Axios Science – April 4, 2019

New studies warn that large, repetitive marine heat waves are disrupting the Great Barrier Reef's ability to regenerate with an abundance and mix of similar species. The paper, published in Nature On Wednesday, depicts a vast complex ecosystem of reefs that is on the verge of "ecological collapse".

Why it's important: The Great Barrier Reef is the world's largest coral reef ecosystem, spanning 1,400 miles north to south of the east coast of Australia. The new findings add to a series of alarming results on the vulnerability of this reef community, long considered too important to fail.

• The new findings also raise the provocative question of what healthy coral reefs will look like, if they exist, as ocean waters continue to heat up as a result of man-made global warming.

What did they do: For this study, researchers from the Australian Center for Coral Reef Excellence examined coral reproduction or reproduction rates from year to year by combining years of in situ measurements on the Great Barrier Reef.

• The researchers compared spawning behavior in the years preceding the marine heat waves that hit two-thirds of the reef in 2016 and 2017, compared to what happened immediately after the heat waves. These heat waves are also known as coral bleaching events for their tendency to turn corals into a ghostly white, because heat stress causes organisms to hunt symbiotic algae that give corals their vibrant colors.
• The study found that such heat episodes undermine the recovery capacity of the reef by causing a sharp drop in reproductive rates.
• By killing adult corals, heat waves reduced the reproductive rate and skewed the equilibrium between coral species.
• According to the study, the number of new corals that settled on the Great Barrier Reef decreased by 89% as a result of the assault on adult corals in 2016 and 2017.
• One species, Acropora, which establishes branches and table corals, has decreased by 93% compared to previous years, with no heat wave.

What they say: The increased frequency of whitening episodes means that more corals could perish before they can recover.

"Another large-scale whitening event in the next few years and this could be a curtain" for many parts of the Great Barrier Reef, co-authored the study's co-author, Andrew Baird, of James Cook University in Australia, by email.

• But the new study is only an overview of reef damage right after a massive shock.

But, but, but: Serious coral bleaching episodes mean that surviving corals may be more heat tolerant and may survive future events if they are allowed to mature first.

Madhavi Colton, program director for Coral Reef Alliance, a nonprofit organization, says she's eager to see what's going on with coral recruitment (the process by which coral larvae 's coral grows. attach to existing coral) beyond a year after bleaching events.

• "I would be interested to see if this trend continues, so we should be more alarmed than we are with a very bad year," she told Axios.

The bottom line: According to Kim Cobb, a climatologist and coral specialist at Georgia Tech, the future of the world's reef ecosystems will be nothing like what we have experienced. "Will they one day go back to where they were before these events? Probably not, "she says.

"Will they look very, very different? Will there be just another type of reef, will it have a different functionality? Maybe yes." She added, however, that the prospects for the world's reefs are "rather dark".

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