Relationship between chronic toxicity and bioaccumulation of copper, cadmium and zinc as affected by water hardness and humic acid

Using daphnids, we have evaluated the effects of water hardness and humic acid (HA) on the chronic toxicity and bioaccumulation of Cu, Cd and Zn. Although changes in water hardness and HA concentration changed bioaccumulation or chronic toxicity, or both, of each of the three metals, there was no consistent relationship between changes in toxicity and changes in bioaccumulation. Only for Cu in hard water was there a positive correlation between toxicity and bioaccumulation. The uptake of 65Zn by molted exoskeletons suggests that changes in water chemistry do indeed modify the bioavailability of metals as would be expected. That is, an increase in the concentration of Ca 2+ or Mg 2+ or the increased chelation of metals by HA should decrease bioavailability. The complex storage, transformation and excretion processes in multicellular animals, however, result in there being a poor correlation among bioavailability, bioaccumulation and toxicity.