Friday , June 18 2021

The study hangs on severe bleaching events in a Pacific coral reef throughout the last century

Ninety-five percent of the corals bleached in November of 2015, but were still alive when the WHO team visited Jarvis during the peak of 2015-16 El Niño. Credit: Thomas M. DeCarlo, Woods Hole Oceanographic Institution

As climate change causes ocean temperature to rise, coral reefs around the world are experiencing massive bleaching events and bats. 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. It is not known exactly how these reefs treat repeated episodes of extreme heat. A new study by the Woods Hole oceanographic organization (WHOI) discovered the story of bleaching in a reef at the Nino's epicenter, revealing how some corals were able to return after experiencing extreme conditions. The study was published on November 8, 2018 in the magazine Communication 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 coral in large areas of the ocean in a very short time," says the scientist WHO, Anne Cohen, who was the principal investigator of the work. "We have seen this global move in the last 30-40 years, and the bleaching events have become more frequent and more serious."

When the water temperature increases to a slight extent, the symbiotic algae that live within the live coral cells begin to create toxic substances and are expelled by the corals. The algae usually provide food and energy corals, as well as their lively colors. Without them, the corals seem to be white "whitening", then starving and dying.

In his study, Cohen's team traveled to Jarvis Island, a small island with little coral reef, 1400 kilometers south of Hawaii, to study the effects of extreme weather in the corals there. Because Jarvis is remote and part of a marine-protected area, he was home to amazingly rich coral reefs, but with its location in the middle of the Pacific, he also experiences more extreme heat waves caused by the El Nino periodic events than the reefs of coral in other places.

"The fact that it was placed directly in the central Pacific ecuador is located at the epicenter of El Niño's dynamics." says NOAA's researcher, Hannah Barkley, who was a graduate student and later was postdoctoral at the Cohen laboratory at the time of the study and is the main author of the work. "It is subject to incredible variability and extreme temperature."

Because there is no observation record of bleaching in the reef at Jarvis before 2015, Cohen and Barkley have become old and massive corals that have lived in the reef for more than 100 years. They took basic samples of the corals, creating a kind of skeletal biopsy that records the history of the reef. After running the cores through a CT scanner, they first found evidence of multiple bleaching events preserved in the physical structure of the reef. The longest nuclei have revealed bleaching since 1912.

"We discovered that when the reef bleached, these great old corals fixed" bands of stress "or a dense layer of calcium carbonate, the bonelike material that make up the coral structure. These bands appear clearly in computerized tomography and correspond to historical waves of heat, "says Cohen. The memory of the latest bleaching events at Jarvis is enclosed in these corals; They can tell us what happened though we were not there to see it for ourselves. "

Jarvis experienced temperatures above the average every four to seven years, dating back to decades or even centuries. The team found that, with each heat wave, the reef experienced a strong bleaching, but it seems that it has recovered quite rapidly every time.

Based on their samples, the group thinks that one of the main reasons for the recovery of the reef are the nearby currents. The topography of the ocean floor, combined with the strength of the commercial winds on the surface, brings cold water and rich in nutrients from the bottom. This vegetation feeds a dense pool of fish and other aquatic life around the reef, which in turn consume algae that compete with corals. In the process, they leave space for young coral polyps to eventually resolve.

"These reefs are resistant, whitened and recovered many times," says Dan Thornhill, program director of the Ocean Sciences Division of the National Science Foundation, who financed the research. "But the 2015-2016 bleaching event was particularly serious, so the island is providing us with new ideas about how some of the world's toughest corals are facing severe bleaching stress."

Understanding how coral reefs like Jarvis can be recovered after extensive bleaching will be essential to understand how other reef ecosystems can grow in the future, says Barkley.

But the Super El Niño of 2015 made Jarvis warm more than he had done before, and the following bleaching was the worst record. 95 percent of the island's corals died.

"The big question for us is whether the reef may burst in all this time," says Barkley. "Even the reefs like the Jarvis that once again had in the past have a limit beyond what they can not recover. What happens in the coming years will really help to understand severe bleaching."

Even so, she is veiled optimistic. "It's easy to look at a place like Jarvis after the 2015 bleaching event and feel depressed. But the historical record we receive from our nuclear samples says we are not beyond hope. Jarvis is just one example: although we're seeing signs From Accelerated whitening and mortality around the world, we have a narrow window to address the effects of climate change on corals. Some reefs may continue through major stress events. "

"The initial signs of recovery are there," says Cohen. "Now we wait, we see and learn."

Explore even more:
The study hovers severe bleaching events in a Pacific coral reef during the last century

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Woods Hole Oceanographic Institution

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