Page 902 - The Toxicology of Fishes

882 The Toxicology of Fishes

the effects of PAHs from those of other compounds through field studies alone. Cause-and-effect
relationships between PAHs and disease conditions in fish can be more definitively established through
controlled exposure studies in the laboratory; however, even this approach does not fully address the
possible modifying effects of other contaminants on the toxicology of PAHs in fish. The additive,
synergistic, or antagonistic effects of various compounds in contaminant mixtures are not well under-
stood, and this issue is of continuing concern in establishing exposure limits and sediment quality
guidelines for marine organisms.

Exposure to PAHs
Bioavailability
The proportion of the total contaminant concentration that is available for uptake by organisms defines
the bioavailable fraction. For neutral hydrophobic organic compounds such as PAHs, organic carbon is
the main variable controlling bioavailability. Hydrophobicity, or the tendency to be water insoluble, is
the primary determinant for partitioning behavior between water, sediment, and tissue (Burgess et al.,
2003). As the hydrophobicity of PAHs increases, the ratio of water to sediment concentrations of a PAH
will decline due to the tendency of the compound to avoid water and seek a nonpolar environment. The
affinity for tissue also increases with hydrophobicity because of lipid in the organism.
The octanol–water partition coefficient (K ) is one physical parameter that can be used to predict the
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partitioning behavior exhibited by PAHs in the environment. A review of several studies was compiled
in Meador et al. (1995a) for 24 of the more commonly measured PAHs to provide estimates of their
K values. The range in K values is almost 4 orders of magnitude (about 5000-fold) from the least
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hydrophobic to the most hydrophobic compound on this list.
Another useful partition coefficient (K ) is the ratio of PAH concentrations in sediment organic carbon
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and water, which is determined by the concentration of PAH per gram of organic carbon in sediment
divided by the concentration of PAH in water. This coefficient is useful for predicting the amount of
waterborne PAH for a given sediment concentration under equilibrium conditions. It has been shown
for many neutral hydrophobic compounds that the K is a good predictor of the K . Several authors
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have developed equations that predict K values from the K for various hydrophobic compounds (Di
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Toro et al., 1991; Karickhoff, 1981; Means et al., 1980). These studies show that the K values range
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from 0.4∗K to 1.0∗K .
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A few studies have found differential partitioning and bioavailability, depending on the type of PAH
and its history of association with sediment. Varanasi et al. (1985), for example, demonstrated that
recently added (spiked) PAH in sediment was more bioavailable to organisms than the PAH in field-
contaminated sediment; however, other studies have shown only about a twofold reduction in bioaccu-
mulation by infaunal invertebrates exposed to sediment that had been aged several months with contam-
inants compared to sediment recently spiked with contaminants (Kukkonen and Landrum, 1998; Loonen
et al., 1997). The source of the PAH is also a factor in determining bioavailability. Farrington et al.
(1983) pointed out that PAHs from oil spills may be less strongly sorbed to sediment than pyrogenic
PAHs and hence more available to organisms. This observation was supported by subsequent work
showing that PAHs from field samples are tightly bound to sediment and only a few percent is available
for equilibrium partitioning (McGroddy et al., 1996; Meador et al., 1995b). The lower water concentra-
tions of PAHs in such sediments can produce much lower than predicted accumulation in organisms;
however, it is not clear if species ingesting sediment with pyrogenic PAHs will also exhibit lower
bioaccumulation (Meador et al., 1995b).

Bioaccumulation/Food Web Transfer
Bioavailability and organism physiology and behavior are the most important variables affecting the
bioaccumulation of chemical contaminants, especially PAHs (Meador, 2003). Of the total environmental
concentration, only the bioavailable fraction is available for assimilation into the organism. Unlike