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This research aims to test the reliability of environmental monitoring by bioaccumulators of pollutants; that is to establish a positive relationship between the pollutant concentrations in the bioaccumulator and those in the water in which it lives. To this end we analysed the contents of Al, Cu, Zn, Fe, Mn and Ca in the soft tissues and shell of Unio pictorum mancus. The filtered water samples from the mussel habitat were analysed for the same metals. The mussels were collected from 15 stations settled in 12 Northern Italian lakes during the first two weeks of July 2001. These results were obtained: a) metal concentrations varied widely with mussel size and among stations; b) a significant positive correlation between the concentration of calcium in the water and in the mussel tissues, but no relationship emerged for the other metals; c) no relationship between the metal concentrations in the tissues and those in the shell was found; d) there was a certain tendency for Mn, Fe and Zn concentrations in the soft tissues to increase with shell size; e) the sequence of the decreasing metal concentrations arranged for the tissues was similar to that of the shell, but rather different from that in the water; and f) the concentration factor values of the trace metals were high for the shell and soft tissues. In highly productive lakes large size mussels dominated, whereas small mussels were more abundant in low productive lakes. Although the metal concentrations in the water of productive lakes were greater than in low productive ones, the metal concentrations in the tissues of the mussels from the latter were generally higher than those in the mussels from the former. We propose some hypotheses to explain this paradox. Finally, our results show that the metal concentrations in the mussels do not reflect the metal concentrations in the water in which they live. It follows that this commonly used but oversimplified monitoring system cannot be recommended. On the other hand mussels may be very useful for other purposes, such as identifying new pollutants or pollutants present in such low concentrations that they cannot be measured with the commonly used methods. The pollutant content of mussels may enable the variations in time of the pollutant level of an environment to be monitored. In addition, the transplantation of mussels from a clean site to a polluted one may be a useful tool for identify the pollutants of the receiving environment.
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