Traditional toxicity bioassays usually assess the effects of single substances on isolated species, which does not, however, allow the identification of potential effects in natural systems. In addition, quantitative estimations of metal uptake in top consumers due to feeding are generally focused on the individual level and do not evaluate the consequences of metal addition at demographic or population levels. An experimental system, composed of the marine microalga Nannochloris oculata Droop, 1955 and the herbivorous rotifer Brachionus plicatilis Müller, 1786, was implemented in order to evaluate (1) whether the interaction of 2 metals, Cd and Zn, causes a greater toxic effect on planktonic species than that obtained through the addition of single metals and (2) whether the toxicity of these metals on a target herbivore population increases through the indirect route of food intake. Factorial experiments were developed, and results indicated that the toxicity on populations was higher when metals were combined, suggesting resource-dependent interaction. Consumption of contaminated algae had a negative effect on the growth rate of B. plicatilis, driven mainly by decreased juvenile and adult survival. This effect was opposite to the direct effect of dissolved metals in the medium. It is suggested that assessment of the interaction between toxic substances, as well as between populations, inserted in trophic networks, is necessary when quantifying the effects of disturbances on marine populations. Finally, our results highlight the relevance of indirect effects using a purely demographic tool focused on population level.