Contamination levels of aquatic ecosystems and ecological effects must be assessed, as stated by the Water Framework Directive. However, linking both issues remains difficult, yet necessary to restoration processes. Recently an active biomonitoring approach has been developed using caged Gammarus fossarum to assess trends of bioavailable contamination levels in aquatic systems. This new tool overcomes most drawbacks associated to currently used methods, allowing accurate comparison of contamination levels between stations or between seasons and years. The present study aimed at calibrating the active biomonitor G. fossarum in order to interprete bioavailable contamination levels in term of risk of ecological disturbance. For this, a national scale investigation has been performed to obtain the bioavailable contamination levels and ecological indices (diversity and abundance) for 117 sites representative of French watercourses. The levels of 11 metals quantified in G. fossarum have been analysed against ecological indicators. No link was revealed between bioavailable contamination levels and global ecological indicators (as I2M2), but clear relationships were established with sensitive macroinvertebrate abundances. This allowed designating concentrations for ? elements above which populations are endangered. These ecological impairment threshold values are discussed accordingly to levels of bioavailable contamination previously determined in G. fossarum at the French national scale.