The rebound in the number of corals growing on the Great Barrier Reef over recent years has paused, with government scientists blaming bleaching, disease and starfish attacks.
The results of the latest annual surveys of more than 100 reefs show a slight decrease in coral cover over the northern and central parts of the reef over the past year.
The Great Barrier Reef – the world’s largest coral reef system – faces an uncertain future as the ocean continues to accumulate heat from burning fossil fuels.
This heat has triggered a series of mass coral bleaching events on the reefs, including four in the past seven years, that can weaken the 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 coral reefs conducted between August 2022 and May this year.
The surveys come after the summer of 2022 saw the first mass coral bleaching occur during La Niña – a weather pattern that usually brings cooler conditions.
The report said: “The effects of the 2022 bleaching event, the fourth in seven years, caused coral loss on some reefs. It is likely that those corals that survived the bleaching were affected by reduced growth and reproduction.”
Last year’s report said that three years of relatively comfortable conditions produced record levels of coral cover in the northern and central regions of the reef.
This coral growth was supported by a group of fast-growing coral species that also tend to be most at risk from bleaching events.
There was an expectation that corals would continue to recover, but the data showed otherwise, said Dr. Mike Emsley, who leads the long-term coral reef monitoring program at Aims.
“This shows that less severe bleaching events are sufficient to cause coral cover to stop,” he said.
The rebound in recent years has been “definitely a good news story,” he said, but that “could turn around very quickly with another mass bleaching event, and there’s still a risk of spiny starfish and coral disease.”
When corals sit in unusually warm water for too long, they break free from the algae that live inside them. Algae provide many nutrients to corals and give them their color.
Corals can recover if temperatures are not so high, but scientists say there are also “near-lethal” effects from bleaching.
Coral reef experts are concerned about the possibility of an El Niño weather pattern this summer, increasing the risk of another mass bleaching event.
The last summer saw mild conditions with low coral bleaching and only one hurricane, said Dr. David Wachenfeld, research program director at Aims.
“However, we are one large-scale disruption away from a rapid reversal of the recent recovery,” he said.
“Coral reefs remain an amazing, complex and beautiful system, but they are at increased risk with climate change leading to more frequent and intense bleaching events, further stressing the resilience of the ecosystem.”
The report said that hard coral cover in the northern part 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 33.8% – almost unchanged from the previous year. In this section, some corals have suffered from diseases and attacks from crown-of-thorns starfish.
“Explorer. Unapologetic entrepreneur. Alcohol fanatic. Certified writer. Wannabe tv evangelist. Twitter fanatic. Student. Web scholar. Travel buff.”