How much coral has died in the Great Barrier Reef’s worst bleaching event?

James Cook University The Conversation AU 29 Nov 16;

Two-thirds of the corals in the northern part of the Great Barrier Reef have died in the reef’s worst-ever bleaching event, according to our latest underwater surveys.

On some reefs in the north, nearly all the corals have died. However the impact of bleaching eases as we move south, and reefs in the central and southern regions (around Cairns and Townsville and southwards) were much less affected, and are now recovering.

In 2015 and 2016, the hottest years on record, we have witnessed at first hand the threat posed by human-caused climate change to the world’s coral reefs.

Heat stress from record high summer temperatures damages the microscopic algae (zooxanthellae) that live in the tissues of corals, turning them white.

After they bleach, these stressed corals either slowly regain their zooxanthellae and colour as temperatures cool off, or else they die.

The Great Barrier Reef bleached severely for the first time in 1998, then in 2002, and now again in 2016. This year’s event was more extreme than the two previous mass bleachings.

Surveying the damage

We undertook extensive underwater surveys at the peak of bleaching in March and April, and again at the same sites in October and November. In the northern third of the Great Barrier Reef, we recorded an average (median) loss of 67% of coral cover on a large sample of 60 reefs.

The dieback of corals due to bleaching in just 8-9 months is the largest loss ever recorded for the Great Barrier Reef.

To put these losses in context, over the 27 years from 1985 to 2012, scientists from the Australian Institute of Marine Science measured the gradual loss of 51% of corals on the central and southern regions of the Great Barrier Reef.

They reported no change over this extended period in the amount of corals in the remote, northern region. Unfortunately, most of the losses in 2016 have occurred in this northern, most pristine part of the Great Barrier Reef.


ARC Centre of Excellence for Coral Reef Studies

Bright spots

The bleaching, and subsequent loss of corals, is very patchy. Our map shows clearly that coral death varies enormously from north to south along the 2,300km length of the Reef.

The southern third of the Reef did not experience severe heat stress in February and March. Consequently, only minor bleaching occurred, and we found no significant mortality in the south since then.

In the central section of the Reef, we measured widespread but moderate bleaching, which was comparably severe to the 1998 and 2002 events. On average, only 6% of coral cover was lost in the central region in 2016.

The remaining corals have now regained their vibrant colour. Many central reefs are in good condition, and they continue to recover from Severe Tropical Cyclones Hamish (in 2009) and Yasi (2011).

In the eastern Torres Strait and outermost ribbon reefs in the northernmost part of the Great Barrier Reef Marine Park, we found a large swathe of reefs that escaped the most severe bleaching and mortality, compared to elsewhere in the north. Nonetheless, 26% of the shallow-water corals died.

We suspect that these reefs were partially protected from heat stress by strong currents and upwelling of cooler water across the edge of the continental shelf that slopes steeply into the Coral Sea.

For visitors, these surveys show there are still many reefs throughout the Marine Park that have abundant living coral, particularly in popular tourism locations in the central and southern regions, such as the Whitsundays and Cairns.

Darkspots

The northern third of the Great Barrier Reef, extending 700km from Port Douglas to Papua New Guinea, experienced the most severe bleaching and subsequent loss of corals.

On 25% of the worst affected reefs (the top quartile), losses of corals ranged from 83-99%. When mortality is this high, it affects even tougher species that normally survive bleaching.

However, even in this region, there are some silver linings. Bleaching and mortality decline with depth, and some sites and reefs had much better than average survival. A few corals are still bleached or mottled, particularly in the north, but the vast majority of survivors have regained their colour.

What will happen next?

The reef science and management community will continue to gather data on the bleaching event as it slowly unfolds. The initial stage focused on mapping the footprint of the event, and now we are analysing how many bleached corals died or recovered over the past 8-9 months.

Over the coming months and for the next year or two we expect to see longer-term impacts on northern corals, including higher levels of disease, slower growth rates and lower rates of reproduction. The process of recovery in the north – the replacement of dead corals by new ones – will be slow, at least 10-15 years, as long as local conditions such as water quality remain conducive to recovery.

As global temperatures continue to climb, time will tell how much recovery in the north is possible before a fourth mass bleaching event occurs.

This article was co-authored by David Wachenfeld, Director for Reef Recovery at the Great Barrier Reef Marine Park Authority.


Great Barrier Reef scientists confirm largest die-off of corals recorded
Higher sea temperatures have led to the worst bleaching event on record, new study finds, with coral predicted to take up to 15 years to recover
Guardian staff and agencies The Guardian 28 Nov 16;

A new study has found that higher water temperatures have ravaged the Great Barrier Reef, causing the worst coral bleaching recorded by scientists.

In the worst-affected area, 67% of a 700km swath in the north of the reef lost its shallow-water corals over the past eight to nine months, the Australian Research Council Centre of Excellence for Coral Reef Studies based at James Cook University study found.

