Page 44 - The Toxicology of Fishes

24 The Toxicology of Fishes

osmotic effects are unlikely to be as significant. Nonetheless, this is a possibility that should
be recognized in more comprehensive evaluations of ammonia bioavailability.
4. The bioavailability of ammonia might also be affected by other ionic constituents of the
exposure water. Ammonia toxicity to fish has been reported to be reduced due to increases in
the hardness or salinity of test water. Causes for this have been suggested in work with the
amphipod Hyalella azteca. Ankley et al. (1995) reported ammonia toxicity to H. azteca to be
lower in hard test water than in soft test water, especially at low pH. Borgmann and Borgmann
(1997) further found that the toxicity of ammonia to H. azteca was reduced by sodium, but
not by calcium, and suggested that this effect is greater at low pH because mechanisms for
transport of ammonium ion across fish gills are affected by the ionic composition of the water,
whereas the passive transport of un-ionized ammonia is not.
5. Because the fraction of the more bioavailable un-ionized ammonia increases with temperature,
it might be expected that total ammonia toxicity would increase with temperature; however,
for fish, there is little effect of temperature on ammonia toxicity expressed on the basis of total
ammonia (U.S. EPA, 1999). Erickson (1985) noted that the effect of temperature on ammonia
toxicity is inconsistent with the relative toxicities of un-ionized ammonia and ammonium ion
inferred from the pH dependence of ammonia toxicity. The effects of temperature on ammonia
toxicity thus apparently reflect multiple influences on ammonia metabolism, excretion, and
toxicological response that are not yet well understood.

Evaluations of ammonia bioavailability, therefore, can involve various considerations beyond the
simple model presented above, depending on the exposure circumstances, the fish species of concern,
and the level of detail and precision desired in an assessment. Nonetheless, the pH dependence of
ammonia toxicity still provides a good example of how the bioavailability of a chemical, to a good
approximation, can reflect a simple weighted average of contributions of its constituent species. This
type of model has broad applicability to bioavailability assessments.

Ionizable Organic: Phenol Derivatives

Some organic chemicals contain functional groups that are weak acids or bases. Such chemicals will
exist as multiple species with different charges at these functional groups, which can affect how readily
they will be accumulated by a fish. The relative amounts of these chemical species will vary with pH,
so the bioavailability of the chemical can also vary with pH.
A group of chemicals that has received considerable study is phenol and its various substituted
derivatives, especially pentachlorophenol (PCP), which has been extensively used as a wood preservative
and is a widespread contaminant of soils and aquatic systems. This class of chemicals consists of a
hydroxyl group (OH) on a phenyl ring (Φ) and can exist as un-ionized molecules (RΦOH) or as phenolate
–
ions (RΦO ), where R denotes other functional groups on the ring. These two chemical species are in
equilibrium with each other according to the following expression:

+ Φ −
K a = [ H ][ R O ] (2.6)
Φ
[ ROH]
where the acid dissociation constant (K ) varies with the nature of R. In the subsequent discussion, the
a
term phenol refers to any member of this class of chemicals.
When based on concentrations in water, the toxicity and accumulation of various phenols in fish have
been found to decrease with increasing pH (see Figure 2.6). The inverse relationship between uptake
rate constants and toxic water concentrations in Figure 2.6 suggests that the degree of accumulation
required to elicit a given level of toxicity does not vary much with pH. This is more evident in Figure
2.7, which shows that LC values for PCP increase with pH by about the same factor as the BCFs
50
decrease, resulting in estimated body burdens at death being approximately constant with pH. Kobayashi
and Kishino (1980) reported that PCP concentrations measured in dead fish were similar at different