Significant concerns about environmental implications and safety of engineered nanoparticles (ENPs) have been raised in the recent years due to their enhanced physical and chemical properties. It is necessary to deepener the understanding on the interactions between ENPs, dissolved pollutants and aquatic organisms in complex environmental settings to decreased the uncertainties related with the cocktail effect and risk assessment. This doctoral work aimed to provide deeper insights into the interactions and effects of mixtures containing toxic metals and ENPs to phytoplankton, zooplankton, and the trophic transfer of pollutants from phytoplankton to zooplankton. To this end, the interactions in the binary or tertiary systems containing titanium dioxide nanoparticles (nanoTiO2) with three different primary sizes (5, 15 and 20 nm), inorganic Hg (IHg) and methyl Hg (CH3Hg+, MeHg) and an artificial two-step food chain consisting of green alga Chlamydomonas reinhardtii and invertebrate Daphnia magna were thoroughly explored.