INFLUENCE OF WATER HARDNESS AND HUMIC ACID ON ZINC TOXICITY AND ACCUMULATION IN DAPHNIA MAGNA STRAUS (ACUTE, CHRONIC)

The interactive effects of water hardness and humic acid (HA) on the acute and chronic toxicity of zinc to Daphnia magna were evaluated. Bioaccumulation of Zn by animals and exuviae as influenced by Zn concentration, hardness, or HA was also assessed. Increases in either water hardness or HA concentration resulted in proportional decreases in both the acute and chronic toxicity of Zn. The effect of either of these variables (hardness or HA) was independent of the other. The relative sensitivity of chronic toxicity indices was dependent on water hardness and HA levels; however, reproduction, body length and dry weight consistently yielded the lowest no-effect concentrations. Although both HA and water hardness decreased the acute and chronic toxicity of Zn, neither had an effect on its bioaccumulation or elimination. Depuration was initially rapid with approximately one half the accumulated Zn lost in the first 24 h after the animals were transferred to "Zn-free" water (biological half life = 4.62 days). Adult females shunted accumulated Zn to offspring but did not deposit Zn in the exoskeleton. Zn adsorption to isolated exuviae increased throughout the 72-h exposure period and was directly proportional to the concentration in solution. Increasing water hardness, HA concentration, or the addition of an antibiotic reduced Zn accumulation by exuviae. Accumulation of Zn by cast exuviae indicates that both antagonism by the divalent cations of hardness and chelation by HA reduces the bioavailability of Zn to daphnids. Since body burdens of Zn in animals were unaffected by either of these variables, Zn accumulation must be regulated by other factors, such as metabolic processes. Until the heavy-metal species which are toxic or available for accumulation are identified, and until it is known how a particular change affects this speciation and the overall physiology of the organism, it will be difficult to predict how changes in water chemistry will affect the toxicity and bioaccumulation of a heavy metal.