Page 39 - The Toxicology of Fishes

Bioavailability of Chemical Contaminants in Aquatic Systems 19

because of preferential metabolism by vertebrates (see benzo(a)pyrene case study below). Similar
considerations influence the ecosystem behavior of trace elements. With few exceptions (notably mercury
as methylmercury and selenium as selenomethionine), concentrations of trace elements tend to decrease
with increasing trophic level due to more efficient regulation by animals at higher tropic levels (Reinfelder
et al., 1998). The overall effect of metabolism (or trace element regulation) on chemical exposure to
higher trophic level organisms is one of the most difficult aspects of bioavailability to predict because
of uncertainties associated with the metabolic capabilities of different species and life stages, as well as
differences in food web structure and function.
Finally, a potential source of complexity in describing and predicting chemical accumulation by fish
at high trophic levels exists when chemical concentrations in water and sediments are not in thermody-
namic equilibrium with one another. This condition arises when the rate of chemical exchange between
water and sediment is slower than the rate of change of chemical inputs to the system. The degree of
disequilibrium between sediments and water is an important ecosystem characteristic that influences the
relative contributions of benthic and pelagic food chains to bioaccumulation in higher trophic level
aquatic organisms. A key element in assessing bioavailability in the presence of such disequilibria are
food-web models that can address the relative contribution of sediment and water-column contamination
to chemical accumulation in fish (Burkhard et al., 2003).

Measures Used in Assessing Bioavailability
Although bioavailability reflects various complexities, as discussed above and in the case studies below,
it still is primarily the simple comparison of the amount of chemical accumulated by an organism to
the amount of chemical the organism is exposed to. As such, a bioavailability assessment generally
addresses the ratio of a measure of accumulation to a measure of exposure and how this ratio varies
among exposure conditions, organisms, and chemicals. Various measures that are used in bioavailability
assessments are addressed in this section.
In some cases, an absolute amount of chemical can be specified for the exposure an organism receives.
This is the case in studies of dietary bioavailability when test organisms ingest a known amount of
contaminated food, and bioavailability can be examined in terms of the fraction of this dose that is
accumulated. As discussed in the dietary section above, measures of this include the dietary absorption
efficiency, the fraction of the total amount of a compound consumed by an animal that is absorbed across
the gastrointestinal epithelium, and the dietary assimilation efficiency, the fraction of the total amount of
a compound consumed by an animal that is retained in body tissues. An analogous absolute measure,
chemical extraction efficiency, can be determined for chemical uptake at fish gills using systems in which
water flow and chemical concentration changes across gills can be directly measured (McKim et al., 1985).
More often, chemical exposure is measured in terms of environmental concentrations, not in terms of
absolute amounts processed by the organism. When the environmental concentration is that in water,
measures of accumulation often used are the bioconcentration factor (BCF) and bioaccumulation factor
(BAF), both of which equal the ratio of the concentration of a substance in the tissue of an aquatic
organism to its concentration in the exposure water. These measures differ in that bioconcentration refers
just to uptake directly from exposure water (via gills, skin, ingestion of water), whereas bioaccumulation
refers to uptake via all exposure routes, including food and sediments as well as water.
Defining a BCF or BAF requires specification of the chemical concentration of interest in both the
organism and the exposure water. For the accumulated chemical, the concentration might be that in the
entire organism or in a specific tissue and might be calculated on a wet or dry weight basis. For organic
chemicals that partition strongly to lipid components in an organism, the concentration might also be
calculated as the mass of chemical in the organism or tissue divided by the weight of lipids (i.e., lipid-
normalized), rather than the total weight. Such a measure is more reflective of chemical activity and can
provide more meaningful comparisons among organisms with different lipid contents. For the exposure
water, the concentration might be the total chemical in a volume of water, or that which is dissolved or
freely dissolved (that portion of the dissolved chemical that is free of any associations with other solutes).
BCFs and BAFs also require specification of a time frame. They might be steady-state values that would
result from long exposures or might refer to accumulation over some specified shorter time frame.