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Deepwater Horizon oil, dispersant toxic to coral in Keys

Published Jan. 11, 2013

Oil from the 2010 Deepwater Horizon disaster would have killed coral reefs in the Florida Keys if the plume had reached that far south, especially when mixed with the dispersant Corexit 9500, according to a new study published Wednesday.

"Dispersant and oil combined is worse than the oil alone," said Kim Ritchie, a senior scientist at Mote Marine Laboratory in Sarasota who is one of the authors of the paper.

The oil and dispersant from Deepwater Horizon likely damaged or killed coral reefs in the Gulf of Mexico, Ritchie said, but no one has reported any studies on that aspect of the disaster. At one point about a month after the spill began, the plume of oil floated over the western edge of a roughly 61-mile expanse of 300- to 500-foot-deep reef south of Louisiana known as the Pinnacles.

Coral reefs provide shelter, breeding grounds and nursery habitat for a variety of fish and other creatures. They are made up of corals, living organisms that work with other corals to create underwater ridges.

The Deepwater Horizon rig exploded and sank on April 20, 2010, killing 11 workers. Two days later oil began spewing from a pipe a mile beneath the surface, and BP and its partners were not able to stop it until July. Before BP could cap the well, 5 million barrels of oil gushed into the gulf.

The Environmental Protection Agency approved BP using a dispersant to try to break up the oil slick before it reached land. The EPA had tested Corexit on fish and shrimp before clearing it for use.

BP sprayed the dispersant directly at the wellhead spewing oil, even though no one had ever tried spraying it below the water's surface before. BP also used more of the dispersant than had been used in any previous oil spill, 1.8 million gallons.

Ritchie, who has spent 20 years studying coral reef organisms, said she and other scientists in the Keys during the 2010 disaster wondered if it would make it to the Keys and, if so, what the effects would be, as well as the effects of using a dispersant.

They collected two species of coral — brooding coral and broadcast spawning coral. Then they bought Corexit from the manufacturer, Texas-based Nalco Energy Services, and acquired some of the oil that spilled from Deepwater Horizon, and exposed the coral to different levels of each both separately and together. Their study was funded by money from the sale of Florida "Protect the Reef" license plates.

That the oil itself was toxic to the corals was no surprise. But no one had tried mixing in Corexit before, Ritchie said.

"The results of the present study clearly indicate that dispersants are highly toxic to early life stages of coral," the scientists concluded in the study, which was published in the scientific journal PLOS ONE.

Ritchie said the results of this study should be taken into account by any federal agency reviewing plans for dealing with a potential spill anywhere near a coral reef — especially off the Florida Keys.

A study released last month on the impact of the oil spill on microscopic marine creatures called rotifers found that a mix of dispersant and oil was more than 50 times more toxic to them than the oil alone.

Information from the Associated Press was used in this report. Craig Pittman can be reached at craig@tampabay.com.

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