| Summary: | The Kostomuksha mining plant (KMP, Republic of Karelia, Russia), which is an important producer of iron pellets, is situated in the upper part of the Kenti–Kento lake–river system. In this water system, Lake Kostomuksha drains its waters through a chain of small lakes into the larger Lake Kuito and on to the White Sea. Effluents from the mining plant have been deposited in Lake Kostomuksha since 1982. For this study, samples of perch (Perca fluviatilis) were obtained from three metal-contaminated lakes with different pollutant concentrations downstream from Lake Kostomuksha. The concentrations of heavy metals (Hg, Cd, Cu, Zn, Ni and Cr) in fish liver and muscle were analysed. Concentrations of Hg, Ni and Cr (1.13, 0.09 and 0.08 ug g–1 dry weight, respectively) in fish liver from the studied lakes were higher than those in the control lake, Kamennoe (0.43, < 0.001, < 0.001 ug g–1), which is not directly influenced by the KMP. In the uppermost lake, Poppalijärvi, the concentration of Hg in perch muscle and liver was > 1.0 ug g–1 dry weight. Compared to the control lake, the electron microscope study of liver tissue from perch in this lake showed an increase in the distance between hepatocytes, a decrease in the number of nuclear pores and the smallest mitochondria of all the lakes studied.
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