Page 152 - The Toxicology of Fishes

132 The Toxicology of Fishes

Piscivores
0.10 0.60
0.30

Omnivores
0.50 0.35

0.15
Benthivores Planktivores

0.85 0.10
0.10
0.90
0.05
Benthos Plankton

Sediment Water

FIGURE 3.31 Schematic representation of a food wed bioaccumulation model. Numbers indicate the fractional contribu-
tion of each contaminant source to the diet of biota. Arrows without numbers designate equilibrium partitioning relationships.

The following section summarizes the model-based equations of a generalized food web model adapted
from Thomann et al. (1992a) and Gobas (1993). This model can be viewed conceptually as a series of
compartments linked by transport pathways (Figure 3.31). Compartments that correspond to biota are
based on assumed feeding habits. Sediment and water compartments act as sources of chemical con-
tamination. Generally, contaminant concentrations in sediment and water are specified as input param-
eters. These concentrations may be based on measured values for a site of interest or obtained as
predictions from a fate and transport model (see Chapter 14). In addition to the pathways shown in
Figure 3.31, the model also calculates chemical concentrations in fish tissues that result from exposure
to a contaminant dissolved in the water column. More complex food web models are available, such as
those based on species- and age-specific bioaccumulation (Connolly, 1991).
Plankton and Benthos
Contaminant concentrations in plankton and benthos are calculated from equilibrium partitioning rela-
tionships based on organism lipid content and the bioavailable chemical concentration in water and
sediment pore water, respectively. For the simplified model in Figure 3.31, contaminant concentrations
are assumed to be equal in phytoplankton and zooplankton. This simplification may be reasonable for
chemicals such as PCBs that partition rapidly and exhibit little or no biomagnification between phy-
toplankton and zooplankton (Gobas, 1993; Oliver and Niimi, 1988). The assumption of equilibrium
partitioning may depend on the rate of uptake relative to the rate of growth of biomass. Swackhamer
and Skoglund (1993) concluded that the establishment of an equilibrium was unlikely under conditions
of rapid phytoplankton growth.

Benthivores, Planktivores, Omnivores, and Piscivores
Predator trophic groups are defined by their prey selection. Contaminant concentrations in predator
trophic groups (C ) are calculated as the ratio of chemical intake (from water [k ] and food [k ]) to
uf
uw
f
chemical elimination (due to excretion [k ] and egestion [k ]) and growth dilution (k ):
g
eg
ex