Analysis of separate and combined effects of heavy metals on the growth of Aporrectodea caliginosa (Oligochaeta; Annelida), using the toxic unit approach

The assessment of polluted soils is often made difficult by the presence of a mixture of toxicants. In this study we try to analyse the effects of mixtures of three heavy metals, cadmium, copper and zinc, on the growth of earthworms, Aporrectodea caliginosa. Laboratory toxicity tests were conducted in which worms were exposed, over a period of 6 weeks, to sulphate salts of metals mixed into a natural soil. EC(50)s and EC(10)s were estimated for each metal separately following a log-logistic curve fitting of growth versus exposure concentration. Cadmium was the most toxic metal followed by copper and zinc; relative toxicities on a molar basis were approximately Cd:Cu:Zn = 1:2:10. Based on single metal EC(50)s, a mixture of one-third of the concentrations of the three metals was prepared to cover the whole effect range and the effects of the mixture were evaluated against the toxic units derived from the separate metal tests. The EC(50) of the mixture was not significantly different from unity (1.33 toxic units). It may be concluded that the effects of mixtures of metals on the growth of earthworms can be described by relative addition of concentrations, although there was a tendency for antagonism.