spill-of-opportunity studies. However, if mesocosm studies are conducted for other dispersant-related purposes, consideration should be given to the addition of carefully designed studies that examine the effects of dispersants or dispersed oil on organisms or groups of organisms that cannot be readily studied in laboratory-scale tests.

DISPERSANT TOXICITY

Early dispersant formulations (prior to 1970) were essentially solvent-based degreasing agents adapted from other uses. These early dispersants proved to be highly toxic to aquatic organisms, as seen following treatment of the Torrey Canyon spill, resulting in an unfavorable public impression of dispersant use that persists today. Concerns about dispersant use after the Torrey Canyon spill were summarized in the previous NRC dispersant review as toxicity of the products themselves, and concern that effective dispersant use would make oil constituents more bioavailable enhancing their toxicity (NRC, 1989). However, the previous NRC report concluded that the acute lethal toxicity of chemically dispersed oil is primarily associated not with the current generation of dispersants but with the dispersed oil and dissolved oil constituents following dispersion (NRC, 1989). There has been little evidence in the intervening years to support a different conclusion.

Dispersants in use today are much less toxic than early generation dispersants, with acute toxicity values (measured in standard 96 h LC50 tests) typically in the range of approximately 190–500 mg/L (Fingas, 2002a) as compared with dispersed oil values in the typical range of 20–50 mg/L. An abundant literature exists on the toxicity of the Corexit dispersants currently approved for use in the United States (Tables 5-2 and 5-3; George-Ares and Clark, 2000). Numerous studies have found current dispersants to be significantly less toxic than oil or dispersed oil in direct comparisons (Figure 5-3; also Adams et al., 1999; Mitchell and Holdaway, 2000; Clark et al., 2001; Fingas, 2002a), although a few studies have reported greater dispersant toxicity compared with oil or dispersed oil toxicity (Gulec et al., 1997). Sensitivity to dispersants and dispersed oil can vary significantly by species and life stage. Embryonic and larval stages appear to be more sensitive than adults to both dispersants and dispersed oil (Clark et al., 2001), with LC50s for both oyster and fish larvae reported to be as low as 3 mg/L for dispersant alone and about 1 mg/L for dispersed oil. However, some studies report higher larval toxicity values (i.e., lower sensitivity) for both dispersant and dispersed oil that are closer to the adult values (Coutou et al., 2001). Variable sensitivity of early life stages to dispersants could be related to species-dependent variability in egg permeability (Georges-Ares and Clark, 2000).



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