Page 62 - The Toxicology of Fishes

42 The Toxicology of Fishes

trout is reduced by the metabolic inhibitor piperonylbutoxide (Sijm et al., 1990). In aquatic food webs,
TCDD does not appear to biomagnify in fish as do PCBs with similar K values. Because dietary
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assimilation efficiencies of TCDD are high (Isosaari et al., 2004; Nichols et al., 1998), this difference
between TCDD and PCBs cannot be attributed to lower dietary bioavailability for TCDD than PCBs,
but rather is likely due to biotransformation in both prey and predator species of fish (Burkhard et al.,
2004; Endicott and Cook, 1994).
This discussion of TCDD illustrated how the bioavailability of hydrophobic organic chemicals is
strongly influenced by their partitioning to organic carbon in aquatic systems. The freely dissolved (most
bioavailable) fraction of such chemicals is reduced by DOC and POC, and much of the chemical is
partitioned into sediments. The importance of dietary exposure increases with chemical hydrophobicity,
especially for organisms at higher trophic levels whose dietary exposures can be affected by biomagni-
fication through the food chain. Bioavailability for such organisms becomes a function, not just of their
exposure to water and food but also of the bioavailability relationships for their entire food chain. Under
these circumstances, chemical accumulation in fish is determined by both chemical speciation in the
water and in the sediments associated with the benthic-based part of the food web. These processes have
been addressed in food-web-based bioaccumulation models that can assess the bioavailability to fish of
hydrophobic organic contaminants throughout entire aquatic systems (Burkhard et al., 2003). The next
case study considers a chemical in which biotransformation is even more important for determining
bioavailability.

Nonionic Organics: Benzo(a)pyrene

Benzo(a)pyrene (BaP) is a polycyclic aromatic hydrocarbon (PAH) with a planar structure composed
of five fused benzene rings, a molecular weight of 252.3 Daltons, and log K of 6.5 (Bruggeman et al.,
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1982). Unlike TCDD, BaP concentrations in most surficial sediments have not declined significantly
since the 1970s (Lima et al., 2003). BaP also provides an interesting contrast to TCDD with regard to
bioavailability. Both chemicals have gained notoriety because of their potential carcinogenicity, but,
although only slightly less hydrophobic, BaP appears to be much less bioaccumulative in fish than
TCDD. Fish BSAFs for PAHs in the environment are about three orders of magnitude less than for
chlorinated aromatic hydrocarbons (Connor, 1984). For more extensive reviews of PAH bioaccumulation
in marine organisms, see Meador et al. (1995).
One factor responsible for lower bioaccumulation of BaP in fish than for TCDD appears to be a much
greater affinity of BaP for organic carbon, despite these two chemicals having similar molecular weights
and hydrophobicities. As noted in the TCDD case study above, partition coefficients of organic chemicals
to DOC are typically about tenfold less than the K for the chemical; however, the binding of BaP with
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dissolved humic material was found to be completely reversible with a partition coefficient approximately
equal to its K (McCarthy and Jimenez, 1985). For POC, partition coefficients for Baltic Sea water
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(Broman et al., 1991) were 20-fold greater for BaP than for TCDD, which is slightly more hydrophobic.
This low availability of BaP in water is also evident in sediments, which is consistent with the low
bioaccumulation observed in fish dependent on benthic food chains. In general, PAHs in sediments seem
to be less bioavailable to benthic invertebrates than other persistent organic chemicals. Bioaccumulation
of PAHs by various benthic-coupled organisms in both field and laboratory exposures is an order of
magnitude less than for PCBs or pesticides, which, unlike the PAHs, tend to reach equilibrium distri-
butions between sediment organic carbon and organism lipid (Tracey and Hansen, 1996). This low
bioavailability of PAHs may be attributable to lower concentrations of PAHs in pore water than expected
based on their K and their total concentrations in the sediment. The uptake of chemicals by benthic
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organisms, however, is not just via pore water. Ingestion of sediment appears to be an important route
for accumulation of PAHs from sediment by oligochaetes (Leppanen and Kukkonen, 1998). The greater
uptake of BaP from ingested sediment particles by an oligochaete in comparison to the less hydrophobic
PAH phenanthrene is attributable to the greater assimilation efficiency of BaP which causes a greater
uptake from the ingested particles than from pore water (Lu et al., 2004). For such organisms, the overall
bioavailability of BaP from sediments will not be as restricted as suggested by the partitioning between
water and sediment.