As climate change causes ocean temperatures to rise, coral reefs around the world are experiencing mass whitening events and deaths. For many, this is their first meeting with extreme heat. However, for some coral reefs in the central Pacific, heat waves caused by El Nino are a way of life. It is not clear how these barriers deal with repeated episodes of extreme heat. A new study by the oceanographic institute Woods Hole (WHOI), has discovered the history of bleaching on a cliff in the epicenter of El Nino, revealing how some corals have been able to return after having faced extreme conditions. The study was published on November 8, 2018 in the journal Communications Biology .
"These huge marine heat waves, which are exacerbated by global warming, are equivalent to an atomic bomb in terms of impact on coral reefs: they kill millions of corals in huge ocean areas in a very short time," says scientist WHOI Anne Cohen, who was the principal labor investigator. "We have seen this game now globally over the past 30-40 years, and whitening events have become more frequent and more stringent."
When the temperatures of the water rise even slightly, the symbiotic algae that live inside the live cells the corals begin to create toxic substances and are expelled from the corals. The algae normally provide the corals with food and energy, in addition to their bright colors. Without them, the corals seem to be "bleached" white, then die of hunger and die.
In their study, Cohen's team went to Jarvis Island, a tiny uninhabited coral reef island 1,400 miles south of Hawaii, to study the effects of extreme climate on corals there. Because Jarvis is remote and is part of a protected marine area, it has been home to incredibly rich coral reefs, but with its location in the middle of the Pacific, it has also experienced more extreme heat waves caused by periodic El Nino events. that from the coral reefs elsewhere.
"The fact that it is located right at the equator in the Central Pacific places it at the epicenter of the dynamics of El Niño." says NOAA researcher Hannah Barkley, who was a graduate student and later a postdoctoral in Cohen's laboratory at the time of the study, and is the lead author of the paper. "It is subject to incredible variability and extreme temperatures."
Since there is no observational bleaching record on the Jarvis reef before 2015, Cohen and Barkley have turned to huge old corals that had lived on the coral reef for more than 100 years. They took basic samples from the corals, creating a sort of skeletal biopsy that records the history of the coral reef. After running the cores through a CT scanner, they found for the first time evidence of multiple bleaching events stored in the physical structure of the reef. The longest nuclei revealed bleaching as early as 1912.
"We discovered that when the coral reef clears, these big old corals lay" strains of tension "or a thick layer of calcium carbonate, the very similar material that constitutes the structure of corals: these bands appear clearly in the CT scan and correspond to the historical heat waves, "says Cohen. The memory of the previous bleaching events on Jarvis is locked up in these corals: they can tell us what happened, even if we were not there to see it alone. "
Jarvis has experienced above-average temperatures every four or seven years, which go back decades or even centuries. The team found that every heat wave had a strong whitening barrier, but it seems to be bouncing pretty quickly each time.
On the basis of their samples, the group believes that one of the main reasons for the recovery of the coral reef are the nearby currents.The topography of the ocean floor, combined with the strength of the trade winds on the surface, brings cold and rich water. of nutrients from the deep, which feeds a dense array of fish and other aquatic species life around the coral reef, which in turn consumes the herbaceous algae that compete with the corals and leave room for new, young coral polyps to settle.  "These rocks are resistant, they have bleached and recovered many mes", says Dan Thornhill, director of the Ocean Scien Division program ces of the National Science Foundation, which financed 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 toughest corals in the world are facing severe bleaching stress."
Understanding how coral reefs like Jarvis Barkley is able to recover after extensive bleaching will be essential to understand how other reef ecosystems may regrow in the future.
But the 2015 Super El Nino caused Jarvis overheating more than ever before, and the bleaching that followed was the worst ever recorded. 95% of the island's corals are dead.
"The big question for us is if the coral reef can bounce back all this time," says Barkley. "Even the coral reefs like Jarvis that have been regenerated in the past have a threshold beyond which they may not recover, and what will happen in the next few years will really help us understand the serious whitening."
However, it is wary and 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 accelerated bleaching and mortality worldwide, we have a narrow window to address the effects of climate change on corals. Some barriers may be able to persist through huge stress events. "
" The first signs of recovery I'm there, "says Cohen. "Now we wait, we watch and we learn."
The study tracks serious bleaching events on a coral reef in the last century