Coral Reefs in Crisis: How Bleaching and Acidification Threaten an Ecosystem

Coral reefs result from the mineral (calcium carbonate) skeleton left behind by coral polyps. Reef-building corals can be found in shallow tropical and subtropical waters (with water temperatures ranging from 70° to 85° F) of the Atlantic, Indian, and Pacific Oceans. The diversity of coral reefs gives them the nickname “rainforests of the seas.” Coral reefs support fish that feed more than a billion people, including many millions in island nations such as the Philippines and Indonesia.

When an aquatic environment nurtures coral reefs, they are among the most productive and diverse ecosystems on Earth. Coral reefs cover much less than 1 percent of the world’s ocean floors, while at the same time hosting more than a third of the marine species presently described by science, with many species remaining undocumented. Some of these organisms may provide new sources of anticancer compounds and other medicines. Coral reefs also protect shorelines from erosion by acting as breakwaters that, if healthy, can repair themselves.

Two snorkelers explore the Florida Reef located in Biscayne National Park in southern Florida. Much of the park is actually underwater, with boundaries that include Biscayne Bay, some of the northern Florida Keys, and the mangrove forests of the Florida shoreline (National Park Service)

Record high temperatures on both land and in the world’s oceans from 2014 to 2017 intensified coral damage, threatening a third or more of reefs with eradication. Kim Cobb, a marine scientist at the Georgia Institute of Technology, was stunned by the damage off Kiritimati Island, near the center of the Pacific Ocean. “The entire reef is covered with a red-brown fuzz,” Cobb said. “It is otherworldly. It is algae that has grown over dead coral. It was devastating.”

The Second-Largest Reef in the World. Most people will have heard of the Great Barrier Reef that stretches over 1,500 miles off the coast of Australia. But who can identify the second-largest reef in the world? Stretching over 600 miles along the warm waters of the Caribbean coasts of Mexico, Belize, Honduras, Guatemala, and Nicaragua, the Mesoamerican Barrier Reef, better known as the Great Maya Reef, is also a sight to behold. The United Nations Educational, Scientific and Cultural Organization (UNESCO) designated the Mesoamerican Barrier Reef a World Heritage Site in 1996. Rainer F. Buschmann

Around Kiritimati, within a broad swath at the center of the El Nino, water temperatures rose from the usual 78° F to 88°. “We are currently experiencing the longest global coral bleaching event ever observed,” said C. Mark Eakin, the Coral Reef Watch coordinator at the National Oceanic and Atmospheric Administration in Maryland. “We are going to lose a lot of the world’s reefs during this event.” Reefs that had required several centuries to grow were being destroyed in a matter of weeks.

A team of scientists replicated oceanic acidic levels at the beginning of the Industrial Revolution and traced them to the present day, learning that rising acidity has been stunting corals’ growth since humankind began reducing oceanic pH. “Acidification-induced reductions in calcification are projected to shift coral reefs from a state of net accretion to one of net dissolution this century,” they wrote. “When ocean chemistry is restored closer to pre-industrial conditions, net community calcification increases. In providing results from the first seawater chemistry manipulation experiment of a natural coral reef community, we provide evidence that net community calcification is depressed compared with values expected for pre-industrial conditions, indicating that ocean acidification may already be impairing coral reef growth.”

“Our work provides the first strong evidence from experiments on a natural ecosystem that ocean acidification is already slowing coral reef growth,” said the study’s lead author Rebecca Albright of the Carnegie Institution for Science. Previous studies had been completed in laboratories. “Ocean acidification is already taking its toll on coral reef communities. This is no longer a fear for the future; it is the reality of today,” Albright added. This research was released as an intense El Nino event raised ocean temperatures to record highs around the world, contributing from 2014 to 2016 to the longest episode of coral bleaching (death) on record.

By the end of the twenty-first century, according to Ken Caldeira of Stanford University, surface acidity around Antarctica will be roughly double pre-industrial levels (a 0.2 decrease in pH), threatening life forms’ ability to maintain their shells. Such a level would put about two-thirds of cold-water corals in corrosive waters. Acidification will affect corals acutely, dissolving their shells at a time when warming temperatures are already threatening their survival. “While bleaching . . . is an acute stress that’s killing them off . . . acidification is a chronic stress that’s preventing them from recovering,” said Joanie Kleypas, a coral reef scientist at the National Center for Atmospheric Research in Boulder, Colorado.

Because coral reefs are among the richest biological areas of the oceans, acidification poses a major long-term threat to aquatic life. Thomas Lovejoy, who coined the term “biological diversity” in 1980, compared the effects of ocean acidification to “running the course of evolution in reverse.” The two most important biological factors for organisms in the ocean are temperature and acidity, Lovejoy said. The effects of changes in both, provoked by human-induced carbon dioxide emissions, reach to the base of the oceanic food chain, with profound long-term implications favoring “lower” forms of life such as jellyfish and other invertebrates. In the very long run, some scientists fear human intervention in the oceanic system may be favoring a return to slime as a predominant life form there, according to the German marine biologist Ulf Riebesell.

The limits of acidity tolerance for many corals are now expected at about 550 parts per million carbon dioxide, a level that will be reached, at present rates of accretion, at about the middle of the twenty-first century. As CO₂ rises above that level, the oceans will encounter a “reef gap,” “when these magnificent, diverse structures disappear from the world.” The last reef gap occurred fifty-five million years ago, during the Paleocene- Eocene Thermal Maximum. The corals did eventually recover, but after several million years. Life will continue during such a gap, as coral reefs “will be replaced by ‘slime-rock’ systems dominated by algal and microbial mats and jellyfish.”

Although coral reefs cover 100,000 m² of ocean, they only amount to 0.1 percent of the watery surfaces. At the same time, such reefs are estimated to contribute—primarily through fishing—close to $1,000,000 per m² to local economies. They support an estimated 25 percent of marine life and, should they disappear, would devastate coastal fisheries and tourism. The survival of these vital habitats thus depends on rapid action. Globally, it requires a reduction of greenhouse gases that primarily contribute to the two main threats to coral reefs: the increasingly warmer and more acidic surface waters. The race to reduce emissions before mid-century is thus vital for the reefs’ survival. Other, more local, actions involve the restoration of reefs through artificial means, such as scuttled ships and additions. Likewise, observing coral species that are more resilient to the lethal heat stress resulting from climate change, especially in the Gulf of Thailand, may allow for a careful introduction into areas of massive coral depletion. Bruce E. Johansen

FURTHER READING:Albright, Rebecca, et al. 2016. “Reversal of Ocean Acidification Enhances Net Coral Reef Calcification.” Nature 530 (February 24): 362-5.

Alkire, William. 1978. Coral Islanders. Arlington Heights, IL: AHM Publication.

Innis, Michelle. 2016. “Climate-Related Death of Coral Around World Alarms Scientists.” The New York Times, April 9, A1.

Kintisch, Eli and Erik Stokstad. 2008. “Ocean CO₂ Studies Look Beyond Coral.” Science 319: 1029.

Kolbert, Elizabeth. 2006. “The Darkening Sea: What Carbon Emissions Are Doing to the Oceans.” The New Yorker, November 20, 66-75.

Zalasiewicz, Jan and Mark Williams. 2016. Ocean Worlds: The Story of Seas on Earth and Other Planets. New York: Oxford University Press.