The toxicity of metal mixtures to the estuarine mysid Neomysis integer (Crustacea: Mysidacea) under changing salinity

Water quality criteria are mainly based on data obtained in toxicity tests with single toxicants. Several authors have demonstrated that this approach may be inadequate as the joint action of the chemicals is not taken into account. In this study, the combined effects of six metals on the European estuarine mysid Neomysis integer (Leach, 1814) were examined. Acute 96-h toxicity tests were performed with mercury, copper, cadmium, nickel, zinc and lead, and this as single compounds and as a mixture of all six. The concentrations of the individual metals of the equitoxic mixtures were calculated using the concentration/addition model. The 96-h LC50’s for the single metals, at a salinity of 5, ranged from 6.9 to 1140 mg/l, with the following toxicity ranking: Hg/Cd/Cu/Zn/Ni/Pb. Increasing the salinity from 5 to 25 resulted in lower toxicity and lower concentrations of the free ion (as derived from speciation calculations) for all metals. This salinity effect was strongest for cadmium and lead and could be attributed to complexation with chloride ions. The toxicity of nickel, copper and zinc was affected to a smaller extent by salinity. The 96-h LC50 for mercury was the same for both salinities. In order to evaluate the influence of changing salinity conditions on the acute toxicity of metal mixtures, tests were performed at different salinities (5, 10, 15 and 25). The 96-h LC50 value (1.49 T.U.) of the metal mixture, at a salinity of 5, was clearly lower than the expected value (6 T.U.) based on the non-additive hypothesis, thus confirming the additive effect of these metals in the marine/estuarine environment. Changin