Sydney: Scientists have revealed that the iconic Great Barrier Reef, off the coast of Queensland, Australia, is experiencing its worst phase of bleaching, with 93 precent the reef been affected.


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Researchers contributing to Australia’s National Coral Bleaching Taskforce found that only 7 percent of the reef has avoided coral bleaching.


 


It found that the northern half of the reef has been hit the hardest, with about 80 percent categorized as severely bleached. The far southern portion has escaped the warmest water, and the area of severe damage there drops to around 1 percent.


“We’ve never seen anything like this scale of bleaching before. In the northern Great Barrier Reef, it’s like 10 cyclones have come ashore all at once,” says Professor Terry Hughes, convenor of the National Coral Bleaching Taskforce that is documenting and studying the event. “Towards the southern end, most of the reefs have minor to moderate bleaching and should soon recover.”


Watch the video of underwater surveys of coral bleaching on the Great Barrier Reef-



Video credit: ARC Centre of Excellence for Coral Reef Studies


Researchers surveyed over 911 individual reefs in a helicopter and light plane to map out the extent and severity of bleaching along the full 2300km length of the Great Barrier Reef, which is made up of 2,900 smaller reefs.


 


“The bleaching is extreme in the 1000km region north of Port Douglas all the way up to the northern Torres Strait between Australia and Papua New Guinea,” says Prof Andrew Baird from the ARC Centre of Excellence for Coral Reef Studies, who has spent the past 17 days at sea.


“North of Port Douglas, we’re already measuring an average of close to 50% mortality of bleached corals. At some reefs, the final death toll is likely to exceed 90%,” adds Prof Baird.


Underwater, teams of scientific divers have confirmed the accuracy of the aerial surveys, and are continuing to measure the ongoing impact of the bleaching.


Researchers have found that the huge footprints of the three mass bleaching events in 2016, 2002 and 1998 are different. “In each case, the location of the most severe bleaching coincides with where the hottest water sits for the longest period,” says Prof Hughes.


Mass coral bleaching occurs when heightened sea temperatures due to global warming cause corals to expel tiny photosynthetic algae, called ‘zooxanthellae’. The loss of these colourful algae causes the corals to turn translucent and white, and ‘bleach’. Mildly bleached corals can recover if the temperature drops and zooxanthellae are able to recolonise them, otherwise the coral may die.


(Source: ARC Centre of Excellence for Coral Reef Studies)