When bleaching events, storms and starfish hit the Great Barrier Reef the result is swathes of dead coral. Now researchers are warning the damage doesn't end there.
Experts say the rubble that's created when dead coral breaks up is a secondary but poorly understood threat to reef regeneration.
Given the shrinking interval between major reef disturbances like the mass bleaching events of 2016, 2017, and 2020, the race is on to unpack the problem and test potential solutions.
University of Queensland Professor Peter Mumby is one of the researchers trying to answer some key questions including why there are so few young corals on rubble-affected reefs like those around the Keppel islands.
"Is it the case that new corals avoid rubble and would rather settle on a healthier patch of reef or is it because when they're rolling around, they don't survive for very long?" he says.
Researchers also need to understand how long it takes for rubble that's constantly shifting with the waves and the tide to stabilise and bond back together with the help of organisms such as sea sponges and ascidians.
And they want to know how interventions, including gathering up rubble in biodegradable bags or installing stabilising metal frames, might aid reef regeneration.
Rubble needs to bond and become a stable lump in order for new corals to get a foothold and begin to grow again.
"The work I've done in various parts of the world has shown me that when you get a reef that ends up being covered in rubble, it can remain in that state for 10 years or more and can really slow down the recovery process," Prof Mumby says.
"In the past we wouldn't have been too concerned about that because there weren't as many disturbances as there are now, and there was plenty of time for recovery.
"But with bleaching events becoming more frequent, now and in the future, we really need to try to do what we can to accelerate the recovery phase once one of these events has happened."
Fellow UQ researcher Tania Kenyon says various stabilisation methods are now being trialled on the Great Barrier Reef, with results expected after about a year of monitoring.
The marine biologist will also be trying to identify the most important organisms that help bind rubble together, with a view to possibly seeding rubble beds with those organisms in the future.
Meanwhile, civil engineers are using wave tanks to work out how strongly bound rubble is over time, and how much wave force is needed to break it apart again.
The shared objective of all this work is to help reef managers know where to target their coral restoration work.
"The idea is you can then look at a map of the reef and say this area here, well rubble is probably not much of a problem because it's so sheltered that it will stabilise itself quite quickly," Prof Mumby says.
"An area over here, on the other hand, has the kind of wave environment that if rubble is established here it's not going to become stable enough for maybe eight years. That might be where you'd prioritise going in to do something manually."
The trial of stabilisation methods is part of the Reef Restoration and Adaptation Program, a partnership between the Great Barrier Reef Foundation, various universities, the Australian Institute of Marine Science and the CSIRO.
Environment Minister Sussan Ley said reef managers around the world were closely watching the work being carried out under the federally funded program.
"We know that coral reefs are under pressure from climate change and through the RRAP program we are researching new ways to help them adapt to changes in ocean temperature and recover from coral bleaching."
Australian Associated Press