Acidification and Toxicity of Metals to Aquatic Biota

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Abstract
Acidification of freshwaters can in principle influence metal–organism interactions in at least two ways: the decrease in pH may affect metal speciation in solution, or it may affect biological sensitivity at the level of the cell surface. We considered the possible effects of environmental acidification (pH 7–4) on (1) metal speciation in solution, (2) metal adsorption at biological surfaces, and (3) metal uptake by and toxicity to aquatic biota. Attention was focused on some 10 metals of potential concern in the context of freshwater acidification (Ag, Al, Cd, Co, Cu, Hg, Mn, Ni, Pb, Zn). For the four metals (Al, Cu, Hg, Pb) predicted to manifest speciation changes in the range pH 7–4, confirmatory experimental data are available for two (Cu, Pb). In the six remaining cases predicted to show little sensitivity to pH changes in this range, supporting experimental evidence exists for four metals (Ag, Cd, Mn, Zn). A pH-dependent biological response is documented over a realistic range of H+ and metal concentrations for 6 of the 10 metals considered (Al, Cd, Cu, Zn, and to a lesser extent, Hg and Pb). These six metals fall into two groups: those for which a decrease in pH results in a decreased biological response (type I behavior: Cd, Cu, Zn) and those for which the dominant effect of acidification is to increase metal availability (type II behavior: Pb). Data for the remaining two metals (Al, Hg) clearly reveal pH effects, but the results are too few and too inconsistent to allow generalizations.