COMPARATIVE ACCUMULATION OF ALKALINE-EARTH METALS BY 2 FRESH-WATER MUSSEL SPECIES FROM THE NEPEAN RIVER, AUSTRALIA - CONSISTENCIES AND A RESOLVED PARADOX

Whole tissue concentrations of Be, Mg, Ca, Sr, Ba and Ra-226 were dete rmined in Hyridella depressa (Lamarck) and Velesunio ambiguus (Philipp i) from a minimally polluted region in the Upper Nepean River, New Sou th Wales. Although the mean tissue concentrations of each metal were c omparable between the two species, their patterns of accumulation were dissimilar. For each metal, tissue concentration was significantly co rrelated (P less-than-or-equal-to 0.01) with tissue dry weight and she ll length in H. depressa, but not in V. ambiguus, in which variability between individuals was high and Mg concentration was inversely corre lated (P less-than-or-equal-to 0.05) with tissue dry weight. However, in each species the Ca concentration was a highly significant (P less- than-or-equal-to 0.001) positive predictor of the concentration of eac h of the other metals. For each species, normalized rates of accumulat ion of the metals, relative to increasing Ca concentration and/or size , were Ra-226>Ba greater-than-or-equal-to Sr>Ca>Mg; these rates were i nversely related (P less-than-or-equal-to 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 tho se previously obtained for V. angasi (Sowerby). The metal solubility r elationships, based on the relative increases in the tissue concentrat ions of each metal over the range of Ca concentrations, held for both species even though V. ambiguus showed no significant (P>0.05) increas e 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 tota l flux is unrelated to mussel size. The use of Ca concentration to pre dict concentrations of the other metals was effective in explaining up to 98% and 95% of the variability between individual mussels of H. de pressa and V. ambiguus respectively. Hence, the problem of inherent va riability between individuals can be eliminated. This will permit any spatial and/or temporal differences in the tissue concentrations of al kaline-earth metals of mussel populations to be more readily discerned .