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

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. However for some reefs in the 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 Nov. 8, 2018, in the journal Communications Biology.

"These huge marine heatwaves, 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 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 inside the cells of the live coral start to create toxic substances and are ejected by the corals. The algae normally provide the corals with food and energy, as well as their bright colors. Without them, the corals appear to be "bleached" white, then starve and die.

In their study, 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.

"The fact that it's placed right at the equator in the central Pacific puts it at epicenter of 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's subject to incredible variability and extremes in temperature.."

https://www.sciencedaily.com/releases/2018/11/181108164318.htm