Coral Reefs Under Attack

In addition to their natural beauty and lure for outdoors-minded tourists, coral reefs are ecologically and economically important ecosystems. Two new studies by scientists at Scripps Institution of Oceanography at UC San Diego have shed light on issues facing the future of coral reefs in a changing world.

In one study, Scripps marine biologist Davey Kline and colleagues at the University of Queensland (UQ) simulated future ocean conditions through a groundbreaking experimental system and found climate change will jeopardize the future of coral reefs more than most realized.

The study, published in the Proceedings of the National Academy of Sciences, details how coral reefs rapidly dissolved once exposed to warmer, more acidic ocean conditions associated with “business-as-usual” carbon dioxide emission rates predicted for the latter half of this century.

“If we don’t reduce our carbon dioxide emissions, our reefs are in trouble due to coral bleaching, reduced coral growth rates, and increased rates of dissolution,” said Kline. “The good news is that reducing our emission levels will increase the chances that our reefs remain diverse and healthy.”

Kline said corals will bleach more and more frequently as CO₂ levels continue to rise. Currently corals suffer massive bleaching events every five to 10 years, he said, and in the future these could become annual events if emissions aren’t reduced.

The collaborative study, led by UQ’s Sophie Dove, concludes that increases in temperature and acidity cause major disruptions to coral reefs, including those growing around the world-famous Heron Island on Australia’s Great Barrier Reef. Dove said even under fairly low emission scenarios, most corals bleached and died.

The nine-month study used computers to control CO₂ content and water temperature to recreate four past and future CO₂ scenarios in 12 mini-reefs constructed at UQ’s Heron Island research center. Dove describes one of the most significant challenges of climate change as being able to accurately reduce future uncertainties.

“By simulating future environments above complex reef systems, we come closer to understanding what might happen as the oceans warm and acidify,” she said. “If we can reduce the uncertainty, then we have a much better chance of making better decisions to help protect and conserve these valuable ecosystems.”
 

Kline designed and built the novel experimental system, which was developed to precisely test corals through varying scenarios of carbon dioxide and temperature levels.

“Traditional experimental treatments are often unsuitable, especially if the experimental organisms come from environments with large daily and seasonal changes in temperature and chemistry,” said Kline. He said testing under multiple scenarios should become the standard to more fully capture changes in future conditions. “The next step in these studies will be to run multi-year experiments to begin to assess if acclimation and adaptation to increasing CO₂ and temperature can occur.”

The study also found for the first time that increases in ocean temperature and acidity not only leads to a reduction in calcification, the process by which corals build reefs, but also the rate at which coral reefs dissolve.

“This has serious implications for the role of coral reefs in providing habitat for thousands of species and their role in protecting coastlines from wave impacts,” said Dove.
In a separate study, Scripps graduate student Aaron Hartmann and his colleagues found a glimmer of good news as corals face increasingly warmer ocean waters. Hartmann and his colleagues led a groundbreaking study on the critical early-stage survival of corals.

In a paper published in the September issue of the journal Ecology, Hartmann and his team reported that large coral offspring tend to be more successful than smaller counterparts, but being large does not protect larval corals from the ill effects of environmental change, including elevated temperatures and changes in salinity.

 “This study provides a new level of detail in the way we think about survival of coral offspring, which are critical to maintaining reefs into the future,” said Hartmann, who worked with scientists at the Carmabi Foundation on the island of Curaçao.

The study revealed that for some coral species that face stressful conditions early in life, negative impacts to their survival follow in later life stages as a consequence.

“This offers a new way to think about how life history characteristics, including offspring size, may predict the tolerance of coral larvae to environmental conditions such as ocean warming on a species-by-species basis,” Hartmann.

On the plus side, the study also revealed that the offspring of a critical but fragile reef-building stony coral species in the Caribbean (Orbicella faveolata) are more tolerant of warming than scientists previously thought.

“The future of Caribbean reefs depends in no small part on the sustainability of this species so it’s interesting that their offspring are surprisingly ‘tough,’ especially when the adults of this species are thought to be relatively ‘weak.’”

Kline’s study was funded by the Great Barrier Reef Foundation, the Australian Research Council (ARC) Centre for Excellence in Coral Reef Studies, ARC Linkage, and a Queensland Smart State Fellowship. Hartmann’s research was supported by the NSF Graduate Research Fellowship Program, Future of Reefs in a Changing Environment project; PADI Foundation; the Carmabi Foundation, and NSF IOS.
 

-- Mario C. Aguilera and University of Queensland