Slick Death: Oil-spill treatment kills coral

Chemicals used to disperse marine oil slicks may harm corals more than the oil itself does, according to a new study. The finding suggests that chemical dispersants should be used near reefs only as a last resort, when oil approaches a shoreline where it might devastate wildlife and plants for decades.

TOUGH BREAKUP. Workers on a barge spray dispersants onto a burning oil spill discharged by a well in the Gulf of Mexico. Such chemicals may be more harmful to corals than the oil is. Corbis

In many cases, authorities first try to clean up oil spills mechanically (SN: 11/18/06, p. 325). If weather conditions are too rough or a slick threatens to wash up on shore, dispersants are usually the next option. Made up of surfactants and solvents, dispersants act as detergents, breaking up oil into droplets that mix into water, scatter with currents, and eventually degrade. However, the dispersed oil droplets readily sink and can lethally contaminate coral.

Baruch Rinkevich of the National Institute of Oceanography in Haifa, Israel, and his colleagues tested whether chemical dispersants, as well as oil droplets, do harm to corals. They report that dispersants kill branching corals or retard their growth. The team also confirms previous research indicating that corals do better when exposed to oil that hasn’t been dispersed.

“Dispersants are very toxic for corals,” Rinkevich says. “It’s a no-win situation, but more knowledge [will add to officials’] evaluation and decisions about what to do in unpredictable situations.”

To test the effects of the dispersants, the researchers pruned 2-inch segments from the branches of two common hard coral species found in the Red Sea and grew them into several large colonies in laboratory tanks. The team then added to the tanks various concentrations of crude oil, one of six commercial dispersants mixed with oil, or one of the six dispersants alone. After allowing 24-hour exposure to the substances, researchers washed the corals, simulating what would happen in the real environment when oil and dispersants wash away. The team then measured coral survival and growth weekly for 50 days.

After 1 week, more than 90 percent of one coral species and about 75 percent of the other survived in the oil-only tanks, whereas virtually all coral died in the tanks containing either the dispersant-oil mix or the dispersant alone. After 50 days, more than 90 percent of the surviving corals from the oil-only tanks continued to grow. Almost all coral from the dispersed-oil and the dispersant-only tanks experienced retarded growth.

The study appears in the Aug. 1 Environmental Science & Technology.

Amy Merten, codirector of the Coastal Response Research Center at the National Oceanic and Atmospheric Administration in Seattle, says that the results contradict the rule of thumb that dispersants are less toxic than oil droplets. It’s important for authorities in charge of spill cleanups to note that coral reacts to the dispersant itself, she says. “There needs to be more consideration of dispersants.” However, Merten adds that under real conditions, coral may not be exposed to dispersants in the same amount, and for the same duration, that it was in the laboratory tests.

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