Interactive Effects of Metals in Mixtures on Biomass Production Kinetics of Freshwater Copepods

The effect of metal mixtures on the biomass production rates of natural assemblages of copepods was predicted by assuming that metal-induced increases in the inverse of the growth rate (i.e. the growth time) are additive. The average observed growth time increase, ΔGTobs in 14 binary mixtures of the metals Cd, Cu, Hg, Pb, Zn, and As was 17% larger than the average calculated growth time increases, ΔGTcal (assuming additivity), indicating a slight but statistically significant synergism. Synergism in multimetal mixtures appears to be cumulative, since the average ΔGTobs was 53% greater than the average ΔGTcal in mixtures containing five metals simultaneously. One of the 15 possible binary mixtures (Zn:As) of the six metals tested demonstrated a large divergence between ΔGTobs and ΔGTcal. For all other metal mixtures studied, most of the toxicity is accounted for by the summation of noninteractive, single, metal effects.Key words: copepods, metal toxicity, metal interactions, biomass production, cadmium, copper, mercury, lead, zinc, arsenic