“Most of the losses in 2016 have occurred in the northern, most-pristine part of the Great Barrier Reef,” Prof Terry Hughes said. “This region escaped with minor damage in two earlier bleaching events in 1998 and 2002, but this time around it has been badly affected.”

Great Barrier Reef: diving in the stench of millions of rotting animals
The southern two-thirds of the reef escaped with minor damage, Hughes said. This part was protected from the rising sea temperatures because of cooler water from the Coral Sea.

Scientists expect that the northern region will take at least 10 to 15 years to regain the lost corals but are concerned a fourth bleaching event could interrupt the slow recovery.

The dire assessment of the reef’s health comes as the Australian government is due to report to Unesco’s world heritage committee on its handling of the reef.

After the federal government submits the report Unesco will decide whether to again consider listing the Great Barrier Reef on its “list of world heritage in danger”.

The government will need to report on how it has funded and implemented its Reef 2050 long-term sustainability plan, as well as how the bleaching event has affected the reef.

Since it last considered including the Great Barrier Reef on its list, the reef has undergone the worst bleaching event in recorded history. According to government agencies, 22% of the reef was killed in one hit, as unusually warm waters bleached and killed the coral.

Climate change poses such a threat to the reef that the former head of the Great Barrier Reef Marine Park Authority has called for a ban on all new coalmines in Australia to protect the reef from climate change.

Graeme Kelleher, who was the first chief executive of the authority, a position he held for 16 years, said: “Australia cannot have a healthy Great Barrier Reef and a continuing coal industry.

“I love the reef and I have worked to preserve it since 1979; I will oppose anything that threatens to destroy it,” he said.


Great Barrier Reef suffered worst bleaching on record in 2016, report finds
Hywel Griffith BBC News 28 Nov 16;

Higher water temperatures in 2016 caused the worst destruction of corals ever recorded on Australia's Great Barrier Reef, a study has found.

Some 67% of corals died in the reef's worst-hit northern section, the ARC Centre of Excellence for Coral Reef Studies report said.

The situation was better in the central section, where 6% perished, while the southern reef is in good health.

But scientists warn recovery could be difficult if climate change continues.

Coral bleaching happens when water temperatures rise for a sustained period of time.

In February, March and April, sea surface temperatures across the Great Barrier Reef were the hottest on record, at least 1C higher than the monthly average.

"Some of the initial mortality was down to heat stress," said study leader Professor Terry Hughes.

"The coral was cooked."

How bleaching occurs

Far more has been lost through gradual starvation, after the coral expelled the colourful algae zooxanthella, which turns sunlight into food.

This is what leads to the white, skeletal appearance of the coral, which is left without its main source of energy.

The study also found that the coral which survived the bleaching have now come under greater threat from predators such as snails and crown of thorns starfish.

This year's mass bleaching was the worst-ever recorded on the Great Barrier Reef, following two previous events in 1998 and 2002.

Professor Hughes is certain that the increased water temperature is the result of carbon emissions, and warns that climate change could bring annual bleaching within 20 years.

"Most of the losses in 2016 have occurred in the northern, most pristine part of the Great Barrier Reef," he said.

"This region escaped with minor damage in two earlier bleaching events in 1998 and 2002, but this time around it has been badly affected."

Where is the damage?

One of the worst-hit areas is around Lizard Island in Far North Queensland, where around 90% of the coral has died.

Dr Andrew Hoey, whose team charted the area, said the impact was far worse than feared after an initial survey in April.

"It's devastating to get in the water somewhere you've been coming for almost 20 years, and it's just knocked it on its head," he said.

"There's very little coral cover left there. It was dominated by the acropora - the branching corals - but we lost most of them."

Lizard Island is home to a research station, where scientists from across the world have come for decades to study marine life
One of its directors, Dr Anne Hogget, said this was by far the worst event to hit the Great Barrier Reef since she started working there in 1990.

"We had bleaching here in 2002," she said. "We thought this was bad at the time, but this has blown it completely out of the water."

She is hopeful that the reef is capable of recovery, but fears it may not be give an opportunity, as sea temperatures continue to rise.

"The trajectory is not good," Dr Hogget explained.

"We keep pumping carbon dioxide into the atmosphere, and this happened absolutely because of that."

What happens next?

On the central and southern parts of the Great Barrier Reef, where bleaching was not as prevalent, there is concern that it has been misreported, with one magazine even publishing an obituary of the reef earlier this year.

Tourism operators like Michael Healey from the Quicksilver Group are keen to point out that many sites were unaffected, but there is concern for the reef's long term health.

"Without the Great Barrier Reef, we wouldn't survive," he said.

"So it is absolutely of the utmost importance that we ensure that our politicians and everyone else in our community and around the world are doing what they can."

The Australian Government has published a long-term sustainability plan for the reef, and pledged financial support for research into coral bleaching.

The 2050 plan identifies the need to help make the reef more resilient to climate change in the future, while trying to lower carbon emissions.

Mr Healy argued even those not financially involved had a stake in the reef.

"I'd say every human on the planet does," he said.

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