Great Barrier Reef coral cover at record levels after mass-bleaching events, report shows

Record coral cover is being seen across much of the Great Barrier Reef as it recovers from past storms and mass-bleaching events. But the new coral taking over is leaving the reef more vulnerable to future devastating impacts, according to the Australian Institute of Marine Science (AIMS).

AIMS’ 36-year Long-Term Monitoring Program has seen continued dramatic improvement in coral cover in the northern and central sections of the reef, following a period without intense disturbances.

The results come off the back of mass coral bleaching events that have happened at an unprecedented frequency — four out of six occurred in the last seven years. Mass bleaching, caused by marine heatwaves, was not known to occur at all prior to 1998.

When the water gets too hot, the algae that live inside the coral and provide it with most of its energy is expelled. If it remains too hot for too long, the coral stars and dies.

“The 2020 and 2022 bleaching events, while extensive, didn’t reach the intensity of the 2016 and 2017 events and, as a result, we have seen less mortality,” AIMS chief executive Paul Hardisty said.

“These latest results demonstrate the reef can still recover in periods free of intense disturbances.”

Eighty-seven reefs were surveyed between August 2021 and March 2022 as part of the report, which showed cover in the north increased from 27 per cent to 36 per cent, and from 26 per cent to 33 per cent in the central section.

That recovery has led to the highest-ever coral cover the Long-Term Monitoring Program has recorded in those sections, which begin north of Mackay.

But Dr Hardisty said the frequent bleaching showed how vulnerable the reef remained.

Despite the good news, the southern section, which extends from the Whitsundays down past the Keppel group of islands, has seen a small reduction in coral cover largely due to an ongoing outbreak of coral-eating crown-of-thorns starfish.

“This shows how vulnerable the reef is to the continued acute and severe disturbances that are occurring more often, and are longer lasting,” Dr Hardisty said.

But even the southern section of the reef remains in relatively good health, with 34 per cent coral cover, a reduction from a recent peak of 37 per cent in 2017.

Increased coral cover could come at a cost

The rapid growth in coral cover appears to have come at the expense of the diversity of coral on the reef, with most of the increases accounted for by fast-growing branching coral called acropora.

Those corals grow quickly after disturbances but are very easily destroyed by storms, heatwaves and crown-of-thorns starfish. By increasing the dominance of those corals, the reef can become more vulnerable.

It is a point acknowledged by Jodie Rummer, a marine biologist at James Cook University in Townsville.

“While it’s great to see increases in coral cover of a particular species, we can’t ignore that the diversity is really what we need to emphasise, and that’s going to be key to a healthy ecosystem over the longer term,” Professor Rummer said .

“While one species might be fast growing and repopulating very quickly, that also might be the most susceptible to some of the stressors that the Great Barrier Reef has faced over and over and over again over the past decade.”

Senior research scientist Mike Emslie, who leads the AIMS Long Term-Monitoring Program, agreed the news was mixed when it came to acropora.

“These corals are particularly vulnerable to wave damage, like that generated by strong winds and tropical cyclones,” Dr Emslie said.

“They are also highly susceptible to coral bleaching, when water temperatures reach elevated levels, and are the preferred prey for crown-of-thorns starfish.

“This means that large increases in hard coral cover can quickly be negated by disturbances on reefs where acropora corals predominate.”

Reef remains in danger from rising temperatures

Around the world, coral reefs face a grim future unless urgent action is taken to drastically halt man-made global warming.

In 2018, the United Nations released a report warning that coral reefs worldwide were projected to decline by up to 90 per cent even if warming was capped at 1.5 degrees Celsius.

Great Barrier Reef campaigner with the Australian Marine Conservation Society Cherry Muddle said while the findings were promising, the reef remained in danger.

“The fact remains that unless fossil-fuel emissions are drastically cut, the reef remains in danger from rising temperatures and more mass bleaching events,” she said.

“In the wake of the State of the Environment report, which showed Australian inshore reefs were in a poor and deteriorating condition due to climate- and water-pollution pressures, it is more important than ever that we ensure urgent action is taken to address all threats to the reef.”