Phytoplankton dynamics in freshwater ecosystems can be faced as a meaningful natural stressor for its main grazers, and henceconstrain bottom-up interactions within the trophic structure of the food-web. Filter-feeding zooplankters, such as Daphnia, copewith resources fluctuations by adjusting their life-history as a function of balances in the energy allocation rules. Additionally,Daphnia populations often withstand interspecific competition within coexisting related species, and it is generally assumedthat body size is a strategic trait conditioning exploitative ability and the related competitive advantage of species. The lifehistoryresponses of one Daphnia magna population and three populations of Daphnia cf longispina to a discrete gradient of algaeconcentration were assessed. This experiment was focused on (i) how these distinct populations regulate life-history when feedingover low, intermediate and high food-levels; and on (ii) whether the large-bodied D. magna can be better in exploiting resourcesover the gradient than can the smaller D. cf longispina populations. We found evidences that body size might not be the singletrait influencing the exploitative ability of related species. The D. cf longispina populations were often better than D. magna inexploiting different levels of resources, and remarkable differences in fitness were found between these similar-sized individualswithin food-level; indeed, these differences were frequently of higher scale than those found between the larger D. magna and anyof the D. cf longispina populations.