Tracer experiments with two diatoms labelled with 13C (Nitzschia palea) and 15N (Fragilaria crotonensis), respectively, were conducted to investigate feeding selectivity and interspecific competition between the herbivore grazers Asellus aquaticus (Isopoda, Crustacea) and Potamopyrgus antipodarum (Hydrobiidae, Gastropoda). Traditional methods such as cell counts and biovolume calculations revealed a similar significant decrease in algal biovolume for all different grazer treatments. In contrast, measurements on isotope signatures showed large differences in tracer uptake and different degrees of assimilation. Thus, this technique enabled the direct quantification of the grazers uptake and assimilation and therefore served as an ideal tool for the detection of passive selectivity. Our results indicate a shift in feeding preferences related to between-species competition and a potential divergence of trophic niches through competitive interactions. Furthermore, this presented study will examine the potential role of passive vs. active selection in marine and freshwater food-webs.