The global bleaching events over the last two years have posed potential threat for Australia’s iconic Great Barrier Reef, scientists have said. The 2,300 kilometre World Heritage Site faced widespread coral decline and loss of habitat than previously thought, they said.
Last year, the Great Barrier Reef experienced the most severe bleaching on record, warming ocean temperatures during March and April are to blame.
“We are very concerned about what this means for the Great Barrier Reef itself and what it means for the communities and industries that depend on it,” said Russell Reichelt, Chairman Great Barrier Reef Marine Park Authority in Australia.
“The amount of coral that died from bleaching in 2016 is up from our original estimates and, at this stage, although reports are still being finalised, it is expected we will also see an overall further coral cover decline by the end of 2017,” he said.
Massive aerial and in-water surveys were carried out and confirmed that there was a pronounced gradation in impacts from north to south.
It has now been confirmed that in 2016 an estimated 29 per cent of shallow water corals died from bleaching.
The coral bleaching spread to deeper corals, beyond the depths divers typically able to reach for survey, however, mortality cannot be systematically assessed. According to researchers, this is up from the original estimated 22 per cent in mid-2016. That time most mortality occurred in the north of the Reef.
The most severe mortality was recorded in the area north of Port Douglas. An estimated 70 per cent of shallow water corals died there. Also, there was significant variability between and within reefs.
Researchers said further coral loss is likely this year from the second consecutive year of bleaching and the impacts of tropical cyclone Debbie.
This is in addition to ongoing impacts from crown-of-thorns starfish, coral disease and poor water quality from coastal run-off.
The bleaching pattern in 2017 was similar to that of 2016 but most severe in the centre of the Reef between Cairns and Townsville. Elevated coral disease is caused by ongoing thermal stress. In early 2017, around a quarter of the Reef were impacted by tropical cyclone Debbie.
Combined with coral bleaching - which is predicted to become more frequent and more severe as a result of steadily rising ocean temperature - the long-term trend of coral decline is expected to continue and accelerate.
Researchers said that the recovery from bleaching is also likely to be slower than from other impacts.
What is coral bleaching?
Coral bleaching occurs when abnormal environmental conditions, like heightened sea temperatures, cause corals to expel tiny photosynthetic algae, called ‘zooxanthellae’.
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The loss of these colourful algae causes the corals to turn white, and bleach. Bleached corals can recover if the temperature drops and zooxanthellae are able to recolonise them, otherwise the coral may die.