Great Barrier Reef has lost more than HALF its corals in the last 25 years due to bleaching caused by climate change, study reveals
- Experts from Australia have been surveying coral populations from 1995–2017
- They found that small, medium and large coral colonies are all dying off
- Human-driven climate change is causing heatwaves that raise sea temperatures
- This bleaches corals of their symbiotic algae, starving them of their food supply
More than half of the corals on the Great Barrier Reef have been lost to ‘bleaching’ due to climate change over the last 25 years, a study has found.
Experts studying the state of the Australian reef system — one of the seven natural wonders of the world — reported that it is disappearing faster than thought.
Human-induced climate change is speeding up so-called bleaching episodes, in which corals lose their vital symbiotic algae, turn white and can ultimate die off.
More than half of the corals on the Great Barrier Reef have been lost to ‘bleaching’ due to climate change over the last 25 years, a study has found Pictured, a coral on the reef
VIRUS AIDS CORAL BLEACHING EVENTS
Researchers from the Oregon State University in the US have found that an infection aids coral bleaching.
The team compared the viral metagenomes of coral colonies in Mo’orea, in French Polynesia, during a minor bleaching event in 2016.
‘After analysing the viral metagenomes of each pair, we found that bleached corals had a higher abundance of eukaryotic viral sequences,’ said paper author and Adriana Messyasz.
‘Non-bleached corals had a higher abundance of bacteriophage sequences,’ he added.
‘This gave us the first quantitative evidence of a shift in viral assemblages between states.’
Eukaryotic viruses are those that affect animals, while Bacteriophages selectively infect bacteria.
The team also found that bleached corals had a greater abundance of a new type of ‘giant’ virus that may aid bleaching, they said.
The full findings of the study were published in the journal Frontiers in Marine Science.
‘We found the number of small, medium and large corals on the Great Barrier Reef has declined by more than 50 per cent since the 1990s,’ said paper author and marine biologist Terry Hughes of the University of Queensland, Australia.
‘The decline occurred in both shallow and deeper water and across virtually all species — but especially in branching and table-shaped corals.’
‘These were the worst affected by record-breaking temperatures that triggered mass bleaching in 2016 and 2017.’
Branching and table-shaped corals provide build structures on the reef that are important for other marine life, such as fish.
Their loss is serving to reduce population sizes and sea food productivity.
So vast that it can be seen from space, the reef off of Queensland’s coastline hosts Earth’s largest collection of living organisms, including some 1,500 fish species.
The Great Barrier Reef is larger than the Great Wall of China and stretches for around 1,400 miles over an area of roughly 133,000 square miles.
When ocean temperatures are too high, corals expel their colourful symbiotic algae that provide them with food — turning them a bleached white.
If the ocean cools quickly enough, the ejected algae can return, but if it stays too hot for too long, the corals begin to starve.
Studying changes in the coral populations can reveal valuable insights into how the reef has responded to past stresses and how it will likely fare in the future.
In their study, Professor Hughes and colleagues documented the changes in the size of individual colonies along the length of reef from between 1995–2017.
‘Both small and large colonies have become increasingly rare,’ said paper author Andy Dietzel of Queensland’s ARC Centre of Excellence for Coral Reef Studies.
‘This indicates declines in reproduction, recovery potential and the long term resilience of coral populations.’
‘We measured changes in colony sizes because population studies are important for understanding demography and the corals’ capacity to breed.’
‘A vibrant coral population has millions of small, baby corals, as well as many large ones — the big mamas who produce most of the larvae.’
‘Our results show the ability of the Great Barrier Reef to recover — its resilience — is compromised compared to the past, because there are fewer babies, and fewer large breeding adults.’
The researchers said that there is an urgent need for better data on the shifting demographics of the corals which make up the Great Barrier Reef.
‘If we want to understand how coral populations are changing and whether or not they can recover between disturbances, we need more detailed demographic data — on recruitment, on reproduction and on colony size structure,’ said Dr Dietzel.
The Great Barrier Reef is composed of more than 2,900 individual reefs and some 900 islands — and is bigger than the entirety of the UK.
The natural wonder is home to around 1,625 species of fish, 3,000 molluscs and 30 different types of whale and dolphin.
‘We used to think the Great Barrier Reef was protected by its sheer size,’ commented Professor Hughes.
‘But our results show even the world’s largest and relatively well-protected reef system is increasingly compromised and in decline.’
The Great Barrier Reef is composed of more than 2,900 individual reefs and some 900 islands — and is bigger than the entirety of the UK. The natural wonder is home to around 1,625 species of fish, 3,000 molluscs and 30 different types of whale and dolphin
Climate change is driving an increase in the frequency of disturbances like marine heatwaves that have the potential to harm the coral that make up the reef.
The team noted steeper deteriorations of coral colonies in the Northern and Central Great Barrier Reef after the mass bleaching events in 2016 and 2017 — alongside damage to southern populations after record high temperatures in early 2020.
‘There is no time to lose — we must sharply decrease greenhouse gas emissions ASAP,’ Dr Dietzel warned.
The full findings of the study were published in Proceedings of the Royal Society B.
Coral expel tiny marine algae when sea temperatures rise which causes them to turn white
Corals have a symbiotic relationship with a tiny marine algae called ‘zooxanthellae’ that live inside and nourish them.
When sea surface temperatures rise, corals expel the colourful algae. The loss of the algae causes them to bleach and turn white.
This bleached states can last for up to six weeks, and while corals can recover if the temperature drops and the algae return, severely bleached corals die, and become covered by algae.
In either case, this makes it hard to distinguish between healthy corals and dead corals from satellite images.
This bleaching recently killed up to 80 per cent of corals in some areas of the Great Barrier Reef.
Bleaching events of this nature are happening worldwide four times more frequently than they used to.
An aerial view of Australia’s Great Barrier Reef. The corals of the Great Barrier Reef have undergone two successive bleaching events, in 2016 and earlier this year, raising experts’ concerns about the capacity for reefs to survive under global-warming
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