European Union represents 45% of world’s pesticide tonnage. Just a little part of pesticides really hit their target organisms; the remaining reaches the environment by several phenomena as leaching, ending up in aquatic ecosystems: the final receptors of microcontaminants. Chemicals stressors induce taxonomic changes on fauna and flora, which are now the focus of many biomonitoring studies. However, water quality indices only take one biological compartment into account, and put combination of abiotic and biotic factors aside. We tested separately four factors (predation, competition, diuron and imidaclopride) on two diatom species exhibiting distinct morphotypes: Planothidium lanceolatum and Gomphonema gracile (normal and teratogen forms), to quantify daily their growth kinetics under varied pressures. The predator used was a nematod from freshwater Aquitaine biofilms: Aphelenchoides bicaudatus. We reproduced experiments combining the factors, to determine if they acted synergistically or antagonistically. P. lanceolatum is strongly impacted by interalgal competition. Nevertheless, under herbicide treatment, A. bicaudatus enhanced its growth thanks to medium enrichment with nutrients (feces, mucus trail agglutinating), and acted negatively on Gomphonema gracile, by grazing it. On the contrary, exposure to imidaclopride improved microalgal defence mechanisms, by forming associations, making them less accessible. Ecological relationships in freshwater biofilms (competition, predation) have non negligible effect on community composition, population behavior and impacts usually observed without considering these factors. These results call for a new approach concerning bioassessment methods of water quality and fundamental studies to improve our understanding of ecosystems functioning, for restoration and environment protection.