In this study, the value of carbon (δ13C) and nitrogen (δ15N) stable isotope ratios were determined in nymphs of a top-predator, the common backswimmer (Notonecta glauca L.), collected in 18 m3 outdoor freshwater mesocosms used to assess the fate and ecotoxicological effects of a diphenyl ether herbicide, fomesafen, applied alone or in combination with Agral 90® (mixture of polyethoxylated derivatives of nonylphenol). Both treatments had a negative effect on δ13C values which may reflect changes in carbon fluxes across food webs in the treated ponds associated with a shift in phytoplankton structure. A decrease in δ15N values was observed in the nymphs collected in mixture-treated ponds, which was presumably due to an increase in the abundance of rotifers and Chironominae larvae in these ponds. These preliminary results indicate that stable isotope ratios may be used as shortcuts to detect qualitative or quantitative shifts in the structure of aquatic food webs caused by pollutants. Carbon and nitrogen stable isotope ratios may be used to detect the impact of pollutants on aquatic food webs.