While the scale of coral mortality is not yet known, recurrent bleaching events profoundly change the composition of coral communities on the reef.
On Tuesday, the Marine Park Authority of the Great Barrier Reef (GBR) published a report showing that over 90 percent of the Australian coral reefs that were surveyed suffered some form of bleaching.
The "Reef Snapshot: Summer 2021-22" pointed out the high accumulated heat stress this summer had caused 654 of the 719 reefs surveyed suffering from bleaching. An aerial survey showed the northern and central regions of the GBR are in a more serious situation.
The pattern of shallow-water coral community bleaching varied from minor (1-10 percent coral cover bleached) to severe (61-90 percent bleached) in the northern region, with severity generally increasing from the northern tip of Cape York down towards Princess Charlotte Bay.
In the central region, the most severe bleaching occurred on both inshore and offshore reefs throughout this region. Coral community bleaching ranged from major (31 percent to 60 percent bleached) to extreme (over 90 percent bleached) on the shallow parts of reefs from Cooktown to the Whitsundays.
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"While the scale of coral mortality is not yet known, recurrent bleaching events profoundly change the composition of coral communities on the reef," said Sarah Hamylton, president of the Australian Coral Reef Society.
"Reports of low coral mortality may reflect shifts in the composition of coral communities, with assemblages being more dominated by resilient and stress-tolerant corals, but an overall loss of biodiversity in the ecosystem."
Despite the La Niña conditions, the GBR's water warmed early in Dec. 2021, exceeding historical summer maximums that typically occur in the hottest summer months. Ocean temperatures continued to accumulate heat throughout the summer until early April 2022, with three distinct heatwaves increasing thermal stress throughout the central and northern GBR.
"To this end, urgent work is still needed to devise a credible national plan to reduce the domestic greenhouse gas emissions that are causing water temperatures to rise, at a speed consistent with the survival of the GBR," Hamylton said.