El Niño Warming Turns Coral Garden in Marine National Monument into a Graveyard

June 1, 2016  

Forereef seascape at Jarvis Island dominated by colonies of plating Montipora corals. Credit: NOAA/Paula Ayotte.
Forereef seascape at Jarvis Island dominated by colonies of plating Montipora corals. Credit: NOAA/Paula Ayotte.

Scientists surveying coral reefs at Jarvis Island in the Pacific Ocean have discovered devastating loss of reef-building corals due to El Niño from May 2015 to April 2016. Dr. Bernardo Vargas-Ángel, leader of the Benthic Ecology and Monitoring Team of NOAA Fisheries' Pacific Islands Science Center Coral Reef Ecosystem Program, estimates that "about 95 percent of the coral colonies died from coral bleaching caused by high and prolonged water temperatures associated with this intense El Niño."

Despite this distressing loss, scientists had their first sighting of a colony of a coral species (Acropora retusa) at Jarvis Island that is listed as threatened under the U.S Endangered Species Act. The fact that this species was found alive when so many Acropora corals were killed is especially remarkable and hopeful.

Scientists from NOAA's Coral Reef Ecosystem Program conducted the April 2015 survey just prior to the onset of the El Niño, and re-surveyed the area with partners from Woods Hole Oceanographic Institution and Rutgers University in May 2016, just as the water returned to near-normal temperatures. For over eight months, water temperatures at Jarvis exceeded levels that kill most corals.

The adjacent photo illustrates the appearance of the unique coral reef ecosystem at Jarvis Island in April 2015 during NOAA's 9th research cruise to monitor the coral reef ecosystems of the U.S. Pacific Remote Islands Marine National Monument since establishing baseline measurements in 2000.

Evidence of Coral Bleaching from November 2015

Seascape of bleached plating Montipora corals on Jarvis' forereef. Credit: Cohen Lab, WHOI.
Seascape of bleached plating Montipora corals on Jarvis' forereef. Credit: Cohen Lab, WHOI.

Jarvis was bathed in warmer-than-normal waters for about seven months when scientists from Anne Cohen's lab at Woods Hole Oceanographic Institution, sponsored by the National Science Foundation, visited the island and reported mass coral bleaching. "At that time, water temperatures were 4°C above normal and 80% of corals were bleached, down to 90 ft depth", says Cohen. When water temperatures reach 1°C warmer than their usual summertime maximum, many corals begin to lose the symbiotic algae living in their tissues, making them look white, which is why they are called bleached.

Coral Loss Findings from May 2016

Widespread coral mortality along the steep sloping Jarvis forereef. Credit: NOAA/Bernardo Vargas-Ángel.
Widespread coral mortality along the steep sloping Jarvis forereef. Credit: NOAA/Bernardo Vargas-Ángel.

When scientists returned to Jarvis Island in May 2016, over a year of high temperatures had left the formerly thriving coral reefs "looking more like a coral graveyard," according to Vargas-Ángel. Many of the previously bleached corals observed in November 2015 were now dead, broken, and covered with a thick mat of filamentous algae.

"One would have never believed that just a year before this was a vibrant and colorful coral reef. Coral mortality was widespread across all reef habitats and depths," said Vargas-Ángel. He reported many remaining live coral colonies were partly bleached and unhealthy, and were being eaten by the snail Drupella.

Some Positive Findings from Coral Survey

Healthy-looking massive coral colony on the eastern terrace. Credit: NOAA/Bernardo Vargas-Ángel.
Healthy-looking massive coral colony on the eastern terrace. Credit: NOAA/Bernardo Vargas-Ángel.

A few hardy and resilient corals survived this El Niño-related bleaching event, including colonies of massive reef-building corals of the genus Porites than live for many decades or centuries. "While many of these died, some looked relatively healthy, providing hope that they, and the exceptional biological productivity and remoteness of Jarvis Island, will be the harbinger to a successful recovery of these unique coral reef ecosystems," said Dr. Rusty Brainard, leader of the Coral Reef Ecosystem Program.

Now that the waters have cooled to more normal temperatures and the usual intense surge of nutrient-rich waters has resumed, NOAA and partners look forward to see how the coral reefs recover during the next few years.

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