nonfloating oils (Scholz et al., 1994). Oils that quickly sink or are suspended in the water column have greater impacts on organisms in the water column because more of the water-soluble fraction of the oil dissolves rather than evaporates. Oil on the surface is primarily weathered by evaporation to the atmosphere and, to a lesser degree, to the water column by dissolution. Oils suspended in the water column or deposited on the bottom are less likely to evaporate but more likely to dissolve, although the water-soluble fraction of heavy oils is usually very low. Consequently, the water column can have higher concentrations of toxic fractions from nonfloating oils than from floating oils. Dissolution tends to be a slower process than evaporation (Lee et al., 1989, p.37), thus increasing potential exposure times. In the Morris J. Berman spill in Puerto Rico, divers observed dead fish, living fish with lesions and tumors, and many lethargic territorial fish in nearshore waters adjacent to the spill site (Vincente, 1994). Mobile species may be able to move to uncontaminated areas, thus reducing their exposure.

Nonfloating oils are often high in polynuclear aromatic hydrocarbons (PAHs), which are the primary source of both acute and chronic toxicity to aquatic organisms. Naphthalene compounds (two-ringed aromatics) have been shown to be more toxic than lightweight aromatics, such as benzene and toluene (Anderson et al., 1987). In terms of the water-soluble fraction, bunker C is as toxic as diesel oil (Markarian et al., 1993). Thus, even though heavy residual oils are not usually considered to be acutely toxic to fish (NOAA and API, 1995), oils that are mixed into the water column without weathering by evaporation on the water surface first may have a higher fraction that dissolves and, therefore, may be more acutely toxic to organisms in the water column.