Corals in the Great Barrier Reef have a lower tolerance to heat stress than expected, contributing to a permanent transformation of the mix of species in some of most pristine regions, a team of international researchers has found.
The scientists examined the impact of the 2016 marine heatwave that alone caused the death of about one-third of the Great Barrier Reef corals, mostly centred on the northern third section.
They studied how much abnormal heat triggers bleaching, the additional heat that killed the corals, and the accumulation needed to cause "an ecological collapse in the transformation of species", said Terry Hughes, Director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University, and the lead of author of the paper published Thursday in Nature.
The thresholds "are lower than we thought they would be", Professor Hughes told Fairfax Media.
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| Bleaching in the northern part of the Great Barrier Reef in 2016 triggered widespread mortality of corals within weeks, scientists found. Photo: AAP |
Some species, such as staghorns and tabular corals, were particularly susceptible, while dome-shaped porites corals were relatively resilient.
"The 2016 marine heatwave has triggered the initial phase of that transition [to heat-tolerant reef assemblages] on the northern, most pristine region of the Great Barrier Reef, changing it forever as the intensity of global warming continues to escalate," the paper said.
The fact the northern section - with fewer people, little fishing and almost no water quality issues - was hit so hard was notable.
"There’s almost nowhere to hide from extreme temperatures," Professor Hughes said. "Even the best-managed, most remote place is vulnerable.”
The Nature paper is one of a series being prepared or already published by Professor Hughes and colleagues at James Cook University that examine the unprecedented back-to-back mass bleaching of the Great Barrier Reef in the summers of 2016 and 2017.
Upcoming papers will examine the impacts of the latter event - which mainly hammered the middle section of the Reef - and the scope for recovery.
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| Fish species are adjusting to the mass mortality of corals - some better than others. Photo: University of Wollongong |
Butterfly fish, for instance, feed on only a couple of coral species. “If their diet disappears, so do they," Professor Hughes said. Parrot fish, though, eat mostly seaweed, and will fare better.
Still, most fish depend on branching corals as a nursery - the type of corals that suffered high mortality in the heatwave.
"People have long predicted there will be significant fisheries impact from losing juvenile habitat and we expect that to unfold over the next five years," Professor Hughes said.
Most of what researchers know about reef recovery has come from the study of the aftermath of cyclones.
Such events, though, tend to carve a swathe through the reef, perhaps 50 to 100 kilometres wide, with patchier damage than mass bleaching. Coral larvae come into the void from either side, typically taking a decade for branching corals to recover.
“The scale of the damage from back-to-back bleaching is vastly bigger," Professor Hughes said. "We don’t know yet where the larvae are going to come from, and in what numbers.”
Links
- Losing Nemo? Wider effects of mass Great Barrier Reef bleaching emerge
- World's Coral Reefs Face New Peril From Beneath Within Decades
- Great Barrier Reef In 'Deep Trouble' As Climate, Other Threats Mount: Official
- 'Wasteful Stunt': Turnbull Government Accused Of Doing Too Little To Save Reef
- Great Barrier Reef: Rising Temperatures Turning Green Sea Turtles Female
- Great Barrier Reef Will Be Dead By 2100, Says David Attenborough's Blue Planet II
- Warming Limit Of 1.2 Degrees Needed To Save Great Barrier Reef: Expert Panel
- While Corals Die Along The Great Barrier Reef, Humans Struggle To Adjust
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