One of the main objectives of ecological risk assessment is to evaluate the effects of toxicants on ecologically relevant biological systems such as populations or communities. However, the effects of toxicants are commonly measured on selected sub-individual or individual endpoints due to their specificity against chemical stressors. Introducing these effects into population models is a promising way to predict impacts on populations. Yet currently employed models are very simplistic and their environmental relevance needs to be improved to establish the ecological relevance of hazard assessment. This study with the gastropod Potamopyrgus antipodarum combines a field experimental approach with a modelling framework. It clarifies the role played by seasonal variability of life-history traits in the population's vulnerability to the alteration of individual performance, potentially caused by toxic stress. The study comprised three steps: (i) characterization of the seasonal variability of the life-history traits of a local population over 1 year with in situ experiments on caged snails, coupled with a demographic follow-up, (ii) development of a periodic matrix population model which visualizes the monthly variability of population dynamics, and (iii) simulation of the demographic consequences of an alteration of life-history traits (i.e., fertility, juvenile and adult survival). The results revealed that demographic impacts strongly depend on the season when alterations of individual performance occur. Model analysis showed that this seasonal variability of population vulnerability is strongly related to the phenology of the population. We underline that improving the realism of population models is a major objective for ecological risk assessment, and that taking into account species phenology in modelling approaches should be a priority. © 2013 SETAC.