Dispersants are chemical formulations composed of solvents, surfactants and other additives that disrupt the solid surface of an oil slick by reducing the surface tension between oil and water. Composed of molecules with a water-compatible (hydrophilic) end and an oil-compatible (lipophilic) end, dispersants link an oil droplet to nearby water molecules and allow the natural agitation caused by waves and wind to pull the droplets apart into increasingly smaller droplets. Unlike the large, free oil droplets that float in a two-dimensional slick at the surface of the water, these smaller droplets eventually become heavier than water and sink (spread in three dimensions) into the water column, or the vertical expanse of water extending from the sea surface to bottom sediments.
The action of dispersants does not change the total amount of oil leaked into the environment. It simply changes the oil's transport, fate and potential effects. Surface oil not only poses the greatest danger to exposed seabirds, turtles and fur-bearing marine mammals, it is also the main threat to biologically sensitive shorelines and economically important beaches. But since both the dispersants and the dispersed oil are toxic (in varying degrees, according to the type of oil spilled and the chemical agent used) to animals living underwater, coral reefs and other marine life,  decisions to use dispersants involve trade-offs between decreasing the risk to water surface and shoreline habitats while increasing the potential harm to organisms in the water column and on the seafloor.  For these reasons, decisions concerning the use of dispersants are usually based on a broad assessment of the overall environmental impact of a specific spill. 
Once the decision to use dispersants has been made, there are additional considerations such as choosing the most effective commercial product and determining the optimal application system.
Dispersant acceptance and product effectiveness criteria vary from country to country. Oil spills in the United States have been limited primarily to offshore marine waters where water salinities are high, so there has been an emphasis on dispersants formulated to work in relatively high salinity water and some are known to lose their effectiveness when applied in fresh or brackish water. In Canada, a number of federal and provincial agencies are responsible for specific aspects of spill response and planning, but as in the U.S., Canada lacks a decision guide and specific criteria for dispersant use in fresh water. France is the only country with published information on dispersant use policies and criteria for their use specific to freshwater.  Australia's Guidelines for Acceptance is a clear and informative publication within that country's National Plan to Combat Pollution of the Sea by Oil and Other Noxious and Hazardous Substances. 
Whatever dispersant formulation(s) the agencies in charge of
a spill select, they must design an application system (aboard
aircraft or marine vessels) capable of meeting several basic criteria,
including the ability: 1. to spray dispersant uniformly on the
oil, 2. to disperse a droplet size that encourages mixing with
the oil and ease of movement to the oil-water interface, 3. to
attain the proper concentration at the oil-water interface, and
4. to deliver sufficient energy to disperse the slick into droplets.
There is evidence that dispersed oil degrades more quickly than
oil that has not been dispersed. So, ultimately, a successful
dispersant operation would end in dispersed oil droplets being
processed in the marine ecosystem and degrading
into naturally occurring substances. This process starts with
the droplets of oil and dispersant being colonized by bacteria
that then begin to degrade them. Next, protozoans and nematodes
(small worms) join the colonies. Eventually, the oil may be further
broken down and incorporated into the food web.
 However, some research raises concerns that certain microbes
may chew up the dispersant molecules instead of dining on the
Bioremediation (the use
of natural microorganisms, plants
or fungi to correct a contaminated or altered environment) may
have a role to play in restoring oil-contaminated environments
and habitats. But when it comes to using microorganisms and their
enzymes to return areas
to their original conditions, there are two opposing schools of
thought. Many scientists agree that naturally occurring bacteria
capable of degrading oil are already present in marine environments,
but the limited availability of nutrients like nitrogen and phosphorus
prevent the oil-eaters from performing to their full potential.
Others, like Ben Lyons, a research scientist and engineer at the small biotech firm Evolugate, think greater potential lies in seeding oil spills with more bacteria. Lyons says, "Our methodology is, you've got to get oil from the actual site where you're going to be putting the microbes." As the experiments go forward, Thomas Lyons (no relation to Ben Lyons), Evolugate's principal research scientist, says that his staff will collect more samples from the waters and marshes along the Gulf Coast and attempt to evolve "designer communities" for each sample.  But, according to Ronald Atlas of the University of Louisville, who has been studying oil-spill bioremediation since 1968, field studies show that adding new microbes is no more valuable than providing nutrients to the ones already there.
Because ecosystems are complex systems of finely tuned, interconnected parts that thrive on very symbiotic relationships, they can be damaged by anything that disrupts the balance of interactions that take place within them. A major known consequence of subsea oil plumes is that they lead to a bloom in oil-chomping microbes that eat the oil, but use oxygen in the process-meaning that oxygen levels in the water can drop rapidly and threaten marine life.  Atlas admits that seeding an oil spill with microbes has no environmental drawback, although providing too much fertilizer can cause die-offs by triggering damaging algal blooms.
For a variety of reasons, bioremediation will likely remain a secondary weapon, most effective after mechanical and physical efforts at containment and cleanup have been exhausted, in the fight to minimize the environmental damage of oil-contaminated water and coastal areas. One downside to using bioremediation in the initial cleanup is that it's a slow process that doesn't satisfy the urgency of first response efforts to minimize environmental threats. Also, bioremediation isn't a practical approach far offshore, where high energy and waves can quickly dilute nutrients the microbes need to thrive.  The final battle line, rescue and rehabilitation, comes after the oil has escaped containment, evaded cleanup and penetrated the living spaces of countless marine and terrestrial wildlife species.
Go To Remediation:
Wildlife Rescue and Rehabilitation