“…This can be explained by the fact that, like most aquatic organisms, M. iridescens can efficiently, at least to a certain extent, regulate uptake and/or excretion of essential metals through homeostatic mechanisms over a wide range of ambient exposure levels, while for the non-essential metals, there appeared to be no, or only a weak, mechanism to regulate or limit uptake or excretion (Voets et al, 2009) tial trace metals with some form of homeostatic control. In this respect, our results are completely in accordance with the findings of other researchers (Bervoets et al, 2004;Bonneris et al, 2005), who also reported that bioaccumulated Cd concentrations were more responsive to the spatial gradients than were those of bioaccumulated Zn, even though the ambient [M] max /[M] min ratios in exposure conditions were less for Cd than for Zn. In a biomonitoring context, M. iridescens would thus be more responsive to the external Cd and Pb exposure gradients than to the Cu and Zn exposure gradients.…”