Researchers at Canberra's Australian National University said they had established that smoke from the fires produces an algae bloom known as red tide which is toxic to most marine life.

Their discovery has also provided the reason for the mysterious deaths six years ago of almost all coral and fish in a 400-kilometre (250-mile) stretch of Indonesia's Mentawai Islands reef, southwest of Sumatra.

In that case, the red tide was caused by thick smoke from wildfires in Indonesia in 1997, the scientists told a conference in Melbourne.

A paper published by the team in the latest issue of Science journal and presented to the Fresh Science 2003 forum revealed that nutrients in smoke produced a large algae bloom, known as a red tide, which suffocated the Indonesian reef ecosystem over a distance equivalent to a quarter of Australia's world-renowned Great Barrier Reef.

By examining the skeletal growth and geochemistry of Porites corals, the team, led by Merilie Abram, was able to track the environmental factors which led to the red tide.

An examination of the fossilised corals also showed the red tide had been the worst to hit the reef in 7,000 years.

"This was an extreme event and almost six years on, the reefs still haven't recovered," Abram said in the paper. "We fear that future fires could further damage the reefs before they can recover."

The researchers found that smoke from the Sumatra fires released almost 11,000 tonnes of iron into the atmosphere.

The iron acted as a fertiliser, increasing water nutrient levels and providing the extraordinary conditions that led to the red tide algal bloom and which caused reef death by asphyxiation.

Cores from rare corals that survived the 1997 red tide showed a clear hiatus in growth at the time of the fires.

"Indonesia's reefs are the most diverse in the world and are an important source of new corals that help to rejuvenate Australia's coral reefs," Abram said.

"They are estimated to generate approximately four billion US dollars a year in tourism, fishing and employment for Indonesia.

"Unfortunately they are also among the most threatened reefs in the world and conservation projects are of high-priority."

"It is expected that this new threat to coral reefs and other coastal ecosystems will increase in the future as global warming and forest clearing lead to more wildfires."

SPACE.WIRE