"The EPA says that the brew of oil and dispersants still swirling in the Gulf of Mexico can be highly to moderately toxic to marine organisms at certain concentrations determined in a lab setting. The agency did not say whether those acute toxic levels are present in the open waters of the Gulf.
According to the agency's lab tests, the mix of Corexit and oil generally falls into the moderate category. The testing also found that the oil-dispersant mix is no more toxic than oil alone, EPA officials said, calling the unprecedented use of dispersant an 'environmental tradeoff that's not to be taken lightly.'
Visit this EPA website for the full press release, as well links to the study as well as an audio recording of the teleconference with reporter questions.
The tests were conducted on two species, the Gulf mysid shrimp (Americamysis bahia), an aquatic invertebrate and the inland silverside (Menidia beryllina) that lives in coastal estuaries. They were tested at juvenile life stages, when most sensitive to toxins. The two species are believed to representative of marine life in the area, according to the EPA.
Mysid shrimp form the basis of many aquatic food chains."
Bob Berwyn reports for the Summit County Citizens Voice August 3, 2010.
July 31, 2010, EPA Comparative Toxicity Report 
EPA Dispersants Testing Page
"Oil Spill Dispersants Shifting Ecosystem Impacts in Gulf, Scientists Warn" (Greenwire) 
"BP "Carpet Bombed" Gulf With Dispersants" (Mother Jones)