Magazine article Oceanus

Hidden Battles on the Reefs

Magazine article Oceanus

Hidden Battles on the Reefs

Article excerpt

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 is often overlooked. 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.

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-bores into the skeleton of living corals to find shelter. Each one leaves 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."

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 (C02) levels in the atmosphere. Not all of the C02 emissions from our cars and factories stay in the atmosphere. The ocean has already absorbed about a third of it. As C02 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, just as putting ankle weights on people makes it harder for them to stay afloat.

Flattened reefs

The bioeroders can affect the health of coral reefs in another way-not just by drowning them, but also by flattening them. When corals and bioeroders are in harmony, the former grow just fast enough to stay near the sea surface, while the latter are busily sculpting the coral skeletons into an intricate, three-dimensional habitat full of nooks and hiding places for the more introverted creatures that call reefs home.

We generally picture coral reefs inhabited by creatures of every color imaginable, with reef fish and sharks swimming about. But a closer look reveals just how many more creatures are hiding within the reef.

When I scuba-dive on reefs, I spend most of my time swimming a few feet above the bottom looking down. During an expedition to the U.S. Virgin Islands, I realized I was completely missing the hidden underworld. One of my jobs was to count adult and juvenile urchins-a baseball-size creature with foot-long needlelike spikes sticking out in all directions. Urchins usually try to hide, but adults never seem to find a hiding place quite big enough, and their spines stick out in plain view. But I had never seen a juvenile urchin.

To find one, I poked my head in all the crevasses of the reef. In every nook, there was some creature hiding out: juvenile urchins, shy reef fish, and mean-looking groupers with sharp teeth. …

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