Last month, the National Oceanic and Atmospheric Administration (NOAA) confirmed that today, more than 83% of the planet’s reefs have been hit by extreme ocean heat since January 2023. This month, the plot thickens, as scientists report that even the most resilient of Australia’s coral reefs are struggling to cope – including the subtropical reefs of Lord Howe Island.

As the world’s fourth – and most severe – mass coral bleaching event continues to unfold, scientists at the University of New South Wales have warned that even those that had so far avoided the kind of coral bleaching events seen on the Great Barrier Reef and Western Australia’s Ningaloo Reef this year are feeling the pressure.

Paige Sawyers, a PhD candidate at the University of New South Wales Sydney has been monitoring the reefs at the island alongside researchers from the University of Newcastle for more than a year. 

In a new paper published this week, she has argued that while Lord Howe Island’s coral reefs have avoided the most extreme bleaching so far, they are still struggling to recover from previous consecutive years of heat stress.

And it’s this, the researchers have warned, that drives the decline of coral reefs. 

“We know that recovery for these southern, subtropical reefs from bleaching and stress events is extremely slow,” said Ms Sawyers. “So while this year we didn’t see the bleaching temperatures of last year, we do have ongoing heat stress – which has occurred exactly when the reef needs to do its fastest recovery.”

These conditions, she has written, are concerning for reefs long-term, as sustained coral losses, disease, and ecosystems change ecosystems permanently. 

“We have never had warming this high for this long – we’re not really sure what to expect after this. That’s what’s so scary.”

Sitting off Australia’s east coast, Lord Howe Island is home to the world’s southernmost coral reef. It’s often considered a more temperate and resilient reef than the Great Barrier Reef. The island itself sits at the crossroads of five major ocean currents, including the East Australian Current, which brings tropical species down from the Great Barrier Reef. This convergence of currents means Lord Howe is a biodiversity hotspot, as well as a key site for understanding how coral reefs might adapt in a warming world.

Reefs are critical marine ecosystems and a home to a third of all marine species. They support coastal communities with food and livelihoods – through fishing as well as tourism. They also serve as a natural buffer from storm surges and coastal erosion.

Yet, while tropical reefs such as the Great Barrier Reef and Ningaloo have been studied for decades, subtropical systems like Lord Howe are understudied. When Ms Sawyers and her team first arrived at the island in January 2024, sea temperatures were already at 26°C – two-degrees above the average. Coral bleaching tends to occur at just a single degree above average. By February 2024, temperatures had surged to 29 to 30°C. And that’s when the bleaching took hold across the site.

“All the susceptible species were bleached,” said Sawyers. “And it happened so fast – just a two-degree spike, and suddenly every site we surveyed was impacted.”

As the emotional strain on reef scientists grows, Sawyers said this need to now be a part of the conversation.

The Great Barrier alone has experienced seven known mass coral bleaching events: in 1998, 2002, 2016, 2017, 2020, 2022, and 2024. Corals are marine animals that get stressed by the heat. When seas are warmer than usual, they expel their microscopic zooxanthellae. These are the symbionts that live within the coral’s tissues, giving them their bright colours and supplying up to 90% of their energy through photosynthesis. Without zooxanthellae, corals begin to starve, turn pale, and bleach.

When water temperature drops, however, recovery can begin. But as the gap between abnormally high temperatures narrows, corals and coral reefs lose their window for recovery.

“It gets to the stage of something more like a heatstroke,” said Sawyers. “At first they expel their zooxanthellae to try to conserve energy. But without recovery, the whole animal starts breaking down. There’s tissue loss, with disease and opportunistic pathogens taking over.”

Researchers had seen signs of recovery at Lord Howe Island in April 2024 when some colour returned to the reef. The following month, however, a sudden and severe low tide struck, exposing the top 20 centimetres of the reef to the air and sun.

“You could see a distinct line where the water level dropped,” said Sawyers. “While corals within the water were fine, everything above it – every single coral that was exposed – was covered in brown algae, dead.”

Occurrences like this are becoming more common. And for corals already weakened by heat, even minor anomalies can be fatal. 

Sawyers said: “It’s a perfect storm for reefs to suffer disease events, be outcompeted by algae, and for the reefs themselves to change.”

A further issue is that the climate is now changing too fast for even restored corals to adapt. However, similar cases in Samoa have seen corals bounce back, offering researchers a glimmer of hope that recovery can be achieved. 

“We need to know what’s out there before it disappears. We need to identify the survivors and understand recovery. Go back to basics,” said Sawyers. “What makes them survive? Because if we can answer that, maybe we can protect what’s left.”

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