Comparative accumulation of Alkaline-earth metals by two freshwater Mussel species from the Nepean River, Australia: Consistences and a resolved Paradox

Abstract

Whole tissue concentrations of Be, Mg, Ca, Sr, Ba and ²²⁶Ra were determined in Hyridella depressa (Lamarck) and Velesunio ambiguus (Philippi) from a minimally polluted region in the Upper Nepean River, New South Wales. Although the mean tissue concentrations of each metal were comparable between the two species, their patterns of accumulation were dissimilar. For each metal, tissue concentration was significantly correlated (P≤0.01) with tissue dry weight and shell length in H. depressa, but not in V. ambiguus, in which variability between individuals was high and Mg concentration was inversely correlated (P≤0.05) with tissue dry weight. However, in each species the Ca concentration was a highly significant (P≤0.001) positive predictor of the concentration of each of the other metals. For each species, normalized rates of accumulation of the metals, relative to increasing Ca concentration and/or size, were ²²⁶Ra >Ba≥Sr>Ca > Mg; these rates were inversely related (P≤0.05) to their solubilities as hydrogen phosphates. This inverse relationship was used to predict a solubility constant for BeHPO₄. The results were consistent with those previously obtained for V. angasi (Sowerby). The metal solubility relationships, based on the relative increases in the tissue concentrations of each metal over the range of Ca concentrations, held for both species even though V. ambiguus showed no significant (P>0.05) increase in tissue concentrations with size. This paradox can be explained as follows. In both species the Ca tissue concentration is a measure of the total influx and efflux of Ca and its analogues through the tissue of an individual over its lifetime; however, in V. ambiguus this total flux is unrelated to mussel size. The use of Ca concentration to predict concentrations of the other metals was effective in explaining up to 98% and 95% of the variability between individual mussels of H. depressa and V. ambiguus respectively. Hence, the problem of inherent variability between individuals can be eliminated. This will permit any spatial and/or temporal differences in the tissue concentrations of alkaline-earth metals of mussel populations to be more readily discerned.