Page 40 - The Toxicology of Fishes

20 The Toxicology of Fishes

Measures analogous to the BAF can also be defined when exposure is referenced to chemical concen-
trations in sediment or food. For example, the biota-sediment accumulation factor (BSAF) was developed
for assessments of hydrophobic chemicals for which concentrations in the water column are difficult or
impossible to adequately measure and for which sediment-based food chains are an important route of
exposure (Ankley et al., 1992). This factor is defined as the ratio of a lipid-normalized chemical
concentration in an aquatic organism to the organic carbon-normalized chemical concentration in the
surface sediment. Organic carbon-normalized sediment concentrations are calculated relative to the
weight of organic carbon in the sediment rather than the total weight. Like lipid normalization, this
provides a measurement that is better related to chemical activity and is more useful for comparing and
predicting accumulation of hydrophobic organic chemicals among different sites and exposure conditions.
As discussed in the previous section on dietary uptake, digestive processes can increase the activity
of chemical within the gastrointestinal tract, resulting in higher concentrations in an organism than in
the food it consumes (i.e., biomagnification). The biomagnification factor (BMF) is the unitless factor
by which the concentration of a substance in an organism at one trophic level exceeds the concentration
in organisms that occupy the next lower trophic level. For organisms at higher tropic levels whose
chemical exposure is primarily via diet, biomagnification can be of great importance for determining
their exposure concentrations. For such organisms, the bioavailability of a chemical relative to its
environmental concentration will be a function of a set of accumulation relationships, including BMFs,
for various organisms throughout the food web.
Chemical uptake rate constants (the ratio of the uptake rate to the exposure concentration) can also serve
as useful measures in bioavailability assessments. Such coefficients are proportional to a BCF or BAF for
short exposures in which chemical elimination is small compared to uptake. For longer exposures, these
coefficients will also be proportional to the BCF or BAF, provided the elimination rate constants do not
vary significantly across the different exposure conditions for which bioavailability is being assessed. If
elimination rates do vary, then uptake rates would not be a good overall measure for assessing bioavailability
but can still be useful for investigating some of the processes regulating bioavailability.
Sometimes chemical accumulation by an organism is not, or cannot be, adequately characterized;
however, if a certain level of toxicity over a specified exposure period can be assumed to reflect a fixed
amount of chemical accumulation, then exposure concentrations causing this level of toxicity can serve
as a measure of bioavailability. By the relationship BAF = (accumulated concentration)/(exposure
concentration), the exposure concentration is inversely proportional to the BAF if the accumulated
concentration is constant. The inverse of a toxic effect concentration therefore can serve as a surrogate
for the BAF. The case studies below for ammonia and copper will illustrate this, using 96-hour LC 50
values (the concentration causing 50% lethality over a 96-hour exposure) as measures of how bioavail-
ability is affected by exposure conditions.

Assessing Bioavailability: Case Studies

To further illustrate the general principles and processes discussed above and to introduce approaches
for assessing and describing bioavailability, this section considers certain aspects of the bioavailability
of selected chemicals with different physicochemical characteristics. Included in these case studies are
(1) an ionizable inorganic compound (ammonia), (2) ionizable organic chemicals (phenols), (3) cationic
metals (copper), (4) organometals (mercury), and (5) nonionic organics with differing properties (2,3,7,8-
tetrachlorodibenzo-p-dioxin and benzo(a)pyrene). Although these examples address specific chemicals,
they are intended to exemplify concepts of bioavailability that are more generally applicable.

Ionizable Inorganic: Ammonia
Aquatic systems can have ammonia concentrations high enough to adversely impact fish as a result of
wastewater treatment plant discharges, degradation of nitrogen-containing organic matter, fertilizer
runoff, and industrial sources. The two major chemical species of ammonia are weakly basic un-ionized