Combined effects of metals; An ecotoxicological evaluation
Additive joint actions of chemicals should be taken into consideration in the development of ecotoxicologically relevant water quality criteria. Existing criteria generally are derived for single chemicals, yet no natural waters exist in which only one compound is present. In order to determine the joint action of eight metals and to evaluate the Dutch water quality criteria for these metals, chronic toxicity tests were conducted with cohorts and populations of Daphnia magna. Four types of effects were distinguished, i.e. survival (LC50), body growth and intrinsic rate of natural population increase (r(m)) which were derived from individual daphnids, and maximum obtainable yield (EC50) from experiments with populations. Arsenic, cadmium, chromium, copper, mercury, lead, nickel and zinc were tested singly and in equitoxic mixtures based on the LC50 (individual D. magna) or EC50 (populations) of the individual metals. The expected toxicities of these mixtures were expressed as toxic units (TU) based on concentration addition. The LC50 and EC50 of the mixtures were 1.8 and 1.6 TU respectively, indicating an additive chronic toxicity of the metals with respect to individual survival as well as population growth of D. magna. Combined at the maximum levels of the present Dutch water quality criteria these metals were severely toxic to D. magna and caused 50% mortality in Salmo gairdneri, the latter exposed for 60 days during embryo-larval development. Even a reduction of these concentrations by a factor of five produced a 10% decrease of the yield of D. magna populations.