Page 50 - The Toxicology of Fishes

30 The Toxicology of Fishes

decrease in toxicity was associated with an increase in the fraction of copper that was complexed by organic
matter. That these effects on toxicity reflect changes in bioavailability is supported by studies that have
demonstrated that accumulation of copper in fish gills and other tissues is reduced by complexation of
copper with organic compounds in exposure water (Buckley et al., 1984; MacRae et al., 1999; Muramoto,
1980; Playle et al., 1993a,b).
Some studies, especially with phytoplankton and zooplankton, have shown a given level of toxicity
to be associated with a constant activity of free copper over a wide range of total copper, suggesting
that some copper complexes contribute little, if anything, to bioavailability (see review by Campbell,
1995). In other cases, however, toxicity on the basis of free copper activity has been reported to increase
as complexation by various inorganic and organic ligands increases, suggesting some contribution of
copper complexes to bioavailability (see reviews by Campbell, 1995; Hunt, 1987; Paquin et al., 2002a;
Sprague, 1985). This bioavailability might arise from release of free copper from a complex in the
microenvironment at the gill surface or by a copper complex directly crossing over, or interacting with,
the gill epithelial surface. In some cases, organic ligands can even increase copper toxicity, presumably
because certain hydrophobic complexes of copper are able to more readily cross lipid cellular membranes
(Florence and Stauber, 1986). The study summarized in Figure 2.9 suggested some bioavailability of
the organic-complexed copper, because, although the addition of humic acid reduced the toxicity of
copper to fathead minnows on the basis of total dissolved concentrations, toxicity on the basis of free
copper ion was increased. The line in the inset in Figure 2.9 indicates the fit of a joint toxicity model
(such as that used for ammonia above), for which the humic-complexed copper is estimated to be about
20% as toxic as the rest of the copper. Such a contribution to bioavailability by copper complexed to
such large ligands could be due to partial dissociation of these complexes at the gill surface, perhaps
because of the reduced pH expected at this surface; however, in the absence of direct evidence from
accumulation measurements, other explanations for the apparent bioavailability of organic-complexed
copper cannot be completely discounted. The organic matter might also affect the speciation of other
ionic constituents important in copper toxicity or exert some direct effect on the gill surface.
Although copper speciation and its role in bioavailability can explain the effects on toxicity of many of
the physicochemical factors cited at the start of the section, some of these factors will have little or no
effect on copper speciation in the exposure water. One such factor is water hardness, which refers to certain
cationic components in the water, primarily calcium and magnesium. Such cations would not affect aqueous
copper speciation except to the extent that they compete with copper for ligands, and such effects are
generally so small that they could not be responsible for any appreciable effects on toxicity. Increased
hardness has been long reported to be associated with decreased metal toxicity, but the nature of these
effects is not particularly clear or consistent. In some cases, reported effects of hardness are actually
combined effects of hardness, pH, and alkalinity, so any effects of the hardness cations were confounded
with those of copper complexation by hydroxide and carbonate. In several studies with fish, however,
hardness was varied in association with anions that do not significantly affect copper speciation. Usually,
reductions in toxicity were still found (Chakoumakos et al., 1979; Erickson et al., 1987; Inglis and Davis,
1972; Miller and Mackay, 1980), an example of which is shown in Figure 2.10. In other cases, little or
no effect of hardness was found (Lauren and McDonald, 1986; Zitko and Carson, 1976). The effect of
hardness can also depend on the relative amounts of calcium and magnesium and on the test species
(Naddy et al., 2002). Figure 2.10 suggests that the incremental effects of hardness are greater at low
hardness than at high hardness. This nonlinearity is further evident in other data for fathead minnows from
this same study (Erickson et al., 1987) and from studies with cladocerans (Gensemer et al., 2002). To
further complicate the situation, Lauren and Macdonald (1986) proposed that apparent effects of hardness
on acute copper toxicity can be affected by inadequate acclimation of fish to the hardness, and that hardness
effects could be manifested more strongly in longer exposures. Erickson et al. (1997), however, found
similar effects of hardness on acute toxicity for both acclimated and unacclimated fish.
Zitko (1976), Zitko and Carson (1976), and Pagenkopf (1983) suggested that hardness cations can
affect metal toxicity by competing for biochemical receptors on fish gills involved in the uptake or effects
of toxic metals, thereby necessitating higher concentrations of the toxic metal to result in enough toxic
metal accumulation on these receptors to elicit effects. The uptake of copper might also be affected by
the effects of calcium on the permeability of paracellular junctions. Such mechanisms would represent