Using oil-dispersing chemicals during the massive 2010 spill in the Gulf of Mexico likely did far more damage than good to a crucial aquatic animal, according to new research that wades into the hotly contested question of whether and when to use the chemicals following an oil spill.

The dispersant used by oil company BP, when mixed with crude oil, was found to be 52 times more toxic than oil alone to some microscopic plankton-like organisms called rotifers.

About 6.8 million litres of the chemical – called Corexit 9500A — were released into the Gulf of Mexico to try to mitigate the devastating underwater petroleum leak caused by the explosion of the Deepwater Horizon offshore oil rig on April 20, 2010.

An oil-covered brown pelican sits in a pool of petroleum off the Louisiana coast on June 5, 2010. Oil-spill recovery efforts sometimes have to choose between allowing oil to flow into sensitive coastal fish and bird habitats, or dispersing it into subsea depths where it can kill plankton-like species. (Sean Gardner/Reuters)

At the time, the U.S. Environmental Protection Agency and other government bodies agonized over whether to use dispersants and, if so, which ones and how much.

Dispersants cause giant pools of spilled oil floating atop the sea to break up into tiny droplets that then dilute with water just below the surface. The process helps creatures including turtles, birds and mammals that need access to the surface, and also ensures less oil flows ashore where it can choke coastal wildlife. However, it increases the amount of oil just below the surface, potentially contaminating the organisms that live there.

Scientists at the Autonomous University of Aguascalientes in Mexico and the Georgia Institute of Technology now say Corexit 9500A is far more harmful than previously thought to a key dweller of those sub-surface depths.

They studied the effect of oil, of Corexit 9500A, and of various mixtures of both on five species of rotifer from the genus brachionus. The rotifers are a core element at the base of the Gulf Coast food chain, where they're eaten by crabs, shrimp and small fish.

The research, released online Friday ahead of its publication in the February 2013 issue of the scientific journal Environmental Pollution, found that on their own, the oil and dispersant were equally toxic. But when combined, the oil and dispersant increased toxicity to one of the rotifer species by a factor of 52.

"What remains to be determined is whether the benefits of dispersing the oil by using Corexit are outweighed by the substantial increase in toxicity of the mixture," said study co-author Terry Snell, chair of Georgia Tech's biology school. "Perhaps we should allow the oil to naturally disperse. It might take longer, but it would have less toxic impact on marine ecosystems."

EPA tested on shrimp and fish

The research paper looked at rotifers because they're a common litmus test in ecological studies of toxicity, due to their sensitivity and quick reactions to contaminants.

Previous studies on oil-spill dispersants, particularly ones done by the Environmental Protection Agency while the BP well was still leaking, looked at other organisms. The EPA's tests were based on shrimp and a small fish that lives in estuaries called a silverside, and they found that nearly all dispersants, when mixed with oil, were no more toxic than the oil alone.

Oil dispersant and a sheen float atop the water off the coast of Louisiana in May 2010, in the midst of the BP spill. The latest study on dispersants says they may cause far more harm than previously thought to aquatic organisms called rotifers. (Matt Stamey/Houma Courier/AP)

That may have led the agency to permit more dispersant to be used than it would otherwise have allowed.

"Our study indicates the increase in toxicity may have been greatly underestimated following the … well explosion," said Roberto-Rico Martinez of the Mexican university, the rotifer study's lead author.

The issue has been hotly debated in the United States ever since the Deepwater Horizon exploded. The Senate's environment and appropriations committees both held hearings on the use of dispersants in summer 2010, and several environmental groups have sued the EPA and the U.S. Coast Guard over the regulation and use of the chemicals.

The new study emerged as three BP managers were in court this week for arraignment on criminal charges related to the disaster. A fourth worker, a former BP engineer, also faces charges.

In all, the British Petroleum oil leak was the largest offshore petroleum spill in U.S. history, sending 4.9 million barrels (584 million litres) of crude into the Gulf of Mexico.