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Study tracks severe bleaching events on a Pacific coral reef over the past century

Ninety five percent of the corals were bleached in November 201 5, but still alive when the WHOI team visited…

Ninety five percent of the corals were bleached in November 201

5, but still alive when the WHOI team visited Jarvis during the peak of the 2015-16 El Nino. Credit: Thomas M. DeCarlo, Woods Hole Oceanographic Institution

As climate change causes ocean temperatures to rise, coral reefs worldwide are experiencing mass bleaching events and die-offs. For many, this is their first encounter with extreme heat. Men for nogle reefs i Central Pacific, heatwaves caused by El Nino are a way of life. Exactly how these reefs deal with repeated episodes of extreme heat has been unclear. A new study from the Woods Hole Oceanographic Institution (WHOI), has uncovered the history of bleaching on a reef in the epicenter of El Nino, revealing how some corals have been able to return after facing extreme conditions. The study was published October 26, 2018, in the journal Communications Biology .

“These huge marine heat waves, which are being exacerbated by global warming, are equivalent to an atomic bomb in terms of impact on coral reefs-they kill millions of corals across huge areas of ocean in a very short time” says WHOI scientist Anne Cohen, who was the principal investigator on the work. “We’ve seen this play out now globally for the past 30-40 years, and bleaching events have become more frequent and more severe.”

When water temperatures rise even slightly, symbiotic algae that live in the cells of the live coral start to create toxic substances and are ejected by the corals. The algae usually provide the corals with food and energy, as well as their bright colors. Cohen’s team traveled to Jarvis Island, a tiny, unpopulated coral reef island 1.400 miles south of Hawaii, to study the effects of extreme climate on the corals there. Because Jarvis is both remote and part of a marine protected area, it has been home to stunningly rich coral reefs, but with its location in the middle of the Pacific, it also experiences more extreme heat waves caused by periodic El Nino events than coral reefs elsewhere.

A very old Porites colony on Jarvis has seen many El Nino’s. Extreme heat caused by the 2015-16 El Nino killed a significant portion of the coral tissue, but some remained alive, and had started to grow back, green and healthy, by 2016. Credit: Hannah Barkley, Woods Hole Oceanographic Institution

“The fact that it is placed right at the equator in the central Pacific puts it at epicenter or El Niño dynamics.” says NOAA researcher Hannah Barkley, who was a graduate student and later a postdoctoral fellow in Cohen’s lab at the time of the study, and is the paper’s lead author. “It is subject to incredible variability and extremes in temperature.”

Because there is no observation record of bleaching on the reef at Jarvis before 2015, Cohen and Barkley turned to massive old corals that had lived on the reef for more than 100 years. De tok kjerneprøver fra korallerne, som skabte en slags skeletal biopsi, som registrerer historien om reefen. Efter at have kørt gennem en CT-scanner, fandt de for første gang bevis på at flere blekehendelser var bevart i fysisk struktur av reefen. The longest cores revealed bleaching as far back as 1912.

“We found that when the reef bleaches, these big old corals lay down” stress bands “or a thin layer of calcium carbonate, the bone-like material that make up The structure of corals. Those bands appear clearly in the CT scan, and correspond with historical heat waves, “says Cohen. Jarvis is locked into these corals-they can tell us what has been going even though we were not there to see it for ourselves. “

Jarvis has experienced above-average temperatures every four to seven years, going back decades or even centuries. The team discovered that with each heat wave, the reef experienced severe bleaching, yet seems to have bounced back fairly quickly each time.

Hannah Barkley extracts a core from a massive Porites colony. The core contains clues about the recovery of the reef after the bleaching event. Credit: Noah Pomeroy, NOAA Fisheries

Based on their samples, the group thinks that one of the main reasons for the reef’s recovery is the currents nearby. The topography of the ocean floor, combined with the force of trade winds on the surface, brings cold, nutrient-rich water up from the deep. Die upwelling voeds a dense array of fish and other aquatic life around the reef, which in turn eat away grassy algae that compete with the corals. In the process, they leave room for new, young coral polyps to eventually settle.

“These reefs are resilient, having been baked and recovered many times,” says Dan Thornhill, Program Director in the National Science Foundation’s Division of Ocean Sciences, which funded the research. “But the 2015-2016 bleaching event was particularly severe, so the island is providing us with new insights into how some of the world’s most resilient corals are faring in the face of severe bleaching stress.”

Understanding how coral reefs like Jarvis er i stand til å komme efter omfattende blekning vil være essensielt for å forstå hvordan andre reef økosystemer kan vokse tilbake i fremtiden, sier Barkley.

But the 2015 Super El Nino caused Jarvis to heat up more than it ever did before, and the bleaching that ensued was the worst on record. 95 percent of island’s corals died.

Coral cores waiting to be CAT scanned at WHOI. Credit: Thomas M. DeCarlo, Woods Hole Oceanographic Institution

“The big question for us is whether the reef can bounce back at all this time,” says Barkley. “Even reefs like Jarvis, who have regrown in the past, have a threshold beyond what they may not recover. Still, she’s guardedly optimistic. What happens over next few years will really help us understand severe bleaching.”

Still, she’s guardedly optimistic. “It’s easy to look at a place like Jarvis after the 2015 bleaching event and feel depressed. But the historical record we got from our core samples says we’re not beyond hope. Jarvis is just one example: even though we are seeing signs of accelerated bleaching and mortality worldwide, we have a narrow window to address the effects of climate change on corals. Some reefs may be able to persist through huge stress events. “

” The initial signs of recovery are there, “says Cohen. “Now we wait, watch and learn.”

Explore further:
Research examines impact of coral bleaching on Western Australia’s coastline

Provided By:
Woods Hole Oceanographic Institution

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