Now that we know that the hole drilled by the Deepwater Horizon will, for the foreseeable time, keep on gushing, we better take a look at the chemicals used to keep the oil from forming a film at the surface. Unsurprisingly, those chemicals are rather toxic by themselves, although they are far less persistent in the environment than crude oil. As far as I know, about 500,000 gallons of dispersants have already been deployed in the Deepwater Horizon disaster, either by ship or by plane.
It is very difficult to tell at this point, what all this means for the marine life in the Gulf, or for that matter the people in the surrounding communities. For one, no-one really knows what chemicals are used and in what quantities, because of the proprietary nature of the products. Also, rumour (and common sense) has it that the calamity is so huge that BP is simply putting out whatever it can get, including stockpiles from a few years back, so there could be a very long list of formulations used. I don’t think anyone really knows what’s out there, and how toxic it really is.
However, the Corexites generally consist of a mixture of non-ionic and anionic tensides, such as dioctyl sulfosuccinate (AOT) or diverse sugars esterated with long fatty acids, e.g. sorbitan monooleate. Also, a solvent is added to the mix, 2-Butoxyethanol in the case of Corexit 9527, or 3-Butoxypropanol in the case of Corexit 9554. I think you get the idea.
Of course there have been extensive studies about the environmental effects of these formulations, since oil spills aren’t exactly rare. For example this study from 1995 found that Corexit 9554 affects mollusks first, at about 10 ppm (tested with Abalone larvae), while fish larvae need 13-15 times higher concentrations, with an LC50 of about 150 ppm. Fish larvae that survived the initial toxicity, however, showed virtually no delayed mortality, which is at least something. Shrimp larvae, on the other hand, were the most resistant to acute toxicity, but showed a significant amount of delayed mortality: About three quarter of all specimen died in the days after exposure. The kelp Macrocystis fell somewhere in between.
This pattern is fairly typical for many surfactant formulations (pdf): They are toxic and mollusks suffer first, other organisms are affected at varying concentrations. This spells trouble, of course, for the oyster banks in the Gulf of Mexico, which just recovered from the hurricane Katrina. Apart from that, all bets are off. Those tests, mind you, usually expose the test organisms to short concentration spikes and see what happens afterward. In this case, there are the issues of enrichment through the food chain and burial in the sedimentas well as synergistic toxicity. The expected long-term presence of a huge amount of those chemicals, together with the oil, which is dispersed but certainly not gone away, make predictions difficult.