A large variety of micropollutants, from pesticides, pharmaceuticals to personal care products, can reach surface waters through the effluent of wastewater treatments plants or leaching off agricultural soils. Measured in Europeans lakes for many decades now, their dissipation through different processes such as sorption, sedimentation and phototransformation has been extensively evaluated, but a knowledge gap still exists regarding the dissipation of micropollutants through biotransformation. This project aims at evaluating the extent of biotransformation of a diverse and relevant range of polar pesticides and pharmaceuticals in lake Greifensee, near Zurich, at different seasons and water depths. The influence of physico-chemical parameters (temperature, pH, dissolved oxygen…) relative to biological parameters such as microbial density, composition and diversity will be evaluated. Statistical approaches are used to identify the most important factors driving the transformation of these micropollutants at different periods of the year. This project will also help identifying which types of micropollutants are readily biotransformed in lake water, and which ones are rather persistent. This evaluation will help foster improvements in risk assessment and management of typical contaminants in the environment.