Deepwater Horizon spill: Dispersant more toxic to corals than oil

Following the spill, coral populations were coated with a dark-coloured flocculent slime that was found to contain oil from the spill and residues from the dispersants

By Deepanwita Gita Niyogi
Published: Friday 10 April 2015

 A coral specimen exposed to oil and dispersant displays declining in health over time. The picture on the furthest right is a healthy control sample.  (Right) The coral species Paramuricea in its natural habitat which was used for the experiment.

The dispersant used as a remedial measure to clean the ocean basin after the 2010 Deepwater Horizon oil spill in the Gulf of Mexico is more toxic to corals than the spilled oil, a new study says.

The research conducted at the US-based Temple University comes on the eve of the spill's fifth anniversary. The disaster, considered the largest marine oil spill, occurred on April 20, 2010 in the Mexican gulf.

Five million barrels of crude oil was approximately released in the water over three months, following the spill. To clean it, 7 million litres of dispersants were applied, according to the study. Dispersants are chemical emulsifiers that increases the rate of oil dispersion.

Experimentation on corals

As part of the study, researchers from the Temple University and the Pennyslyvania State Univeristy (USA) exposed three cold-water coral species from the gulf to various concentrations of both the dispersant and the spilled oil. The dispersant is toxic to the corals at lower concentrations than the oil, the study concluded.

The finding, “Response of deep-water corals to oil and chemical dispersant exposure,” was published online in the journal Deep-Sea Research II.

Dispersants are normally applied to the water's surface. But for the first time dispersants were applied at a depth during an oil spill, according to the study.

“Applying the dispersants at depth was a grand experiment being conducted in real-time,” said Erik Cordes, associate professor of biology at Temple Univeristy, who has been studying the Gulf of Mexico coral communities for more than a decade.

The disaster and its effect on corals

Following the spill, Cordes and his team members discovered several damaged coral populations that were coated with a dark-loured flocculent slime that was found to contain oil from the spill and residues from the dispersants, the study says.

“We wanted to know if the damages that had been witnessed could have been caused by the oil, the dispersant itself, or a combination of both,” said Danielle DeLeo, a Temple University doctoral student, who was the study's lead author.

"We know that the corals in the gulf were exposed to all of these different combinations, so we have been trying to determine the toxicity of the oil and the dispersants, and see what their impact would be on the corals.”

The researchers exposed the corals to a range of concentrations for both the dispersant and the oil to determine a lethal dose for each of them. They were surprised to find that the lethal concentration is much lower for the dispersant, meaning that it is more toxic than the oil.

“The concentration of dispersant needed to produce a significant effect on the corals is lower than the concentration of oil (and so) it is considered to be more toxic. It takes a higher concentration of oil to produce the same effect,” Cordes told Down To Earth.

The study says the coral specimens showed “more severe declines in health in response to dispersant alone and the oil-dispersant mixtures than the oil-only treatment”.

Cordes said that his lab will carry out additional studies to try to replicate the concentrations of oil and dispersant that the corals were exposed to during the gulf oil spill.

He added that the finding could assist in developing future strategies for applying dispersants at oil spills that may be more helpful than harmful to the environment.

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