Since climate change causes ocean temperatures to rise, coral reefs throughout the world are experiencing massive whitening and dehydration. For many, this is their first encounter with extreme heat. However, for some reefs in the central Pacific, the heat waves caused by El Nino are a way of life. Just as these reefs relate to repeated episodes of extreme heat, it was not clear. A new study by the Oceanic Institute of Woods Hole (WHOI) revealed the history of bleeding on the ridge in the epicenter of El Nina, revealing how some corals returned after facing extreme conditions. The study was published on November 8, 2018 in a journal Biology of communication.
"These huge ocean heat waves, which are exacerbated by global warming, are equivalent to atomic bombs in terms of impact on coral reefs – in a very short time, killing millions of corals across vast oceans," says WHOI scientist Anne Cohen, the chief investigator at work. "Today we have seen this game globally over the last 30-40 years, and bleeding events have become more and more difficult."
When the water temperature is even slightly increased, the symbiotic algae living within the coral cortex begin to create toxic substances and eradicate corals. Algae normally give the corals food and energy as well as their bright colors. Without them, the corals are "white" white, then hungry and dying.
In his study, Cohen's crew traveled to the island of Jarvis, the small, unpopulated island of the coral reef, 1400 miles south of Hawaii, to study the effects of the extreme climate on the corals there. Since Jarvis is a remote part of the marine protected area, it is home to the stunningly rich coral reefs – but with its location in the middle of the Pacific Ocean, it also experiences the extreme heat waves caused by El Nino's periodic events of coral reefs elsewhere.
"The fact of being at the equator in the central Pacific places it in the epicenter of the dynamic dynamics of El Niño." says NOAA researcher Hannah Barkley, who was a student and later a postdoctoral student at Cohen's Laboratory at the time of her studies, and is the lead author of paper. "It is subject to incredible variability and extreme temperatures".
Since there was no bleaching on the Jarvis before 2015, Cohen and Barkley turned to massive old corals that have lived on the ridge for more than 100 years. They took samples of coral cores, creating a kind of skeletal biopsy that records the reef history. After conducting the cores through the CT scanner, they found for the first time evidence of multiple bleaching events held in the physical reef structure. The longest cores discovered bleaching in 1912.
"We have discovered that when the ridge is ejected, these big old corals drop" stress straps, "or a dense layer of calcium carbonate, a glittering material that makes the coral structure. These belts clearly appear in the CT and fit with historical heat waves," says Cohen . Remembering the past whitening events at Jarvis is locked in those corals – they can tell us what's going on, though we were not there to see it for ourselves. "
Jarvis experienced above average temperatures every four to seven years, going back decades or even centuries. The team discovered that with each wave of heat, the ridge was experiencing serious bleeding, but it seemed to be moving each time pretty fast.
Based on their patterns, the group thinks that one of the main reasons for the reef currents is nearby. Ocean floor topography, combined with the strength of trade winds on the surface, brings cold water rich in nutrients from the depths. This upwelling feeds on dense fish and other aquatic life around the reefs, which in turn eradicate grass algae that compete with corals. In this process, they leave room for new, young coral polyps to finally be established.
"These reefs are resistant, bleaching and recovering many times," says Dan Thornhill, director of the National Science Foundation, Ocean Science, funded research. "But the bleeding event from 2015 to 2016. It was particularly difficult, so our island provides new insights into how some of the most deadly corals in the world come under serious bleeding stress."
Understanding how coral reefs like Jarvis can recover after extensive bleaching will be key to understanding how other reef ecosystems will come back in the future, says Barkley.
But in 2015, Super El Nino had Jarvis warming more than ever before, and the bleeding that followed was the worst. 95 percent of Coral islands died.
"The big question for us is whether runners could always resist," says Barkley. "Even reefs like Jarvis who have come back to the past have a threshold above which they can not recover. What will happen over the next few years will really help us understand serious bleeding."
Still, she is kept optimistic. "It's easy to look at a place like Jarvis after the bleeding event for 2015 and feel depressed, but the historical record we get from our core samples says we're not in. Jarvis is just one example: although we see signs of rapid whitening and mortality around the world, we have a narrow window to address the effects of climate change on the corals. Some rocks may be able to keep up through huge stress events. "
"Initial signs of recovery exist," says Cohen. "We are now waiting, watching and learning."
The study follows serious bleeding events on the coral reef of the last century