Australian scientists are throwing new light on the killers responsible for the mass death of corals under global warming.
And one of their interesting findings is that 'fat' corals cope with stress and survive the onslaught a lot better than those which have been on a diet.
In a world-first study, researchers from the Australian Research Council Centre of Excellence for Coral Reef Studies (CoECRS) have shown how hot water, sunlight and starvation combine to bring about the mass die-offs known as coral bleaching.
Research by Dr Ken Anthony and Dr Sean Connolly of James Cook University and Professor Ove Hoegh-Guldberg of The University of Queensland has explained for the first time why some bleaching events are more devastating than others.
Researchers have known for some years that high sea water temperatures are responsible for coral bleaching, by causing the corals to shed their zooxanthellae (symbiotic single-celled plants) which provide their main food source. Unless these plants re-colonise the coral quickly, it dies.
Recently it has emerged that fierce sunlight can also cause corals to bleach and die, even when sea water temperatures are cool.
In a major experiment involving 1600 corals kept in 16 1000-litre tanks on Orpheus Island on the Great Barrier Reef, the team studied the interplay of various factors – temperature, light, coral condition and water quality – to determine which conditions produced the highest risk of coral death.
"We found that a combination of warm water, strong light and corals that were low on energy was most likely to result in high mortality," Dr Connolly said.
"Corals store their spare energy in lipids, or fats, and they can live for a while on these reserves if their zooxanthellae abandon them.
"So you might say that 'fat' corals have a better chance of survival when stressed than 'skinny' corals," Dr Anthony said.
The team also discovered that turbid water, containing sediment, provided a temporary food source for corals while the symbiotic algae were absent, and may also have acted as a sun-screen.
"Murky water is a bit of a two-edged sword," Professor Hoegh-Guldberg said. "On the one hand it can smother corals in sediment and help weeds to establish, which destroys the reefs by out-competing the corals.
"But murky water at the right time – when a bleaching event is in progress – can also be a life-saver. We've often noticed how coastal corals in more turbid waters survive bleaching better than corals in sparkling clear waters. The shading and added nutrition turbid water provides may be the explanation."
Since sediment in the Great Barrier Reef lagoon is mainly stirred up by the wind, wind conditions at the time of a bleaching event may thus play a role in how much coral survives or dies.
Dr Anthony said the new insights into mortality rates in bleached corals were both of scientific interest and of value to reef managers in better understanding and predicting conditions for bleaching and coral death, which is expected to occur with greater frequency under global warming.
This could enable them to develop management strategies that will reduce other stresses on corals at a time when they are particularly susceptible to climate change extremes, so they can recover more quickly from bleaching, he says.