Joint toxicity of similarly and dissimilarly acting chemicals to Daphnia magna at different response levels.

The toxicity of two amines, nonylamine and decylamine, which have a narcotic effect on organisms, and ethylparathion (enzymatic inhibitor) on Daphnia magna has been studied when acting singly and in joint toxicity tests. This was done on the basis of an isobologram method where curves of constant response, i.e., isoboles are plotted vs. the concentrations of the two toxicants. The concentrations that immobilized 50, 10, and 0% of the test population in 48 h (IC50-48h, IC10-48h, and IC0-48h) were calculated. It is shown that nonylamine-decylamine mixtures follow a near concentration addition model of joint toxicity, while ethylparathion-decylamine mixtures follow a less than additive or near independent action model. The study was performed using three toxicity indices: additive index, sum of toxic units, and similarity parameter lambda. An analysis of the similarity parameter lambda is done to evaluate whether it has a constant value for IC50-48h, IC10-48h, and IC0-48h. The results suggest that it is quite probable that a constant value of lambda can be used to characterize all response levels, as the isoboles corresponding to the average value of the similarity parameter fit within the 95% confidence intervals of toxicant concentrations at all response levels. The average value of lambda is about 0.80 for the nonylamine-decylamine test and 0.30 for the ethylparathion-decylamine test.