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More ocean acidification, less coral?

Co-authored by Erica Cirino

Scientists have known for about 15 years that ocean acidification has made it more difficult for hard corals and shelled marine organisms to survive. To grow, hard corals as well as clams, oysters, and others pull calcium and carbonate molecules out of the water and join them together to create calcium carbonate (limestone), the basis for their limestone “skeletons,” their shells and hard parts.

When extra carbon from the burning of fossil fuels is released into the atmosphere, it’s deposited into the oceans. This carbon acidifies seawater, making less carbonate available for use by creatures that build skeletons from limestone.

Two years ago, University of Miami marine scientist Chris Langdon set out to better understand to what extent ocean acidification has affected hard corals’ ability to grow their skeletons. So he studied some local hard corals located along the Florida Reef Tract. Thanks to acidification, Langdon expected to find reefs to be accumulating limestone at an abnormally slow rate.

Satellite view of the Florida Keys and its coral reef tract. Credit: Jeff Schmaltz (MODIS Rapid Response Team, NASA/GSFC)
Satellite view of the Florida Keys and its coral reef tract. Credit: Jeff Schmaltz (MODIS Rapid Response Team, NASA/GSFC)

But what he found was even worse: That acidification is more extreme than scientists believed it to be. It’s causing much of the coral reefs in the upper Florida Keys to lose large amounts of limestone, which is dissolving away from the body of the reef tract—and fast.

Specifically, limestone is dissolving mostly during the fall and winter months, and most seriously on reefs in the upper Keys where the loss of limestone exceeds the amount corals can create annually. The results of Langdon’s two-year study were published in the journal Global Biogeochemical Cycles in May.

It wasn’t the reefs that led Langdon to this conclusion but the water around them. He and his colleagues collected water samples along the 124-mile Florida Reef Tract north of Biscayne National Park to the Looe Key National Marine Sanctuary and analyzed them in the lab to determine how much calcium they contained. In water surrounding corals, a higher calcium concentration corresponds with the dissolution of limestone.

A lack of limestone makes reefs more vulnerable to stress. Think of the limestone in a coral reef like the foundation of a house: With a weak foundation, a house is more vulnerable to destruction from natural disasters such as earthquakes, tornadoes and hurricanes. It’s similar for reefs, which are made more vulnerable to stressors when their limestone foundations begin to dissolve.

“The limestone is not lost as a thin uniform layer off of the top of the reef each year,” says Langdon. “Rather the loss happens in nooks and crannies and the undersides of the corals weakening the framework so that when a storm or hurricane comes along the structure can crumble.”

Coral reefs in the Florida Keys have an economic value of $7.6 billion, in terms of the benefits they provide in supporting fish and other marine creatures, tourism and storm protection for Florida’s coasts. A loss of coral diminishes the number of marine organisms that can survive near it. It also accelerates the erosion of shorelines due to the regular action of waves, and even more severely during storms.

Corals in the Florida Keys Reef Tract. Credit: National Park Service
Corals in the Florida Keys Reef Tract. Credit: National Park Service

Marine scientist and former Safina Center fellow Ellen Prager ranks climate change and associated ocean acidification as number-one of the “top-five” ocean issues she thinks people need to be worried about.

“Across the globe, corals and coral reefs are under attack,” says Prager. “Climate change, pollution, overfishing, development, and invasive species are causing unprecedented stress and mortality to corals.”

Not only does climate change cause acidification, but it causes ocean water to warm—one of the major causes of another deadly problem for corals: bleaching.

“Conservative estimates are that coral reefs [worldwide] are annually worth some $375 billion dollars in the services they provide such as in tourism and recreation, fisheries, wave protection, and more,” Prager wrote recently in a call to action on climate change. “It’s an annual net worth that one would hope is too big to fail!”