The Great Barrier Reef has lost half its corals, with a decline in the number of shallow and deep water corals across almost all species in the past two decades.
The decrease was most marked in branching and table-shaped corals, which were the worst affected by record-breaking temperatures that triggered mass bleaching in 2016 and 2017. The study was completed before another mass coral bleaching event this year, which has probably exacerbated the decline.
The number of small, medium and large corals dropped more than 50 per cent between 1996 and 2017, researchers from the Townsville-based ARC Centre of Excellence for Coral Reef Studies, who assessed coral communities and their colony size, have said.
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| Half of the Great Barrier Reef's corals have been lost. Credit: Tony Chase |
These surveys also showed fewer baby coral and large breeding corals, lead researcher Dr Andy Dietzel and co-author Professor Terry Hughes said in a study published in Proceedings of the Royal Society on Wednesday.
There is no time to lose - we must sharply decrease greenhouse gas emissions ASAP“The loss of big corals is important because they make all the babies - they’re responsible for a huge proportion of the breeding that’s done every year by adult corals,” Professor Hughes said.
Study authors
The climate crisis has driven marine heatwaves, with the research recording steeper deterioration of coral colonies in the northern and central Great Barrier Reef after mass coral bleaching in 2016 and 2017.
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| Dead coral at the Great Barrier Reef. Credit: Tony Chase |
Corals rely on algae known as zooxanthellae to provide the bulk of their energy and much of their vibrant colour. When exposed to sustained abnormal heat - measured in so-called degree-heating days - corals begin to expel the algae, leading to mass bleaching.
Branching and table-shaped corals provide shelter and habitat for reef inhabitants such as fish, and their loss reduces fish abundance and the productivity of coral reef fisheries.
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| Bleaching on the Great Barrier Reef corals near Cooktown in March 2020. Credit: Via Terry Hughes |
Better data on the demographic trends of corals is needed to understand how their populations are changing, the researchers found.
“We used to think the Great Barrier Reef is protected by its sheer size—but our results show that even the world’s largest and relatively well-protected reef system is increasingly compromised and in decline,” Professor Hughes said.
The authors are concerned about the shrinking gap between bleaching events because there is little opportunity for corals to rebound.
“There is no time to lose - we must sharply decrease greenhouse gas emissions ASAP,” the research paper states.
Global temperature increases would need to stabilise between 1.5 and two degrees for the reef to remain, even if it was quite different to what exists now, Professor Hughes said. “If it’s three or four degrees then forget about it.”
This year, February had the highest monthly sea surface temperatures ever recorded on the Great Barrier Reef since the Bureau of Meteorology’s records began in 1900.
The warming ocean is affecting reefs worldwide, including off the coast of Brazil, in parts of Melanesia and Indonesia.
Links
- (AU) Australia Cannot Wait For Climate Decision To Save Reef: Foundation
- Why This Year Is Our Last, Best Chance For Saving the Oceans
- (AU) This Sculpture Is A Mood Ring For The Great Barrier Reef
- (AU) If We Can Put A Man On The Moon, We Can Save The Great Barrier Reef
- (AU) We Just Spent Two Weeks Surveying The Great Barrier Reef. What We Saw Was An Utter Tragedy
- (AU) Scientists Trial Cloud Brightening Equipment To Shade And Cool Great Barrier Reef
- Big Parts Of The Great Barrier Reef Are Dying
- More Than Half Of Remote Reefs In Coral Sea Marine Park Suffered Extreme Bleaching
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