A recovery in the number of corals growing on the Great Barrier Reef over recent years has paused, with government scientists blaming bleaching, disease and attacks by starfish.
Results from the latest annual surveys of more than 100 individual reefs show a small drop in coral cover over the northern and central parts of the reef over the past year.
The Great Barrier Reef – the world’s biggest coral reef system – faces an uncertain future as the ocean continues to accumulate heat caused by the burning of fossil fuels.
That heat has caused a series of mass coral bleaching events over the reef, including four in the past seven years, that can weaken corals and affect their ability to reproduce.
The report from the Australian Institute of Marine Science (Aims) details the results of in-water surveys of 111 reefs carried out between August 2022 and May this year.
The surveys came after the summer of 2022 which saw the first mass coral bleaching on record to occur during a La Niña – a climate pattern that usually brings cooler conditions.
The report said: “The effects from the 2022 bleaching event, the fourth in seven years, caused some coral loss on some reefs. It is likely that those corals which survived bleaching have been affected by reduced growth and reproduction.”
Last year’s report said three years of relatively benign conditions produced record levels of coral cover in the reef’s northern and central areas.
That growth in corals was underpinned by a group of faster-growing coral species that also tend to be the most at risk from bleaching events.
Dr Mike Emslie, who leads the long-term reef monitoring program at Aims, said there was an expectation the reef would have continued to recover, but the data showed otherwise.
“This demonstrates that even less-severe bleaching events are enough to cause a pause in coral cover,” he said.
He said the recovery in recent years was “definitely a good news story”, but this “could turn around very quickly with another mass bleaching event, and there’s still the risk from crown-of-thorns starfish and coral disease”.
When corals sit in unusually warm water for too long, they seperate from algae that live inside them. The algae provide much of the corals’ nutrients, and give them their colour.
Corals can recover if temperatures are not too extreme, but scientists say there are also “sub-lethal” effects from bleaching.
Reef experts are concerned an El Niño climate pattern could take hold this summer, further raising the risk of another mass bleaching event.
Dr David Wachenfeld, the research program director at Aims, said the most recent summer had delivered mild conditions with low coral bleaching and only one cyclone.
“However, we are only one large scale disturbance away from a rapid reversal of recent recovery,” he said.
“The reef remains a wonderful, complex and beautiful system, but it is at increased risk with climate change driving more frequent and severe bleaching events, putting increasing pressure on the ecosystem’s resilience.”
The report said hard coral cover in the northern section of the reef, from Cape York to Cooktown, was estimated at 35.7%, down from 36.5% in 2022.
Between Cooktown and Proserpine, the reef’s central region, coral cover was estimated at 30.8%, down from 32.6% last year.
In the southern region, from Proserpine to Gladstone, coral cover was at 33.8% – virtually unchanged from the previous year. In this section, some reefs suffered from disease and attacks from crown-of-thorns starfish.
