Pesticides are increasingly used worldwide to protect crops. However, only a small fraction of pesticides really hit their target organisms, with the remaining fraction reaching the environment by several phenomena such as leaching, and ending up in aquatic ecosystems: the final receptor of micropollutants. Chemical stressors induce changes in taxonomic composition of fauna and flora which are now the focus of many biomonitoring studies. Interspecific competition and predation are structuring factors of community composition. But the combined effects of biotic relationships (competition, predation) and pesticides are rarely accounted for. We tested four factors (Predation, Competition, Diuron (Herbicide) and Imidacloprid (insecticide)) separately on three distinct morphotypes of two diatoms species: Planothidium lanceolatum and Gomphonema gracile (normal and teratogen forms), to quantify the daily growth kinetics of each under varied pressures. The predator used was a nematode, cosmopolitan in soils and aquatic ecosystems (Aphelenchoides bicaudatus). We reproduced experiments combining the factors in binary and ternary combinations. Diuron had lower toxicity than expected, while imidacloprid affected the growth of non-target diatoms. Interalgal competition had marked negative effects on diatom growth kinetics, which increased as supplementary pressures (nematodes and/or pesticides) were added. These results demonstrate that ecological relationships in freshwater biofilms (competition, predation) have a non negligible effect on community composition, population behavior and impacts usually observed. Multistress conditions including the presence of pesticides in freshwaters are expected to affect biodiversity in ways that are hard to predict from simple toxicity assays.