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Corals Won’t Survive A Shallow-Water Oil Spill Treated With Dispersants
A shallow-water oil spill, even one that doesn’t reach the magnitude of the Gulf oil disaster, could kill miles of coral reefs. And it won’t be the oil alone that does it.
At the University of Miami’s Rosenstiel School of Marine and Atmospheric Science, doctoral candidate Rachel Silverstein conducted a pilot study of the effects of high doses of oil and dispersant on staghorn (Acropora cervicornis) and star corals (Montastraea cavernosa), two inhabitants of the Florida Keys and a wide range of tropical reef systems.
Since 2006 staghorn coral has been classified as threatened under the Endangered Species Act. The International Union for the Conservation of Nature and Natural Resources (IUCN) lists staghorn coral as critically endangered.
Star coral is a reef builder that is abundant throughout the Caribbean and tropical Atlantic, and can be found at the Flower Garden Banks in the Western Gulf of Mexico.
Silverstein created four treatments: 1) a 2% concentration of oil in sea water, likey much higher than would normally occur at a spill site (except, perhaps, close to a spill’s “ground zero”), 2) seawater with 2% Corexit 9500, the dispersant used during the BP oil spill, 3) a mixture of both dispersant and oil —2% oil in sea water, the dispersant added in a in a 1:10 ratio, per the manufacturer’s recommendation, and finally 4) a control treatment of just sea water with no oil or dispersant.
She used a Pulse Amplitude Modulation fluorometer to measure how well the symbiotic algae that live inside the corals photosynthesize after exposure to the three treatments.
But when the corals were treated for 12 hours with mixtures including dispersant or oil plus dispersant, there was no coral to measure.
“Basically, I found that with the oil alone there was no effect,” said Silverstein. “But, with the dispersant, and the oil plus dispersant in a 12-hour exposure, the tissue mostly just dissolved.”
Detergents, the primary ingredients in oil dispersants, are thought to eliminate the outer mucus layer that coral polyps secrete to protect against bacterial infection and to hold and transport nutrients.
Corals regularly and naturally slough off and replace this mucus layer. When it’s gone, the corals are susceptible to a host of damaging entities — from naturally-occurring bacteria to the toxic chemicals used to break up and sink floating oil sludge.
When dispersants are used the resulting oil-and-dispersant soup (i.e., dispersed oil) mixes into the water column. It doesn’t just float on the surface, which would be relatively harmless from a coral’s viewpoint.
In shallow water, from about 20 feet to 60 feet deep, where coral reefs abound, dispersed oil can cause a coral catastrophe. In deep water, like the site of the Gulf oil disaster, where dispersant was used in massive, never before seen quantities, the potential for coral death increases as the dispersed oil drifts towards coral reefs in the Florida Keys and other warm water locations.
“Corals rely a lot on their mucus layer to protect them, and the dispersant, which is like a detergent, probably just dissolves that mucus layer away,” said Silverstein. She hypothesizes that after the mucus is gone, “all the toxins [in the dispersant] can access the coral to break down the tissue.”
Silverstein reflected on the use of dispersants saying, “They say it’s 27 times safer than dish soap, but the Corexit safety manual recommends using nitrile [oil, fuel and chemical-resistant synthetic rubber] gloves to handle it.”