| Summary: | Seasonal changes in dry weight based Cd, Cr, Cu, Fe, Hg, Ni and Zn concentrations were measured in the soft tissue of freshwater mussels Anodonta anatina and Pseudanodonta complanata. The animals were collected from uncontaminated Lake Höytiäinen, and from three areas of central Lake Saimaa, with varying anthropogenic impact. In addition, two transplantation experiments with A. anatina were carried out in Lake Saimaa. Heavy metal levels in the mussel soft tissue were low. Statistically significant seasonal fluctuation was found in the concentrations of Cd, Cr, Cu, Hg, and Ni in A. anatina. The apparent increase in Cd, Cr, Hg and Ni concentration in July was probably related to spawning. The two species differed in metal levels at different stages of their seasonal cycle. The effect of sex, size and sediment contact on trace metal concentrations was negligible for mussels of 60–80 mm shell length. Higher levels of Fe and Zn were, in general, found in P. complanata. Transplanted mussels did not have the same metal concentrations as natural populations but the order of metal levels in different areas in central Lake Saimaa was similar to native mussels. Therefore transplanted A. anatina can be used to indicate areas of differing metal bioavailability. Use of male mussels and sampling in autumn can be recommended.
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