This is the first use of dispersants at depth.

EPA- "effects of underwater dispersant use on the environment are still widely unknown, which is why we are testing to determine its effectiveness first and foremost."
US National Wildlife Federation head, Larry Schweiger, said the method of using underwater dispersant at the source of the leak was untested and could have devastating effects.
National Academy of Sciences, 2005- These dispersants "do not actually reduce the total amount of oil entering the environment."
Studies of dispersant after the Exxon Valdez spill suggest that the toxicity of chemically dispersed oil is similar to that of physically dispersed oil.
Small droplets of oil and dispersant are toxic to microorganisms as well as spawning fisheries, and could affect shrimp larvae as well.

http://priceofoil/2010/05/07/epa-says-effect-of-dispersants-at-depth-unknown/

The Deepwater Horizon Response Center-
"Coast Guard and EPA approve use of dispersant subsea in further effort to prevent oil from reaching U.S. shoreline. Agencies reserve authority to stop use of the dispersant at any time."
"The use of the dispersant at the source of the leak represents a novel approach to addressing the significant environmental threat posed by the spill. Preliminary testing results indicate that subsea use of the dispersant is effective in reducing the amount of oil from reaching the surface- and can do so with the use of less dispersant than is needed when the oil does reach the surface. This is an important step to reduce the potential for damage from oil reaching fragile wetlands and coastal areas."

"While BP pursues the use of subsea dispersants, the federal government will require regular analysis of its effectiveness and impact on the environment, water and air quality, and human health......
"The federal government will work with caution and strong oversight and reserves the right to discontinue the use of the dispersant method if any negative impacts on the environment outweigh the benefits."

In other words, the dispersant use is experimental; it smacks of doing something that may be no better (or may be worse?) than doing nothing.

http://www.deepwaterhorizonresponse.com/go/doc/2931/551271/