Healthy Coral in the Capricorn Group of Islands, Southern Great Barrier Reef, Nov. 2016.  Image: Tory Chase, ARC Centre of Excellence for Coral Reef Studies.

During the past two years, Australia's iconic Great Barrier Reef suffered its largest die-off ever recorded — with an average of 67 percent of corals in one area declared dead, scientists said on Monday. The cause was unusually warm waters that pushed the intricate ecosystems past their limits of survival...

The worst affected area is a 430 mile section of reefs in the northern part of the reef that has lost an average of 67 percent of its shallow water corals in just the past 8 to 9 months alone. 

These corals, located off the coast of Cooktown, are now a ghostly white, having expelled the symbiotic algae that gave them their vibrant colors and helped feed a huge array of marine species.

It had previously been known that the northern sections of the Great Barrier Reef, which is really a collection of thousands of smaller reefs, had seen high levels of coral bleaching. However, the extent of the die-off had not been fully assessed until now, and was predicted to be less severe. 

Across the central and southern regions of the Great Barrier Reef, scientists from the Australian Research Council's Center of Excellence for Coral Reef Studies at James Cook University found that the losses were less severe.

“Most of the losses in 2016 have occurred in the northern, most-pristine part of the Great Barrier Reef," said Terry Hughes, director of the ARC Center at James Cook University, in a statement. "This region escaped with minor damage in two earlier bleaching events in 1998 and 2002, but this time around it has been badly affected."

In one part of the northern Great Barrier Reef, located farther offshore from the more heavily visited areas that were so hard hit, the loss of reefs were lower — at about 26 percent — than they were to the south and west. 

Coral loss map. Image: ARC Centre of Excellence for coral reef research

“We found a large corridor of reefs that escaped the most severe damage along the eastern edge of the continental shelf in the far north of the Great Barrier Reef,” Hughes said. “We suspect these reefs are partially protected from heat stress by upwelling of cooler water from the Coral Sea.”

The least damaged sections of the Great Barrier reef are further south, in the central and southern areas off the coast of Queensland. 

“The good news is the southern two-thirds of the Reef has escaped with minor damage. On average, six percent of bleached corals died in the central region in 2016, and only one percent in the south. The corals have now regained their vibrant color, and these reefs are in good condition,” said Andrew Baird, who led teams of divers to re-survey the reefs during October and November, in the statement.

However, these southern areas may be hit by bleaching in the next few months, according to current forecasts.

The U.N. recognizes the Great Barrier Reef as a World Heritage Site for its staggering amount of marine biodiversity. In addition to the tiny life forms the reefs nurture, from algae to mollusks, they are also home to more than 1,600 species of fish, according to the Great Barrier Reef Marine Park Authority. 

Coral bleaching occurs when coral expels the algae that lives in its tissue, giving it color and nutrients. This action, caused by stresses such as increased water temperatures and pollution, leaves the coral skeleton exposed, making it more susceptible to heat stress, disease and pollution.

Bleached corals can recover if they are not exposed to further stress, but scientists are finding that some reefs are more resilient than others. 

Forecast showing areas with at least a 60 percent chance of coral bleaching through March 2017.Image: NOAA Coral reef watch

The unprecedented coral die off at the Great Barrier Reef is not an isolated incident. Since 2014, coral bleaching has stalked the seas worldwide, with repeated bouts of heat stress dealing a fatal blow to large sections of formerly vibrant reefs from Hawaii to the Bahamas, and the Indian Ocean to Indonesia. 

This is all part of the largest and longest-lasting global coral bleaching event on record, and it will continue through early 2017, according to forecasts from the U.S. National Oceanic and Atmospheric Administration.

While a sizable contribution to this event came from El Niño, which is a naturally-occurring phenomenon, record warm ocean temperatures also have had a human element too, due to the buildup of greenhouse gases in the atmosphere. 

Huge volumes of heat have been added to the oceans in recent years, with that heat being drawn to upper layers of the seas during El Niño events. 

In the hardest hit areas of the Great Barrier Reef, it could take 10 to 15 years for corals to regenerate, but another severe bleaching event is possible during that timeframe, which would stunt any recovery.

The Australian government has been eager to downplay the damage to the reef, given that tourism there employs 70,000 people and generates $5 billion in income per year.

A tragic consequence of global warming is evident in the Great Barrier Reef where much of its beautiful coral is dying off. Scientists have wondered how exactly coral bleaching happens. One researcher, Brett Lewis of Queensland University of Technology was able to capture coral bleaching in unprecedented detail and published what the results in Coral Reefs. The resulting time-lapse video is an amazing reminder of how pressing this issue is.

By Andrew Freedman

Nov 28, 2016

source: http://mashable.com/

original story HERE

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