National Geographic News has an excellent article on the use of chemical dispersants used by BP oil after the explosion of the Deepwater Horizon oil rig on April 20, 2010 and the subsequent massive oil spill that is moving toward shore in the Gulf of Mexico.
Gulf Oil Spill Turns To Chemicals
By Ker Than
April 30, 2010
Breaking oil into droplets
Oil dispersants have been available to combat spills since the mid-1980s. They are detergent-like chemicals that break up oil slicks on the surface of the water into smaller droplets, which can then be broken down by bacteria in the water and by other natural processes. Dispersants also help prevent the oil droplets from coalescing to form other slicks.
According to the U.S. National Research Council, oil spill dispersants do not actually reduce the total amount of oil entering the environment. Rather, they change the chemical and physical properties of the oil, making it more likely to mix into the water column than to contaminate the shoreline.
The NRC report says that evaluating the environmental trade-offs associated with dispersant use is “one of the most difficult decisions that oil spill responders and natural resources managers face during a spill.” The reason is the increased oil exposure for fish, as well as for corals and creatures that live in the lowest level of the water, such as oysters, the report said.
"Dispersant only alters the destination of the toxic compounds in the oil," redirecting its impact from feathered and fur-bearing animals on shore to organisms in the water column itself and on the seafloor, Richard Charter, senior policy adviser for marine programs at the conservation group Defenders of Wildlife, said in an email.
"No good answers to a mess this big, only degrees of damage to various life-forms," Charter said.
I'll be updating this as I learn more.