Page 150 - The Toxicology of Fishes

130 The Toxicology of Fishes

is an association between toxicity and bioavailability. The source of this organic carbon (e.g., mud, plant
material) may influence bioavailability and bioaccumulation (DeWitt et al., 1992). In some regions, soot
carbon from the combustion of fossil fuels may also contribute to sediment binding of organic compounds
(Accardi-Dey and Gschwend, 2003; Persson et al., 2002). Although generally present in small amounts
(a few percentage points of total carbon), the affinity of soot carbon for some hydrophobic compounds
may be considerably higher than that of organically derived carbon.
The extent of chemical bioaccumulation from sediment is typically expressed using either a bioaccumu-
lation factor or a biota-to-sediment accumulation factor (BSAF) (Spacie et al., 1995). The sediment BAF
is the ratio of the chemical concentration in fish to chemical concentration in sediment. Sediment BAFs
have units of sediment mass per unit of tissue weight (wet weight/wet weight) and can be viewed concep-
tually as the sediment mass containing the amount of chemical concentrated in 1 gram of fish tissue. The
BSAF is the ratio of the lipid-normalized chemical concentration (ng chemical per g lipid) in fish to the
organic carbon normalized concentration in sediment (ng chemical per g organic carbon). Thorsen et al.
(2004) adjusted this ratio further to account for chemical binding to soot carbon in sediments and determined
that this binding was unlikely to have a major impact on BSAFs for PAHs in two freshwater bivalves.
Additional work is required to determine whether such adjustments are needed for other compounds.
Biota-to-sediment accumulation factors are used to reduce variability in measured bioaccumulation
due to variation in organism lipid content and sediment organic carbon concentrations. BSAFs in fish
typically range from 0.1 to 10 (Bierman, 1990; Burkhard et al., 2004; Rubinstein et al., 1984; Schuytema
et al., 1988). Values greater than 1 are often interpreted as evidence for biomagnification of chemical
residues coupled with little or no biotransformation. Values less than 1 have been attributed to biotrans-
formation, although other factors such as growth and low dietary assimilation efficiency may contribute.
Methods for estimating the bioaccumulation of sediment-associated contaminants include steady-state
exposures, equilibrium partitioning (EqP) theory, and kinetic models. Each approach has been used
extensively to estimate BSAFs for benthic invertebrates. Chemical accumulation by benthic invertebrates
is important because consumption of these organisms by fish provides a route for translocation of
sediment-associated contaminants to aquatic food webs. Direct exposures also have been employed to
study the bioaccumulation of sediment contaminants by fish that live in intimate contact with these
sediments (Hellou et al., 1995; Payne et al., 1995). BSAFs for high trophic level fish are generally
predicted using a food web model that includes a benthic component (see below).
In each case, it should be noted that the BSAF definition makes no assumptions about a chemical
equilibrium between the organism and sediment. Although it may be reasonable to assume that an
equilibrium exists between sediments and benthic invertebrates, this assumption becomes more prob-
lematic for high-trophic-level fish. Field-derived BSAFs must be interpreted, therefore, in the context
of changes in toxicant loadings to the environment, as well as other potential sources of disequilibrium.

Equilibrium Partitioning Theory of Bioaccumulation from Sediments
The equilibrium partitioning (EqP) theory of bioaccumulation from sediments is based on the assumption
that chemicals partition between sediment, pore water, and aquatic organisms in accordance with thermo-
dynamic principles (Di Toro et al., 1991). The theory predicts that the equilibrium concentration of a
chemical in sediment pore water (C ) is controlled by the concentration of the chemical in sediment (C ),
w
s
the organic carbon–water partition coefficient (K ) of the chemical, and the fraction of organic carbon
oc
(OC ) in sediment according to:
s
C w = C s ( K OC s) (3.126)
oc
Bioaccumulation of contaminants by sediment-dwelling biota (C biota ) is determined by the lipid content
of the organism (L biota ), C , and OC : s
s
C biota = L biota( C OC s) (3.127)
s
Substituting Equation 3.126 into Equation 3.127 and adopting the assumption that K = K gives the
oc
ow
relationship: