Ecological risk assessment seeks assessing toxicants’impacts on populations or communities. Nevertheless, toxicity is commonly measured on sub-individual or individual endpoints. Introducing these effects into population models allows to apprehend potential demographic impacts. The environmental relevance of currently employed models could be improved to set up the ecological relevance of hazard assessment. With the mudsnail, our study illustrates how the combination of a field experimental approach with a modelling framework allows to decipher the seasonal variability of population vulnerability. We proceeded in three steps: (i) we characterized the seasonal variability of life-history traits from a native population during one year by means of in situ experiments (caged snails), coupled with a demographic follow-up, (ii) we developed a periodic matrix population model picturing the monthly variability of population dynamics, (iii) we simulated demographic consequences of life-history trait alterations. Results reveal that demographic impacts strongly depend on the season of toxic events. Model analysis allows understanding this pattern consistently to the phenology of the mudsnail life cycle. Thus, we underline that improving the realism of population models is a major concern for ERA, and that taking into account species phenology in modelling approaches should be one priority notably for time-varying contaminations.