El Niño Tests Coral Survival Limits

October 12, 2015

SYSTEM SHOCK:<br/> El Niño Tests Coral Survival Limits

A FIRST pulse of abnormally warm water hit Panama’s Pacific coast in April and corals immediately began to fade

A FIRST pulse of abnormally warm water hit Panama’s Pacific coast in April and corals immediately began to fade. By August, a species of hydrocoral near Coiba Island—a national park and World Heritage Site—had almost completely died off in water up to six meters deep. Bleaching, the ghostly white state that can precede coral death, extended to almost half the surface of the affected coral colonies at the same depth.

Temperatures might not return to normal until early 2016 and another superheated blast of El Niño-fueled water will strike soon. Scientists are mobilizing for one of the largest eastern tropical pacific coral die-offs in almost two decades.

They’re actually pretty excited about it, but not for immediately obvious reasons. This is only the fourth major El Niño since 1973, when Peter Glynn first surveyed Panama’s Pacific coral as a staff scientist at the Smithsonian Tropical Research Institute. The last two major events decimated coral communities and both times coral recovered—just in time to get clobbered again.

But scientists are still at a loss to explain this remarkable resilience in a place that is already one of the world’s most stressed-out environments for coral. This year’s El Niño presents a rare opportunity to get some answers.


Climate change trends suggest tomorrow’s oceans will be warmer and more acidic due to increased atmospheric carbon dioxide, which may exacerbate the already dramatic global decline in coral reef coverage.

Panama’s eastern tropical Pacific, with its naturally higher acidity and seasonal temperature swings, already mirrors some predicted conditions. Throw in an “extreme thermal anomaly” and Panama’s Pacific coast becomes one of the best wet labs available to study if, in fact, we’re in the midst of a global coral apocalypse.

Before-during-and-after observations of the past two El Niño events in the eastern Pacific suggest coral can rapidly adapt to thermal shocks. If proven true, “this will drastically change predictions of the fate of coral reef ecosystems globally over the next 100 years of climate change,” noted scientists, including Glynn, now at the Rosenstiel School of Marine and Atmospheric Science at the University of Miami.

Which is not to say coral will endlessly adapt to a quickly changing, human-altered environment.

“There will be a limit to resilience mechanisms beyond which reefs can no longer adapt and ecosystem functions break down,” they warned.


The 1982–1983 El Niño was the first to show that heat stress provokes bleaching and mortality—about 75 percent of the coral in surveyed areas around Coiba died. The 1997–1998 event claimed only 13 percent of living coral overall, even though heat conditions and pre-event coral coverage were similar.

In spite of the rapid bleaching, it is still difficult to predict how this year’s event will compare, said Juan Maté, who studied the 1997–1998 event as a Ph.D. student and is now manager of scientific affairs at STRI. “Some species are more resistant than others,” said Maté, after returning from a survey to Panama’s Gulf of Chiriquí, where Coiba is located. “We think some are acclimatizing.”

Bleaching occurs when corals are stressed-out by heat or other environmental disruptions and lose the single-celled organisms called zooxanthellae that live inside their soft tissue structure. The zooxanthellae photosynthesize, providing much of the nutrients the corals need to live and grow. Coral can survive brief periods without these symbionts, but must re-assimilate them to recover from bleaching.

Even under exactly the same environmental conditions, not all corals were bleaching at the same rate in the Gulf of Chiriquí. Identical species in what are presumably identical environmental conditions of temperature, light, nutrient availability and so on, responded very differently to April’s heat blast. Some were completely bleached, others appeared perfectly healthy. There were also marked differences between species, with massive coral species more bleached than branching corals. Bleaching also decreased with depth, but past major El Niño events have found bleaching can extend to lower depths.

Ana Palacio, a University of Miami Ph.D. student at the Rosentiel marine school, believes the zooxanthellae will help explain the different reactions. With a hole punch, hammer and clippers, she took more than 300 live coral tissue samples from the long-term coral survey sites. Using genetic tests unavailable during the last major El Niño event, Palacio will determine the pedigree and quantity of zooxanthellae in each sample. Lab work has shown that different symbionts influence a coral’s ability to withstand heat stress. This is the first time these tests will be done in a natural setting.

“We’re finding out how the symbiotic community changes during the El Niño event,” Palacio said. “Do they recover with the same symbionts or acquire new ones when they recover?”


Genetic tools are not the only things to have changed in the last three decades. The ocean environment is generally warmer, more acidic, more polluted and fish-depleted. All of the above can harm coral and complicate understanding and comparing the impact of different El Niño events. Peggy Fong, a marine biologist at University of California, Los Angeles, who has been studying these corals since the 1990s, says only a multidisciplinary team can tackle how long-term change influences short-term events like El Niño.

“That’s the million-dollar question,” said Fong, who is the project’s principal investigator and surveyed more than 1,000 coral colonies to assess bleaching progress during the August expedition. “But this is where all the long-term monitoring data come into play. We are all now cognizant of ‘shifting baselines’ and need to take this into account—likely on many axes.”

These data include fish abundance and diversity, changes in coral and algal cover, shifts in zooxanthellae distribution and diversity, and ocean chemistry such as CO2 saturation and temperature. Once these factors are picked apart, coupled with this El Niño’s observations, and then carefully pieced back together in mathematical models, the data may reveal just how much natural and human-made heat stress corals can tolerate. With any luck, scientists won’t have this opportunity again until the 2030s.


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