Diver Margaux Hein surveys dead branching corals at Day Reef, near Lizard Island. Photo: Greg Torda, ARC Centre of Excellence Coral Reef Studies Dead coral at Yonge reef, near Lizard Island. Photo: Greg Torda, ARC Centre of Excellence Coral Reef Studies
Healthy reef between Mackay and Townsville escape. Photo: Tane Sinclair-Taylor, ARC Centre of Excellence Coral Reef Studies
Crown-of-thorns starfish attached to healthy coral on a reef between Mackay and Townsville. Photo: Tane Sinclair-Taylor, ARC Centre of Excellence Coral Reef Studies
Extensive bleaching of Acropora corals on the reef crest of North Direction Island, April 2016. Photo: Andrew Hoey, ARC Centre of Excellence Coral Reef Studies
Dead bleached corals on the reef crest of North Direction Island in October 2016. Photo: Andrew Hoey, ARC Centre of Excellence Coral Reef Studies
A healthy southern reef. Photo: Tane Sinclair-Taylor, ARC Centre of Excellence Coral Reef Studies
Fresh surveys of the Great Barrier Reef six months on from a mass coral bleaching have found large-scale damage north of Cairns, where a growing coral death rate due to heat stress is being exacerbated by disease and predators, scientists say.
Researchers from the Australian Research Council Centre of Excellence for Coral Reef Studies have released a map with new pictures and video that show the aftermath of the extreme underwater heatwave last summer.
The southern half of the reef is in good condition, but the scientists say ongoing surveys at the top end – stretching north of Cairns to Papua New Guinea – confirm it was the worst bleaching episode recorded.
The Great Barrier Reef Marine Park Authority earlier this year estimated 22 per cent of coral died across the length of the reef due to heat stress.
Greg Torda, from the Centre of Excellence, based at James Cook University in Townsville, said millions of corals died from heat stress in March and ongoing surveys showed many more had slowly died in the months since.
“On the reefs we surveyed close to Lizard Island [off the coast of Cooktown, in far-north Queensland], the amount of live coral covering the reef has fallen from around 40 per cent in March to under 5 per cent now,” Dr Torda said.
The scientists reported finding damaged living coral in that area under attack from snails and affected by disease.
In the central section of the reef, between Mackay and Townsville, Andrew Baird said there was still close to 40 per cent coral cover at most sites.
“The corals that were moderately bleached last summer have nearly all regained their normal colour,” Professor Baird said.
The high mortality rate across the reef came at the end of what was then the warmest year on record, and during an El Nino, when sea temperatures are particularly warm in the eastern Pacific. (This year has been warmer across the globe, but the El Nino cycle is over.)
Scientists warn that as the planet warms due to climate change, bleaching of many types of coral is expected to become more frequent and severe.
Coral bleaching occurs when the stressed coral host ejects the tiny marine algae, known as zooxanthellae, that lives inside its tissue and gives it its colour and the bulk of the energy needed for it to grow and reproduce.
Most corals rely on zooxanthellae to feed. Without it they start to starve and their tissue becomes transparent.
Corals can regain their zooxanthellae and colour if the temperature returns to normal, though some never fully recover. When heat stress continues for eight weeks or more, bleached coral often die. If the mortality rate on a reef is high it can take reefs years or decades to recover.
There was historically significant coral bleaching at reefs across much of the globe last year. Scientists say along the Great Barrier Reef it was worse than previous episodes in 1998 and 2002.
The current surveys are due to be completed in mid-November.
See more video and pictures of the reef here.
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This story Administrator ready to work first appeared on Nanjing Night Net.