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14The aquatic environment is continuously under threat because it is the final receptor and sink 15 of waste streams. The development of industry, mining activities and agriculture gave rise to 16 an increase in metal pollution in the aquatic system. Thus a wide occurrence of metal mixtures 17 exists in the aquatic environment. The assessment of mixture stress remains a challenge 18 considering that we can not predict the toxicity of a mixture on the basis of single compounds.35 common carp are able to cope with these low metal concentrations, at least during a one week 36 exposure.

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To improve our understanding of underlying toxic mechanisms, it is important to evaluate differences in effects that a variety of metals exert at concentrations representing the same toxic level to the organism. Therefore, the main goal of the present study was to compare the effects of waterborne copper (Cu(II)), zinc (Zn(II)) and cadmium (Cd (II)) on a freshwater fish, the common carp (Cyprinus carpio), at concentrations being 0%, 25%, 50% and 100% of the 96 h LC50 (the concentration which is lethal to 50% of the population in 96 h). All the exposures were performed for a period of 1 week at 20°C. Our results show a rapid increase in the amount of copper and cadmium accumulated in the gills, while zinc only started to increase by the end of the experiment. All three metal ions increased metallothionein gene expression in both gills and liver. However, clear adverse effects were mainly observed for the Cu exposed group. Cu caused a decrease in Na level in gill tissue; it altered the expression of genes involved in ionoregulation such as Na+/K+‐ATPase and H+‐ATPase as well as the expression of oxidative stress‐related genes, such as catalase, glutathione reductase and glutathione S‐transferase. Zinc and cadmium exposure did not alter the ion levels in the gills. In addition, no obvious effect of oxidative stress was observed, except for a transient increase in glutathione reductase at the highest cadmium concentration.

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