Coral Reefs Face the Dual Threats of Ocean Acidification and Erosion

As coral tissues die off, the exposed calcified skeleton becomes vulnerable to organisms that eat away at the dying reefs

Three green sea turtles, Chelonia mydas, on a coral reef, Heron Island, Great Barrier Reef, Australia. (Andre Seale / Alamy Stock Photo)
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Coral reefs are the rainforests of the sea. As the most diverse marine ecosystem, reefs are home to thousands of plant and animal species that provide billions of dollars in income via the fishing, tourism and pharmaceutical industries. They also protect coastal land from erosion and damage associated with storms. But much like rainforests, coral reefs are in danger due to climate change.

David Kline, a staff scientist at the Smithsonian Tropical Research Institute, says the threat of disappearing coral reefs is far more urgent than scientists ever realized. In 2010, he led a team of researchers in simulating a future climate change scenario to examine the effects of ocean acidification on coral reefs at the University of Queensland’s Heron Island field station on the Great Barrier Reef. The results of the study were recently published in the journal Nature Ecology and Evolution.

While previous studies of ocean acidification’s impact on coral reefs have taken place in artificial aquarium settings, Kline conducted this 200-day trial in a natural reef ecosystem using a Free Ocean Carbon Enrichment (FOCE) system. The FOCE system, pioneered by a team at the Monterey Bay Aquarium Research Institute, delivers computer-controlled pulses of carbon dioxide-enriched seawater to mimic ocean acidification.

By studying the reef itself, the team was able to examine the effects of organisms that feed on coral in conjunction with the effects of ocean acidification. Healthy, live coral tissue typically protects its calcified skeleton from a process called bioerosion, where organisms like parrotfish and worms either eat away at exposed coral skeletons or enter the skeletons and feed from within.

The study was the first ever to use the FOCE system to study a coral reef in situ, and the results were bleak. “We found that the effects of ocean acidification, compounded by bioerosion, are likely going to be worse than previously predicted,” Kline says.

Coral skeletons are made of calcium carbonate, which dissolves like chalk in a glass of vinegar when exposed to acidic seawater. In Kline’s future climate change scenario, the dissolution rate for dead coral colonies not protected by live tissue nearly doubled due to ocean acidification and bioerosion. Live corals continued to grow under acidic conditions, but their calcification rate failed to outpace the rate of dissolution, resulting in a net growth rate of almost zero.

FOCE Tool
Kline’s team used a Free Ocean Carbon Dioxide Enrichment system (FOCE) to study the effect of ocean acidification on coral reefs. (David Kline)

Bradley Eyre, a professor of biogeochemistry at Southern Cross University in Lismore, Australia who is not associated with the study, says the rate at which live corals produce new carbonate slows due to ocean acidification, while the rate of dissolution increases. “As such, it is expected that coral reefs will become net dissolving and maybe net erosional by the end of the century,” he writes in an email.

By observing both live and dead coral colonies, Kline’s team modeled a grim reality for reef ecosystems. According to the National Oceanic and Atmospheric Administration (NOAA), more than a quarter of the world’s live coral has died in the past three decades due to widespread coral bleaching. Bleaching occurs when the algae living inside coral tissue become stressed and leave the organism, often due to increased ocean temperature or pollution. Although coral can survive a bleaching event, they will eventually die without the food and protection provided by algae.

One of the study's co-authors, Kenneth Caldeira, who is a senior scientist at the Carnegie Institution for Science, describes coral death as a “double whammy” for reefs. Not only does the coral structure cease to grow after it has died, but the existing structure also begins to erode away. The upside, he says, is that keeping corals alive offers solutions to both sides of the problem.

In fact, Kline’s team found that live coral tissue simultaneously protects coral skeletons from bioerosion and offsets dissolution with tissue growth. They used their experimental data to estimate the point at which dead and live corals begin to dissolve, ultimately concluding that live coral coverage slowed the effects of ocean acidification. “The more living coral tissue you can protect on the reef, the more resilient the coral reefs are going to be to ocean acidification,” Kline says.

But given the current rates of climate change, protecting existing live coral is not enough. Kline says the future of coral reefs was less apparent before the third and most severe global bleaching event, which lasted from 2014 through 2017 and affected 75 percent of the Earth’s corals, according to NOAA. Now, Kline says reef management and replanting are necessary to restore coral reefs.

“Ten years ago, if you had asked coral biologists if replanting would really be essential, they would have probably laughed and said: ‘Oh, no, that won't be necessary,’” Kline says. “But now, in light of the massive amounts of corals that have died in bleaching events and the threat that ocean acidification is going to pose, coral reef scientists have turned around and said: ‘No, we have to consider every option available to try to increase the amount of living coral.’”

Kline says options for preserving coral reefs include planting new coral and restoring existing reefs. He points to a recent XPRIZE competition that encouraged global innovators to develop coral-saving technology as an example of recent efforts toward rebuilding reefs.

For Kline, who has been working on coral reefs ever since his first dive on the Great Barrier Reef during his senior year of college, there has never been a more critical case for saving his favorite ecosystem. “If we want to have healthy coral reefs for our children—not even our grandchildren anymore—we have to do something right now. And that includes more active approaches, like planting corals.”

About Andrea Michelson
Andrea Michelson

Andrea Michelson is a digital intern with Smithsonian magazine. She is currently a senior at Northwestern University, where she studies journalism and global health.

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