Biomonitoring, assumed to be an integrative measurement of the chemical exposure of aquatic organisms, is not straightforward for essential metals because they can be actively regulated by animals. Although increasing bioaccumulation with exposure levels is a crucial endpoint for the development of biomonitors, it is rarely verified in real environments, where the metal concentrations are rather low and vary little. This study was designed at the scale of a river basin to assess the ability of Gammarus pulex indigenous populations to accumulate Cu, Zn and Mn in realistic exposure conditions. During two annual campaigns, water and gammarids were collected at various sites contrasted in terms of physicochemistry and contamination. The results show significant relationships between metal concentrations in animals and in freshwaters established by conceptual models of bioaccumulation, but with patterns specific to each metal (base level, internal regulation and maximal accumulation). In particular, a saturation process of Cu accumulation occurs at environmental exposure levels, unlike Mn and Zn. Statistical analyses performed from field data show that Cu and Zn bioaccumulations may be influenced by a complex combination of geochemical variables, unlike Mn. We conclude that G. pulex is a useful candidate to monitor metal bioavailability in freshwaters due to its responsiveness to low exposures of surrounding environments. Nevertheless, a reliable quantification of bioavailability of essential metals requires characterizing some geochemical effects on metal bioaccumulation.