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Hidden Battles on Reefs: How Will Corals Fare in a Changing Ocean?

Hidden Battles on Reefs: How Will Corals Fare in a Changing Ocean?

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Tom DeCarlo, a graduate student in the MIT-WHOI Joint Program in Oceanography is investigating how changing ocean conditions, including lower pH levels, will affect the health of vital coral reefs.

The research team has been studying this relatively acidic coral reef in the Palauan archipelago. Seawater pH on this reef today represents acidification levels predicted for tropical western Pacific by the end of the 21st century. (Photo by: Tom DeCarlo, WHOI)

The research team has been studying this relatively acidic coral reef in the Palauan archipelago. Seawater pH on this reef today represents acidification levels predicted for tropical western Pacific by the end of the 21st century. (Photo by: Tom DeCarlo, WHOI)

How do you drown a coral reef? The very idea seems unfathomable for animals that spend their entire lives under water. But the deep ocean is actually riddled with ‘drowned’ coral reefs—the remains of ancient reefs that slipped into the dark ocean depths and starved without sunlight.

To stay afloat in the ocean, people work hard to keep their heads just above water. Corals do the opposite, striving to stay just under the surface of the ocean. Coral reefs need to get it just right—submerged in the sea, but shallow enough for the corals’ symbiotic photosynthetic algae to soak up sunlight. Too deep and the ecosystem wastes away without solar energy to make food. Too shallow and corals dry out at low tide.

This delicate balance is achieved by a constant tug-of-war, which most people overlook. Day in and day out, as corals build their skeletons up toward the sea surface, other organisms are eroding and dissolving the skeletons to build their own homes, cutting down the corals’ hard work.

Graduate student Tom DeCarlo has sailed the world studying coral reefs, from the Caribbean Sea to the South China Sea. (Photo by: Pat Lohmann, WHOI)

Graduate student Tom DeCarlo has sailed the world studying coral reefs, from the Caribbean Sea to the South China Sea. (Photo by: Pat Lohmann, WHOI)

Today, the chemistry of our ocean is rapidly changing. How sensitive are corals and bioeroders to these changes? How will changing ocean conditions favor or thwart each side in this battle? Will they tip the delicate balance and expand the graveyards of drowned coral reefs?

An Undersea Tug-of-War

Corals make their skeletons out of calcium and carbonate ions dissolved in seawater, constructing massive structures as large as your car, maybe even your house. But at the same time, a collection of other organisms called bioeroders—mollusks, worms, and sponges—bore into the skeleton of living corals to find shelter, each leaving distinctly shaped tunnels or borings in the reef framework.

Corals usually win this war, keeping the reef near the sea surface and within reach of sunlight. But if the balance is tipped and the reef becomes eroded down far below the sea surface, there is no longer enough sunlight for corals to survive, and they will ‘drown.’

On all coral reefs, an unseen tug-of-war is going on. While corals grow toward the sunlit surface, organisms called bioeroders—mollusks, worms, and sponges—bore into coral skeletons to find shelter. The bioeroders also sculpt nooks and crannies in the skeletons that create hiding places for other creatures. If all is in balance, the coral reef becomes a vibrant, diverse habitat (top). But if factors hinder coral growth or give bioeroders the upper hand, coral reefs will become flattened and die -  E. Paul Oberlander - WHOI

On all coral reefs, an unseen tug-of-war is going on. While corals grow toward the sunlit surface, organisms called bioeroders—mollusks, worms, and sponges—bore into coral skeletons to find shelter. The bioeroders also sculpt nooks and crannies in the skeletons that create hiding places for other creatures. If all is in balance, the coral reef becomes a vibrant, diverse habitat (top). But if factors hinder coral growth or give bioeroders the upper hand, coral reefs will become flattened and die – E. Paul Oberlander – WHOI

This delicate balance between calcium carbonate production and removal is threatened by ocean acidification, the decline in seawater’s pH driven by rising carbon dioxide (CO2) levels in the atmosphere. Not all of the CO2 emissions from our cars and factories stay in the atmosphere. The ocean has already absorbed about a third of it. As CO2 enters the ocean, a well-known series of chemical reactions takes place and carbonate ions, the currency of coral reefs, begin to disappear. Reducing the amount of carbonate ions available in seawater for corals makes it harder for them to build their skeletons up to the surface. It’s like putting ankle weights on people makes it harder for them to stay afloat.

A Trans-Pacific Quest

Changing ocean conditions are bound to affect the bioeroders, too. Will lower-pH seawater hinder them, or will it help them and tip the balance even further against corals?

As a member of Anne Cohen’s lab at Woods Hole Oceanographic Institution (WHOI), I’ve been investigating the impacts of ocean acidification on coral reefs. Our team has searched the ocean for reefs that are able to tolerate living in seawater that naturally has a pH lower than the seawater typically found in the tropics. This search took us from the Pacific coast of Panama, across a vast stretch of the equatorial Pacific Ocean, and into the South China Sea—just shy of half the circumference of the globe.

Some of these reefs are among the most remote places in the world. In satellite images, they are odd sand-colored pixels. To seafarers, they are dull piles of sand and a few palm trees, and to cartographers, they are annoying specks. Traveling to and working on these remote reefs is not easy. A bunch of wooden planks tied to eight plastic pipes passes for a research vessel. Yet, just below the sea surface, these reefs are underwater jungles teeming with life.

To continue reading about Tom DeCarlo’s findings, go to the original story.

This research is funded by the National Science Foundation (NSF), the Nature Conservancy, and the National Oceanic and Atmospheric Administration’s Coral Reef Conservation Program. Tom DeCarlo is supported by an NSF Graduate Research Fellowship.

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