Six months on from the worst coral bleaching event to ever hit the Great Barrier Reef, many of the corals in northern third of the Reef have died off, according to new underwater surveys.
Researchers from the ARC Centre of Excellence for Coral Reef Studies at James Cook University conducted fresh surveys on the same 83 reefs that had first been surveyed in March this year. They found that reefs hit hardest by the bleaching event have slowly died from starvation, predation and disease.
“Millions of corals in the north of the Great Barrier Reef died quickly from heat stress in March and since then, many more have died more slowly,” says Dr Greg Torda, who led one of the survey teams near Lizard Island in far-north Queensland.
“Six months after the peak bleaching, the corals now have either regained their algal symbionts and survived, or they have slowly starved to death without the nutrition the algae provided them.”
The new surveys, which were released today as part of a new map of the Great Barrier Reef, show that although the southern half of the Reef remains in good condition thanks to it escaping the worst the bleaching, the northern half of the reef stretching from Cairns to Papua New Guinea remained in a state of devastation.
“In March, we measured a lot of heavily bleached branching corals that were still alive, but we didn’t see many survivors this week,” says Dr Andrew Hoey, also part of the Lizard Island survey team.
For example, the amount of live corals in reefs near Lizard Island had dropped from an already low figure of 40 per cent in March to below 5 per cent today.
Coral bleaching occurs when extreme environmental conditions, such as higher than normal ocean temperatures, cause corals to become stressed and eject the photosynthesising algae companions (zooxanthellae) that give them nutrients and their rich colour.
If these conditions persist and the zooxanthellae do not return, the bleached corals can starve to death. The large-scale devastation of this year’s coral bleaching event makes it worse than the bleachings that occurred in 1998 and 2002.
The researchers say that one of the reasons why the mortality rate has been so high in these reefs is because the effects of bleaching have been exacerbated by the presence of disease and coral-eating predators such as snails.
However, Professor Andrew Baird, who led survey teams in the central section of the Reef, says the southern half remained in much better shape than the northern section.
“There is still close to 40 per cent coral cover at most reefs in the central Great Barrier Reef, and the corals that were moderately bleached last summer have nearly all regained their normal colour,” he says.
The researchers will announce the final death toll of the northern section of the Reef once all of the underwater surveys are completed in November.