Two tropical cyclones and flooding in the first three months of the year, as well as an extensive and serious mass bleaching event have brought some of the “largest recorded losses” to Queensland coral reefs in almost 40 years.

Following what the Australian Institute of Marine Science (AIMS) has called a ‘summer of disturbance events’, data collected from recent in-water surveys has indicated “substantial annual declines” in coral cover across 12 of the 19 coral reefs between Lizard Island and Cardwell along the Queensland coastline.

Scientists belonging to the Long Term Monitoring Programme have been analysing data collected from the first in-water surveys, conducted in August and October this year, to better understand the impact of climate-change driven events on the Great Barrier Reef coral populations in local waters.

While results have only been collected from a percentage of the reefs populating this region of the Queensland coast, already they have showed “substantial losses” of coral cover on 12 reefs, ranging from 11% declines in the least affected areas to as much as 72% in those that bore the largest brunt of this summer’s ‘disturbance events’.

More than one third of coral cover was lost across the Cooktown-Lizard Island Sector (plummeting from the recorded high of 31.4% cover one year ago to the 19.3% recorded today), marking the largest annual decline scientists from AIMS have recorded in 39 years.

Meanwhile, in the Cairns sector, coral cover has declined by just over a third across the five reefs surveyed to date. In the Innisfail sector, cover has remained similar to pre-summer levels across the four reefs surveyed.

“During February and March 2024, all the reefs we recently surveyed in this north Queensland region were subjected to levels of climate change-driven heat stress that causes bleaching,” said AIMS Long Term Monitoring Programme leader, Dr Mike Emslie.

“The heat stress got so high in some areas that mortality is not a surprising outcome.”

But bleaching isn’t the only culprit. Reefs surveyed in the Cairns sector, for instance, were subjected to both bleaching and Tropical Cyclone Jasper as it crossed the reef in December 2023. With it, the cyclone brought waves more than four metres high to which the Reef was exposed for somewhere between 33 and 40 hours.

Findings have only been collated from across a percentage of the total reef count along the Queensland coast, so far and AIMS scientists have noted there are some areas that managed to escape the ‘significant losses’ experienced elsewhere and by other coral species. 

“We saw mortality from coral bleaching that was variable within reefs,” said Dr Emslie. “Some areas were more impacted by heat stress in the top few metres, compared to corals deeper down the reef slope which were largely unaffected.

“Coral types also fared differently. Mortality was most common in the table Acropora corals – a fast-growing coral that has been partly responsible for the recent recovery on the Reef but is also susceptible to the kind of disturbance events we’ve seen this summer.

“Whereas, other coral types – such as branching Acropora were less affected, while massive corals – like Porites – were the least affected of all. This points to the variability and dynamics on coral reefs.”

It may also offer some clues to the survivalist ability of the recent discovery over in the Pacific Ocean’s Solomon Island last week, of the world’s largest coral, a ‘mega coral’ measuring in with a 183-metre circumference (making it larger than the average blue whale) and dating to somewhere between 300 and 500 years old.

AIMS’ acting research programme director, Dr Manuel Gonzalez Rivero said that the preliminary results from these in-water studies in Queensland provide a first glimpse of impacts from the 2024 Austral summer, suggesting that the Reef’s resilience is “being severely tested” right now. 

“We don’t yet have a full picture of how each region of the Reef has fared – our continued monitoring will capture this,” he said. “These initial results show the vulnerability of the Reef to bleaching events, which are increasing in frequency, footprint, and intensity under climate change. Its resilience is being severely tested.

“The 2024 mass bleaching event on the Reef, its fifth since 2016, forms part of the fourth global bleaching event impacting both the northern and southern hemispheres of the Atlantic, Pacific, and Indian Oceans during 2023 and 2024, documented in more than 60 countries and territories worldwide.”

Scientists agree that the driver behind events such as these is climate change and that the future of reefs around the world relies on strong greenhouse gas emissions reduction, management of local and regional pressures, and the development of approaches to help reefs adapt to and recover from the impacts we are already seeing.

To round out its picture of the health of local coral ecosystems, the team at the Long Term Monitoring Programme are currently collecting data on reefs in the Southern region of the Great Barrier Reef. AIMS is also collecting detailed data on reef communities, physiology, and genetics.