Today there exists a large consensus that ecological models with populations rather than individuals as targets to protect have a high potential to be implemented into ecotoxicological risk assessment [1]. Indeed, they revealed particularly useful to solve the challenging issue of extrapolating toxic effects measured at the individual level towards possible contamination impacts on populations. Nevertheless, with the perspective to project or predict population answer, the environmental relevance of these models has to be improved, accounting for variability in population vulnerability and making them transferable with native species. In this way, the extrapolation process could be also used within a diagnostic framework. Our work was focused on two phylogenetically and ecologically contrasted species (the Crustacean Amphipod Gammarus fossarum and the Mollusk Gastropod Potamopyrgus antipodarum), widely present in European rivers and for which ecotoxicological (sub)individual biomarkers are available. We first developed environmentally realistic size-structured population models including both in situ caging and laboratory data, and relating individual-level demographic parameters (survival, growth, reproductive activity, fecundity, size at maturity) to the dynamics of reference native populations all along a year. Second, this methodology was coupled with in situ biotests or laboratory bioassays, in order to allow a population-level assessment of water quality or chemical compound toxicity. We showed thus the relevance to take into account the phenology of population dynamics (seasonal variability), by illustrating how it strongly controls the demographic sensitivity to toxic alteration of individual fitness traits in native species. More precisely, the poster will exemplify how sensitivity analyses of population endpoints were able to improve the comprehensive evaluation of possible contamination impacts on ecologically relevant biological levels.