A fire coral before and after bleaching, from left to right. (XL Catlin Seaview Survey)

Global coral ‘bleaching’ threatens reef survival

By Kathleen Ferraro

Many of the ocean’s coral reef populations are fast declining in what is the longest episode of coral bleaching on record.

Global warming, the current intense El Nino, overfishing and land-based pollution are all contributing to rapid coral bleaching, or a potentially fatal loss of pigmentation caused by environmental stress, according to National Oceanic and Atmospheric Administration scientists. With a majority of the world’s reefs threatened by some combination of these factors, extensive coral damage could mean certain coral species’ extinction, a decline in ocean biodiversity and a restructuring of ocean ecosystems, said an NOAA scientist.

“We are having a global bleaching event, a mass bleaching event,” said Paulo Maurin, national education coordinator and fellowship manager for NOAA’s Coral Reef Conservation Program. “When you look at the bleakest reefs, they will, by the middle of the century, not stay around.”

Coral bleaching is a loss of pigmentation that causes energy drain that may eventually lead to a coral’s death. Greta Aeby, a researcher at the Hawai’i Institute of Marine Biology, likened bleaching to anorexia.

“If [humans] refuse to eat for a long time, they use up any lipid stores and then their body has to start breaking down their organs, and that starts killing them. It’s the same with coral,” Aeby said.

An infographic outlining the coral bleaching process, describing the transition from healthy to dead coral. (XL Catlin Seaview Survey)
An infographic outlining the coral bleaching process, describing the transition from healthy to dead coral. (XL Catlin Seaview Survey)

Coral reefs are found in all of the world’s oceans. Shallow coral reefs occupy just 0.015 percent of the ocean floor, yet are home to over one-quarter of ocean biodiversity—or a broad variety of species—according to NOAA. Deep-sea coral communities likewise host many species, though less is known about these reefs due to incomplete exploration.

NOAA estimates that 500 million people worldwide rely on coral reefs for food and coastal protection. Reefs also sustain fisheries and local tourism industries, contributing nearly $30 billion to world economies annually, according to an NOAA statement. Significant reef death could hit local economies and the seafood industry hard. Eroded reefs could also spell danger for coastal areas.

“With coastal protection, wave energy breaks on the coral reefs that are a little ways from shore,” Aeby said. “And if that protective layer is getting eroded every year, on top of global climate changes making storms worse than ever, that’s a double whammy for any coastal state.”

Ocean acidification and rising ocean temperatures are two culprits in coral bleaching and death.

“The oceans are warming because of excess CO2 in the atmosphere, and they are acidifying because the excess CO2 that doesn’t stay in the atmosphere goes into the ocean,” Maurin said. “That’s making it harder for coral reefs to calcify, to make their calcium carbonate structures. So when you combine one with the other, bleaching is basically sapping out all of corals’ energy.”

Global warming and the current El Nino are contributing to the longest coral bleaching event ever observed, which began in 2014 and is forecast to last well into 2017, according to NOAA.

Disease and heat stress prompted by resulting rising ocean temperatures have made coral bleaching happen more frequently, to the point where corals are unable to recover before the next bleaching event begins, said NOAA.

“It was a very strong El Nino last year, so we saw some bleaching in many places. And now we’re beginning to see it again. So those El Ninos together, back to back, haven’t given the reefs a whole lot of time to recover,” Maurin said. “Just like when you’re sick and as you’re coming on your way up, you get hit with another illness, you don’t have the strength to fight it that you would otherwise. That’s what is happening with coral bleaching.”

A coral reef in American Samoa before, during and after a bleaching event. (XL Catlin Seaview Survey)
A coral reef in American Samoa before, during and after a bleaching event. (XL Catlin Seaview Survey)

Beyond environmental factors, human impact affects reef systems. Overfishing significantly cuts fish populations, which in turn throws the reef’s ecosystem out of balance, according to Aeby. Specifically, a decline in herbivorous, or plant-eating, fish species means an abundance of algae enters the reef and threatens coral, Maurin said. Destructive fishing techniques also contribute to coral destruction and breakage.

Land-based pollution also contributes to reef systems’ decline. Sediment, trash and sewage wash into the sea, creating poor water quality that further contaminate coral ecosystems, Aeby said.

With trends like this occurring worldwide, experts predict that the most threatened coral reef systems could disappear as early as the middle of this century, according to Maurin.

This is problematic for reef systems, which host a quarter of all marine life, he said. As corals bleach or die, this habitat becomes inhospitable to many creatures. In fact, when a reef dies, you see a decline in that ecosystem’s fish population within five years, according to Maurin.

A green turtle swims in a bleached reef in Hawai`i. (XL Catlin Seaview Survey)
A green turtle swims in a bleached reef in Hawai`i . (XL Catlin Seaview Survey)

And changes in species’ populations can throw the entire ecosystem’s food chain out of whack. Disturbances in even the tiniest food sources, like plankton, cause changes all the way up to the biggest predator and change the balance of life through an entire reef system, said Maurin.

Fortunately, work is being done to prevent further damage. Take, for instance, the scientists at Maui Ocean Center in Kihei.

Maui’s coral population has experienced significant deterioration. The island’s Honolua Bay saw a 33 percent decline in coral cover over the course of 12 years, according to Hawaii’s Department of Land and Natural Resources data.

Certain areas around the island are showing less than 10 percent coral cover—the proportion of reef surface covered by live coral—a significant decline that has only occurred over the past two decades, said John Gorman, head curator at the center. He noted that high nutrient introduction and sediment deposit are causing algae to flourish in local reef systems. This, coupled with a decline in overall herbivorous fish populations, further endangers Maui’s coral.

With local coral species threatened, marine biologists at the center are undertaking coral propagation research to preserve local reef ecosystems. They research the different growth rates between corals transplanted into the ocean immediately after their removal versus corals grown in a laboratory and transplanted into the ocean later in life. The research aims at identifying the most effective way to regrow and replant coral, thereby repairing and preserving reefs as quickly and efficiently as possible, said center officials.

Other local preservationists are doing similar work, experimenting with coral growth to develop species of coral that can better adapt to changing ocean conditions.

In addition to research, Maurin pointed to policy advocacy as a means for regulating overfishing and, eventually, helping reef systems recover. Aeby also noted that educating local coastal populations about the dangers of coral bleaching could instigate preservation and pollution regulation on a community level.

“It is a serious issue and the reefs will die if it’s not addressed,” Aeby said. “It’s a critical part of our islands, from fisheries to coastal protection to economies.”

Photo at top: A fire coral before and after bleaching, from left to right. (XL Catlin Seaview Survey)