution.

"Coral bleaching needs to be mapped at the global scale," Dekker adds. "High-spatial resolution satellites can only do it on a few reefs due to cost and coverage constraints. We need a system that has appropriate coverage and revisit frequency, with a sufficient amount of spectral bands and sensitivity. There is no more suitable system than MERIS."


Heron Island is a World Heritage listed Marine National Park, a coral cay located on the Great Barrier Reef. (Photo courtesy Travel Online Australia)
The Australian team studied Heron Island reef at the southern end of the Great Barrier Reef, site of a University of Queensland research station.

Validating MERIS Full Resolution mode results, they found that the observations of changes in live coral cover were related to an existing bleaching event.

Theoretical studies indicate that for each complete 300 meter pixel of coral under one meter of water it is possible to detect a two percent bleaching of live coral.

The researchers say MERIS should remain sensitive to detecting from seven to eight percent bleached coral even under 10 meters of water.

"MERIS Full Resolution covers the world every three days, a bottleneck for global monitoring could be data processing," Dekker concludes.

"However, satellite sensors measuring sea surface temperature, such as Envisat's Advanced Along Track Scanning Radiometer, can be applied to prioritize reefs that are subject to sea temperature heating anomalies," he said.

Australia's Great Barrier Reef Marine Park Authority has expressed interest in the project. Australian scientists plan to progress to perform MERIS monitoring of bleaching events up to the scale of the whole Great Barrier Reef.