In situ bioassays are recognized as relevant tools for stressor and effect diagnosis in aquatic environments by allowing realistic exposure scenarios. However, most in situ bioassays are based on individual-level response assessment and do not enable to predict and/or assess the risk toward higher biological levels, such as population. Today, one of the challenges is to develop methods to extrapolate from effects measured in individual to population-level effects. This study aimed to enhance ecological relevance of this extrapolation step by using caging methods. The first step was to establish in situ relationships between life history traits (potentially used as markers of contaminant effects) and population dynamics of Gammarus fossarum (recognised as an ecologically relevant species). For this two methodologies were employed for a native population living in an uncontaminated stream: individual caging & demographic census. The second step was to assess the biological water quality of Ardières river, based on individual ecotoxicological marker measurements in transplanted organisms and then the projection of observed effects on the demography of the native population used as reference.This approach (coupling in situ bio-tests and population modeling) allows : (1) to enhance ecological relevance of diagnostic assessment, (2) to propose integrated ecotoxicological indicators.