The ocean heatwave that killed a WA reef

Bleached coral

Record-breaking sea surface temperatures in 2016 bleached up to 80 per cent of the Kimberley’s super-tough coral and nearly 30 per cent of coral off Rottnest Island, scientists say. Despite being home to some of the world’s most stress-resistant coral, in-shore Kimberley reefs, were devastated by the most severe global bleaching event ever recorded, a survey of the entire WA coastline has found.

The researchers from UWA, the ARC Centre of Excellence and WA Marine Science Institution found Ningaloo Reef, which is still recovering from major bleaching in 2010-11, was not affected.

The 2016 global bleaching event was the third and longest on record and the Kimberley region was the hardest hit.

Between March and May 2016, the world’s oceans were 0.75C above the 20th century average of 16.1C.

UWA biologist Dr Verena Schoepf says vast areas of the Kimberley’s coral reef are now dead and overgrown with algae.

“We found a concerning 57 to 80 per cent of corals on inshore Kimberley reefs were bleached in April 2016 – this included Montgomery Reef, Australia’s largest inshore reef,” Dr Schoepf said.

“Our research also found that there was mild bleaching at Rottnest Island – 29 per cent of corals were moderately bleached.”

Coral bleaching occurs when warm sea temperatures cause corals to expel tiny colourful algae, called ‘zooxanthellae’ turning them white.

If the water temperature drops, bleached corals sometimes recover, but if the zooxanthellae are unable to recolonise the coral, it dies.

Dr Schoepf said bleaching patterns occurred at the same time as WA sweltered through heat waves.

“This is the first documented regional-scale bleaching event in WA during an El Nino year and the first time we have been able to measure the percentage of impacted corals in 2016,” Dr Schoepf says.

“Temperate corals at Bremer Bay (South-West) experienced no bleaching.”

“Coral reefs in WA are now at risk of bleaching during both El Nino years, such as in 2016, and La Nina years, such as 2010/11,” Dr Schoepf says.

“But the geographic footprint differs – the northwest is at risk during El Nino years, whereas Ningaloo Reef and reefs further south are at risk during the La Nina cycle.

“As bleaching events become more common in the future, it is critical to monitor how bleaching events impact coral reef resilience, and how long it takes reefs to recover from such catastrophic events.”

The research has been published in the journal Scientific Reports.