The interactive toxicity of cadmium, copper, and zinc to<i>Ceriodaphnia dubia</i>and rainbow trout (<i>Oncorhynchus mykiss</i>)

Traditionally, aquatic toxicity studies examine the toxicity of a single chemical to an organism. Organisms in nature, however, may be exposed to multiple toxicants. Given this is a more realistic exposure scenario in situ, the authors sought to understand the interactive toxicity of multiple metals to aquatic organisms. The authors performed a series of studies using equitoxic mixtures of cadmium, copper, and zinc to 2 aquatic organisms, rainbow trout (Oncorhynchus mykiss) and the waterflea, Ceriodaphnia dubia. Single metal toxicity tests were conducted to determine the acute median lethal concentration (LC50) values for O. mykiss and short-term, chronic median effective concentration (EC50) values for C. dubia. All 3 metals were then combined in equitoxic concentrations for subsequent mixture studies using a toxic unit (TU) approach (i.e., 1 TU = EC50 or LC50). For C. dubia, the mixture study showed greater-than-additive effects in hard water (TU-based EC50 = 0.74 TU), but less-than-additive effects in soft water (TU-based EC50 = 1.93 TU). The mixture effects for O. mykiss showed less-than-additive effects in both hard and soft waters, with TU-based LC50 values of 2.33 total TU and 2.22 total TU, respectively. These data are useful in helping understand metal mixture toxicity in aquatic systems and indicate that although in most situations the assumption of additivity of metal mixture toxicity is valid, under certain conditions it may not be sufficiently protective.