Marine heatwaves pose greater threat to coral reefs than once thought

Marine heatwaves are a much bigger threat to coral reefs than previously thought, as new research reveals a previously unrecognized impact of climate change on coral reefs.

Image credit: UNSW

A new study by a team of researchers in Australia, the UK and the US shows for the first time what really happens to corals during marine heatwaves – and it reveals that coral animals aren’t the only ones that are affected. As their skeletons also start to decay within a few weeks, the 3D coral framework that provides a home to many other animals on the reef is also at risk.

Heatwaves are killing the coral animal itself

Back in 2016, the team’s research showed that just a 0.5C increase in ocean temperature changes the extent of mortality that happens in coral during bleaching.

In their latest study, published last week in the journal Current Biology, the team now find that severe marine heatwaves not only trigger bleaching events as we have known them – a breakdown of symbiosis – but in fact can lead to heat-induced mortality of the coral animal itself. They suggest that severe heatwave-induced mortality events should therefore be considered a distinct biological phenomenon, with more direct damage different from coral bleaching.

“Until now, we have described coral bleaching as an event where the symbiotic relationship between coral and its microbes breaks down and corals lose their main source of nutrition, and the coral can die if the symbiosis is not restored,” says lead author Tracy Ainsworth from the University of New South Wales.

“But what we are now seeing is that severe marine heatwave events can have a far more severe impact than coral bleaching: the water temperatures are so warm that the coral animal doesn’t bleach – in terms of a loss of its symbiosis – the animal dies and its underlying skeleton is all that remains.”

Devastating for the skeleton left behind

“We find that the skeleton is immediately overgrown by rapid growth of algae and bacteria,” says co-author Bill Leggat of the University of Newcastle.

By using CT scanning of the coral skeleton, the researchers have found that this process of rapid algal and bacterial colonisation is “devastating not just for the animal tissue, but also for the skeleton that is left behind, which is rapidly eroded and weakened”, adds Leggat.

The new bio-optical techniques allow the researchers to visualise and study the rapid transition in the coral microbiome for the first time, showing howing the microbial communities go from symbionts to harmful coral skeleton-dissolvers. As the frequency and intensity of heatwaves are expected to increase, adopting these techniques more broadly will be “central to understand how this process occurs on reefs globally”, add University of Technology Sydney scientists David Suggett and Emma Camp.

‘Unknown unknowns’ could cause more damage

“Climate scientists talk about ‘unknown unknowns’ – impacts that we haven’t anticipated from existing knowledge and experience,” explains Scott Heron from James Cook University. The rapid dissolving of coral skeletons following severe heatwaves fits into this category.

“As we begin now to understand this impact, the question is how many more of these ‘unknown unknowns’ might there still be that could bring faster and greater damage to coral reefs from climate change,” he says.

Fortunately, such events are predictable, and scientists are already using climate models and satellite data to predict and detect conditions that cause coral bleaching. “By focusing on especially severe marine heatwaves, we should be able to predict this direct coral death, too,” says Mark Eakin of NOAA’s Coral Reef Watch.

The team hopes that their research will motivate the public to tell decision makers how important coral reefs are to them and to voice the immediate need to preserve coral reefs now.

“Given that the degradation of coral reefs will result in the collapse of ecosystem services that sustain over half a billion people, we urgently need actions both globally and locally that protect and conserve these truly wonderful places,” says Tracy Ainsworth.

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