Heatwaves ‘cooking’ corals to death

Climate change is warming our oceans, and even a 1-degree Celsius rise is enough to completely transform complex ecosystems such as the Great Barrier Reef. A new study reveals that corals were more or less ‘cooked’ to death by the marine heatwave of 2016. Barbara Barkhausen reports from Sydney.

The various colours demonstrate different bleaching responses during a mass coral bleaching event. (Image credit: ARC Centre of Excellence for Coral Reef Studies/Gergely Torda)

Living coral are colourful creatures, a vibrant maze offering shelter to countless species of fish and other marine life. But when they bleach, they become ghostly, fragile skeletons that are crushed into tiny pieces by the next storm.

The Great Barrier Reef has been hit by such devastating bleaching two years in a row, and the extended marine heatwave of 2016 was especially catastrophic. According to a recently published study in the journal Nature, 29 per cent of the 3,863 reefs of the Great Barrier Reef lost two-thirds or more of their corals that year. In some cases, up to 90 per cent of corals died off.

One of the Australian researchers told media that the corals were literally cooked to death. “They didn’t die slowly of starvation, they died directly of heat stress,” said Terry Hughes from James Cook University (JCU). “They cooked because the temperatures were so extreme.”

Nearly a third of corals lost

During a bleaching event, corals turn white because their symbiosis with a type of algae, which supplies the cnidarians with energy and gives them their bright colours, is interrupted.

“When corals bleach from a heatwave, they can either survive and regain their colour slowly as the temperature drops, or they can die,” explained Hughes. “Averaged across the whole Great Barrier Reef, we lost 30 per cent of the corals in the nine-month period between March and November 2016.

Bleaching linked to heat exposure

The scientists mapped the geographical pattern of heat exposure from satellites and measured coral survival along the 2,300-kilometre length of the Great Barrier Reef after the extreme marine heatwave of 2016.

They found that the amount of coral death was closely linked to the amount of bleaching and level of heat exposure. The northern third of the Great Barrier Reef, a region that was home to 50- to 100-year-old corals, was especially hard hit. Until then, the region was considered relatively unspoiled.

Table and staghorn corals died

The coral die-off also caused radical changes in the mix of coral species, said co-author Andrea Baird. “Mature and diverse reef communities are being transformed into more degraded species, with just a few tough species remaining.”

During the 2016 marine heatwave, many table corals and staghorn corals, which make up a significant part of the reef structure, died. Table corals in particular offer protection for smaller reef dwellers, acting as a kind of “kindergarten” in which young fish can grow up undisturbed.

Some 1,500 fish species make their home among the 400 coral types, fish that later grow up to become an important source of food for humans.

Climate change will makes things worse

The current damage is a harbinger of what climate change has in store for the Earth’s reef ecosystems in the future.

“Failure to curb climate change, causing global temperatures to rise far above 2 degrees Celsius, will radically alter tropical reef ecosystems and undermine the benefits they provide to hundreds of millions of people, mostly in poor, rapidly developing countries,” warn the authors.

Climate change is the greatest threat to the Great Barrier Reef. A separate study in April 2016 conducted by leading Australian climate and reef experts showed that coral bleaching on the Great Barrier Reef has become 175 times more likely due to climate change.

Should greenhouse gases continue to rise in the atmosphere, the scientists pessimistically predict that bleaching will occur every two years by 2030.

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