TOWNSVILLE, Queensland, Australia, May 23, 2002 (ENS) - Coral bleaching in the Great Barrier Reef Marine Park may be the worst on record, scientists said today after the most comprehensive aerial survey ever conducted. The survey is aimed at helping unravel the implications of global warming for reef management.

"Our aerial surveys found that nearly 60 percent of the reef area in the marine park was heat stressed to some extent as indicated by bleaching," said Dr. Ray Berkelmans from CRC Reef who led the aerial survey team. Coral bleaching is the loss of color from corals that can lead to their death.

"Until now, the coral bleaching episode in 1998 was the worst on record, but the 2002 event was probably worse because more reef area was affected," said Dr. Berkelmans.

Bleached stag plate coral (Photo courtesy CRC Reef)

Coral bleaching is a response to stressful environmental conditions in corals, says Dr. Terry Done of the Australian Institute of Marine Science and CRC Reef. It is caused when the small algae that live inside the bodies of most corals - their zooxanthellae - leave the corals. Some corals can recover from bleaching by regaining their zooxanthellae, but others may die.

Not all reefs in the Great Barrier Reef Marine Park were bleached equally, and the bleaching was not evenly distributed throughout the park, the aerial surveys found. Generally, the most severely bleached areas were in the southern part of the park, while the northern sections were less affected.

"The most severe bleaching occurred on reefs close to shore in both bleaching events, but the 2002 event has affected a greater area of reefs further offshore," said Dr. Berkelmans.

The aerial surveys found that bleaching was worst in the Princess Charlotte Bay region, near the Turtle Island Group, on inshore reefs from Cape Upstart to the Whitsundays in some reefs in the Sir James Smith Group and in the Keppel Island area.

Moderate to very high bleaching was seen inshore and offshore from around Cape Flattery to Mackay.

Diver explores underwater corals to check for bleaching. (Photo courtesy CRC Reef)

"Our underwater surveys found that few reefs escaped bleaching, but it appears likely that most reefs will recover with only minor death of corals, said Dr. Paul Marshall of the Great Barrier Reef Marine Park Authority (GBRMPA) who led the underwater surveys.

"We did find that some of the most severely bleached reefs were devastated with 50 percent and 90 percent of coral dead at some sites," he said.

"Australia has been lucky to see another major bleaching event without widespread death of corals, but the devastation we have seen at some sites provides a vivid warning of what could happen if hot water events become more frequent and severe. Although we can't control the weather, the GBRMPA is working to reduce other stresses to coral reefs," said Dr. Marshall.

"We may be witnessing the beginning of a slow motion degradation of the reef system that will only get worse in coming decades," said Dr. Done.

Reefs can recover from coral bleaching, the scientists say. If a reef is only briefly and slightly stressed, only a few scattered corals will die. That reef can recover within a few years as new corals arrive and settle in the empty spaces vacated by the dead corals.

Healthy corals on the Great Barrier Reef (Photo courtesy GBRMPA)

Recovery will be hampered, the scientists say, if the reef is in a location that has high levels of dissolved or organic nutrients and low numbers of grazing fishes, sea urchins and snails. This combination allows some seaweeds to establish which form a carpet that is impenetrable to coral larvae and delays the commencement of recovery by corals.

A change in the climate so that the reef is repeatedly exposed to high temperatures on an annual basis will also hamper recovery.

CRC Reef is the Cooperative Research Centre for the Great Barrier Reef World Heritage Area incorporated as CRC Reef Research Centre Ltd. Scientists from this center together with the Australian Institute of Marine Science said they will keep a careful watch on the health of the corals and improve their understanding of how global warming impacts the world's longest reef.