Page 149 - The Toxicology of Fishes

Toxicokinetics in Fishes 129

Mechanism of Biomagnification
Field sampling efforts suggest that in some cases measured BAFs in fish exceed the BCF that would
have been expected from an equilibrium distribution of chemical between the fish and water. These
observations have been attributed to processes that accompany the digestion of contaminated food items.
According to the digestion hypothesis, absorption of dietary lipid and reductions in meal volume increase
chemical activity in the gut contents above that of the ingested meal, resulting in an inwardly directed
diffusion gradient and a potential for biomagnification of chemical residues (Connolly and Peterson,
1988; Gobas et al., 1993a,b, 1999). Under these circumstances, the gills become a route of net chemical
elimination and not uptake, and the extent of bioaccumulation and biomagnification is determined by
the balance between chemical uptake within the gut, branchial efflux, biotransformation, and growth.
The extent of biomagnification is defined using a biomagnification factor (BMF), which is the lipid-
normalized chemical concentration in a predator divided by that of its prey. For a given trophic transfer
step, a compound is said to biomagnify when the BMF is greater than 1. Within a simple food chain,
BMFs for each trophic transfer step can be multiplied. Under these circumstances, the BAF for fish that
occupy the highest trophic level will exceed the partitioning-based BCF prediction by an amount equal
to the product of BMFs for all relevant trophic transfers.
Support for the digestion hypothesis has been obtained in studies with several fish species. In feeding
studies with guppies and goldfish, the ratio of feces to food fugacity increased with chemical log K ,
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attaining a maximum value of 4.6 (in guppies) for the pesticide mirex (as described below, fugacity is
a measure of chemical activity; see Gobas et al., 1993a). The ratio of intestinal contents to food fugacity
in a natural population of white bass (Morone chrysops) also increased with chemical log K , attaining
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a value of about 2.2 for 2,2′,3,4,4′,5,5′-heptachlorobiphenyl (Russell et al., 1995). In rainbow trout and
rock bass (Ambloplites rupestris) exposed to 2,2′,4,4′,6,6′-hexachlorobiphenyl, the chemical fugacity in
chyme following uptake of dietary lipid exceeded that of food by a factor of 7 to 8 under both laboratory
and field conditions (Gobas et al., 1999).
Nichols et al. (2004a) developed a PBTK model for the dietary uptake of hydrophobic chemicals by
fish that accounts for the absorption of dietary lipid and reductions in meal volume. The model was then
used to simulate chronic exposures to a set of hypothetical high log K compounds (Nichols et al.,
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2004b). The results of this effort showed that a log K -dependent diffusion resistance acts along the
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entire length of the GIT to limit dietary uptake of high log K compounds. This decrease in diffusive
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uptake appears to be responsible for a log K -dependent decrease in absorption efficiencies, BAFs, and
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BMFs at log K values greater than about 6 (Figure 3.9). Metabolism and growth were predicted by
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the model to result in lower BMFs at all log K values. In either case, however, BMFs continued to
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increase with chemical log K (assuming constant diffusion resistance). In field sampling efforts, BAFs
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for persistent organochlorines in fish have been shown to increase with log K up to a log K value
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of 6 or 7 and then level off or decline at higher log K values (Burkhard, 1998; Thomann, 1989).
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Bioaccumulation Referenced to Sediment
Aquatic sediments are formed from the deposition of particles and colloids and can act as both a sink and
source of contaminants. Long-term contaminant input can lead to sediment concentrations that exceed the
water concentration by several orders of magnitude. Both metals and hydrophobic organic compounds
bind to sediments, but the nature of these interactions differs. Under reducing conditions, metals tend to
form insoluble complexes with sulfide, and the acid volatile sulfide (AVS) content of sediment has been
used to normalize for differences in apparent toxicity of some sediment-associated metals (Carlson et al.,
1991; Di Toro et al., 1990). In oxic sediments, however, organic material may provide the major site for
metal binding (Fu et al., 1992). Other factors that may influence metal bioaccumulation from sediment
include: (1) speciation, (2) transformation to organic derivatives, (3) interactions of different metals, and
(4) sediment chemistry (salinity, iron oxide content, redox potential, and pH) (Bryan and Langston, 1992).
The binding of organic contaminants to sediments has been related to the organic carbon content, clay
type and content, cation exchange capacity, pH, and particle surface area of the sediment (Knezovich
et al., 1987). In many systems, the organic carbon content of sediment (typically 0.5 to 3% of sediment
mass) predicts contaminant toxicity (Bierman, 1990; Di Toro et al., 1991). Inferred by these observations