With climate change causing ocean temperatures to rise, coral reefs are experiencing massive bleaching events and deaths worldwide. For many, this is their first encounter with extreme heat. For some reefs in the central Pacific, heat waves caused by El Nino are a way of life. How exactly these reefs deal with repeated episodes of extreme heat is unclear. A recent study by the Woods Hole Oceanographic Institution (WHOI) revealed the history of bleaching on a reef in the epicenter of El Nino and showed that some corals were able to return to extreme conditions. The study was published on October 26, 2018 in the journal Communications Biology .
"These massive sea-heat waves, exacerbated by global warming, are equivalent to a nuclear bomb impact on coral reefs – killing millions of coral over vast ocean areas in no time," says WHOI researcher Anne Cohen, who directed the work of the researcher. "We've seen that this has gone global over the past 30 to 40 years, and bleaching events have become more common and more serious."
When water temperatures even rise slightly, symbiotic algae that live in the cells of life corals begin to form toxins and are ejected from the corals. The algae usually provide the corals with food and energy as well as their bright colors. Without them, the corals seem to be bleached white, then starve and die.
In their study, Cohen's team traveled to Jarvis Island, a tiny, uninhabited coral reef island 1,400 miles south of Hawaii, to study the effects of extreme climate on the local coral. Being both isolated and part of a marine reserve, Jarvis has housed amazingly rich coral reefs – but its location in the middle of the Pacific also causes it to experience more extreme heatwaves caused by periodic El Nino events than coral reefs elsewhere.
"The fact that it is placed right on the equator in the central Pacific puts it at the center of El Niño's dynamics." says NOAA researcher Hannah Barkley, who was a graduate student and later a postdoctoral researcher in Cohen's lab and is the lead author of the paper. "It is subject to incredible variability and extreme temperatures."
With no observational data on bleaching on the reef at Jarvis by 2015, Cohen and Barkley turned to massive old corals that had lived on the reef for more than 100 years. They took core samples of the corals and created a kind of bone biopsy documenting the history of the reef. After passing the cores through a CT scanner, they found for the first time evidence of multiple bleaching events remaining in the physical structure of the reef. The longest cores revealed bleaching as early as 1912.
"We found out that these big old corals bleach the reef 'stress bands' or a dense layer of calcium carbonate, the bone-like material that forms the structure of the coral Scan are clearly visible and consistent with historical heat waves, "says Cohen. The memory of past bleaching events on Jarvis is trapped in these corals – they can tell us what was going on, even though we were not there to see it for ourselves.
Jarvis has experienced above-average temperatures every four to seven years The team discovered that the reef bleached strongly with each heatwave, but bounced back pretty quickly each time.
Based on their samples, the group believes that one of the main reasons for the reef's recovery is the nearby flow. The topography of the seabed, combined with the trade winds on the surface, brings cold, nutrient-rich water from the depths. This buoyancy nourishes a dense collection of fish and other aquatic life around the reef, which in turn eat grass algae that compete with the coral. They leave room for new, young coral polyps, who can finally settle.
"These reefs are resilient and have often bleached and regained," says Dan Thornhill, program director of the Department of Marine Sciences at the National Science Foundation. which funded the research. "But the bleaching event of 2015-2016 was particularly severe, and it provides us with new insights into how some of the world's most resilient corals suffer in the face of heavy bleaching stress." After extensive bleaching, the reefs may recover in the future, says Barkley.
But the 2015 Super El Nino made Jarvis warming more than ever, and the subsequent bleaching was the worst since records began. 95 percent of the island corals died.
"The big question for us is whether the reef can jump back all the time," says Barkley. "Even reefs like Jarvis that have grown back in the past have a threshold behind which they may not recover, and what happens in the years to come will really help us understand severe bleaching."
Nevertheless, she is cautiously optimistic. "After the bleaching event of 2015, it's easy to visit a place like Jarvis and be depressed, but the historical records of our core samples show that we're not beyond mere hope." Jarvis is just one example: though we have signs of bleaching and Mortality around the world has a tight window of opportunity to deal with the effects of climate change on corals, and some reefs can sustain major stress events. "
" The first signs of recovery are there, "says Cohen. "Now we wait, see and learn."
Research investigates the influence of coral bleaching on the coast of Western Australia