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Coral bleaching has become a sad reality amid rising ocean temperatures due to climate change.
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Severe bleaching means coral could die, now 93 per cent

Scott Sutherland
Meteorologist/Science Writer

Monday, April 25, 2016, 12:51 PM - The world's largest coral reef system is in trouble, as scientists report that the Great Barrier Reef is undergoing the worst mass coral bleaching event ever recorded, with 93 per cent of the reef ecosystems affected.

Flying up and down the northeastern coastline of Australia, scientists with the ARC Centre of Excellence for Coral Reef Studies have been documenting the conditions on the Great Barrier Reef, and have now returned with the dire news.

Over 900 coral reefs were surveyed, with the worst of the bleaching located in the northern region.

"Between 60 and 100 per cent of coral are severely bleached on 316 reefs, nearly all in the northern half of the Reef," Terry Hughes, director of the ARC Centre, said in a press release.

In addition, scientists have reported nearly 50 per cent coral death in the northern reef. Meanwhile, most of the reefs in the southern end have minor to moderate bleaching and should soon recover.

”Of all the reefs surveyed, only 7 per cent have escaped bleaching," added Hughes.  

In an earlier press release, Hughes noted it has been the saddest trip of his life. "

Photos courtesy: ARC Centre of Excellence for Coral Reef Studies

Under normal conditions, corals coexist in a mutually-beneficial relationship with tiny photosynthetic algae, known as zooxanthellae. The corals provide a safe, nutrient-rich living space for the algae, while the algae provide the corals with their main source of energy, consume waste from the corals to keep the environment clean, provide elements for the corals' calcium carbonate skeletons, and are even responsible for the corals' vibrant colours.

Although zooxanthellae are adaptable to different conditions, given time, and there are many different species of these algae, they are susceptible to temperature shocks. If the water temperature rises too high, too fast, they die. Without the colourful algae, the corals turn white, and this is known as coral bleaching.

If the coral bleaching event is not prolonged, different - typically, more resilient - zooxanthellae species will repopulate the corals fairly quickly, preventing them from starving. Although stressed, the reefs once again become colourful, functional ecosystems, at least until the next event.

If the extreme ocean temperatures persist, enough algae can die off that the corals can't hold out until the next algae population takes over. They starve and die.

Only time will tell what the exact impact this event will have on the Great Barrier Reef, however the extreme conditions are already taking their toll.

This dire report follows just weeks after Dr. Jodie Rummer, a senior research fellow at the ARC Centre, returned from Lizard Island Research Station, in the northern part of the Great Barrier Reef, to report on conditions that she described as "catastrophic."

"I witnessed a sight underwater that no marine biologist, and no person with a love and appreciation for the natural world for that matter, wants to see," Rummer said in an ARC Centre press release on March 21, 2016. "The bleaching now is not just restricted to the hard corals. There’s also extensive bleaching in the soft corals, and it is also affecting anemones and giant clams."

"This year, the combination of El Niño, climate change, and an extended period of hot summer days when the tide was exceptionally low has caused many of the corals that survived last year's cyclone to lose their symbiotic algae and start bleaching."

While El Niño is having an effect on the region of the Great Barrier Reef, resulting in slightly elevated ocean temperatures and lower ocean levels (thus lower than normal tides), climate change has pushed ocean temperatures above normal, making reef systems more vulnerable to bleaching events.

"What we're seeing now is unequivocally to do with climate change," Prof. Justin Marshall, a University of Queensland neuroscientist and marine biologist who runs the Coral Watch citizen science website, told Australia's ABC News. "The world has agreed, this is climate change, we're seeing climate change play out across our reefs."

Sources: ARC Centre of Excellence for Coral Reef Studies | Washington Post | ABC News | Coral Watch

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