Page 41 - The Toxicology of Fishes

Bioavailability of Chemical Contaminants in Aquatic Systems 21

FIGURE 2.4 Effects of pH on the relative abundance of un-ionized ammonia vs. ammonium ion at 5°C (dashed lines)
and 25°C (solid lines).

+
ammonia (NH ) and its conjugate acid, ammonium ion (NH ), which are in equilibrium with each other
3
4
according to the following expression:
+
K a = [ H ][ NH ] (2.1)
3
+
[ NH ]
4
The acid dissociation coefficient (K ) for this reaction ranges from approximately 10 at 0°C to 10 –9
–10
a
at 30°C (Emerson et al., 1975). The relative amounts of these species, therefore, can vary markedly
across the ranges of pH and temperature found in natural aquatic systems (Figure 2.4).
Ammonia toxicity has been found to increase greatly with increasing pH for various fish species (see
Figure 2.5A). This increased toxicity is correlated with large increases in the fraction of total ammonia
that is un-ionized; therefore, un-ionized ammonia is presumably responsible for this increased toxicity
and, because it is present at much lower concentrations, is apparently much more bioavailable than the
ammonium ion. This has not been directly demonstrated based on uptake of ammonia at fish gills, but
un-ionized ammonia is considered to be a primary component of ammonia excretion, despite its low
concentration relative to ammonium ion, because its lack of charge and smaller size result in a higher
permeability than ammonium ion across biological membranes (Wood, 1993).
When a single chemical species is the predominant bioavailable form, it can be useful to express
toxicity on the basis of just that species. This results in relationships that are more constant relative to
environmental variables and can better demonstrate the role speciation has in toxicity. Ammonia toxicity
is therefore often reported on the basis of un-ionized ammonia. When the LC values in Figure 2.5A
50
are recalculated based just on the concentration of un-ionized ammonia (Figure 2.5B), the overall
variation of LC values is reduced, and the LC values are roughly constant for pH = 7.8 to 9.0. Such
50
50
constancy of the LC values suggests that un-ionized ammonia is the predominant source of toxicity
50
for this pH range, despite it being present at much lower concentrations than ammonium ion. .
At near-neutral and acidic pH values, however, toxicity on the basis of un-ionized ammonia is not
constant, but rather increases with decreasing pH (Figure 2.5B). This suggests that ammonium ion has
enough bioavailability to cause it to be the predominant source of toxicity when concentrations of un-
ionized ammonia are extremely low. Some bioavailability of ammonium ion is expected based on studies
of ammonia excretion, which have demonstrated that ammonium ion can be excreted at fish gills by
diffusion along an electrochemical gradient from blood to water and by carrier-mediated exchange with
sodium or hydrogen ion (Wood, 1993).
These data qualitatively suggest that ammonia toxicity is a joint function of both un-ionized ammonia
and ammonium ion. A quantitative description of this relationship can be derived as follows. If the
uptake rate of each ammonia species is proportional to its environmental concentration and if the