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Record February heat pushes Reef corals closer to big bleaching event

The next few weeks will be critical if the current moderate bleaching is not about to become severe.Credit:NOAA

“The threat of [mass bleaching] is severe and imminent enough to redo the survey,” he said.

The monitoring will cover the length of the Great Barrier Reef and cover similar regions to those in 2016 and 2017, the other two mass bleaching events of recent years.

“It just got progressively hotter from non-dangerous levels in December to record heat in February,” Professor Hughes told The Sydney Morning Herald and The Age. “To my mind, the south [of the Reef] is hotter than I’ve ever seen and it’s much more vulnerable than the north.”

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Corals respond to excessive accumulated heat by expelling algae – known as zooxanthellae that provide them with their brilliant colours and most of the energy. After prolonged heat stress, the corals begin to die.

A spokeswoman for the Marine Park Authority said the agency had received the Bureau’s analysis of February sea-surface temperatures.

“As we are already doing, we will closely monitor Reef health in the coming months to track ongoing temperatures and Reef health,” she said.

Last month, the authority’s chief scientist David Wachenfeld said much of the park was about 2 to 3 degrees above normal.

Sea-surface temperatures reached record-high levels on the Great Barrier Reef, topping those in 2016 and 2017 when there was mass coral bleaching.

Sea-surface temperatures reached record-high levels on the Great Barrier Reef, topping those in 2016 and 2017 when there was mass coral bleaching.Credit:Bureau of Meteorology

Heat stress is measured in so-called degree-heating days. Corals would typically start bleaching at about 30-50 degree-heating days, and would begin to die at 50-70 such days, Dr Wachenfeld said.

At temperatures of 3 degrees above average, corals in those regions would be accumulating 21 degree-heating days each week.

During the mass bleaching events of 2016 and 2017, the Great Barrier Reef lost about half its total coral cover, with the centre and the north hardest hit.

Lizard Island waters were already seeing coral bleaching in February. More areas up and down the Great Barrier Reef will be at risk in coming weeks without a big change in the weather.

Lizard Island waters were already seeing coral bleaching in February. More areas up and down the Great Barrier Reef will be at risk in coming weeks without a big change in the weather.Credit:Lyle Vail/Australian Museum via WWF

The south largely avoided bleaching in 2016 because the remnant of severe tropical cyclone Winston brought heavy cloud cover and mixed in cooler waters.

“We’ve already got moderate or mild bleaching” along much of the Great Barrier Reef, Professor Hughes said, adding that next couple of weeks are likely to be critical.

“We could get lucky and get a cool second and third week of March,” he said. “It’s getting really late in the year for that kind of reprieve” such as from a cyclone.

Weatherzone said on Thursday that there were early indications of a tropical cyclone forming “somewhere near Queensland next week”.

Aerial surveys will likely begin at Princess Charlotte Bay on Cape York Peninsula where corals have already been bleaching for several weeks, Professor Hughes said.

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