SUMMARY 1. In reservoirs of the south‐central United States, the exotic cladoceran Daphnia lumholtzi is common during warm midsummer conditions, when cyanobacteria are abundant and native Daphnia are rare. In the current study, we employed surveys, field experiments, and a life table experiment to investigate the role of food quality in explaining the distribution and phenology of D. lumholtzi, relative to two native species (Daphnia parvula and Daphnia mendotae). 2. During May–September 2000 in eutrophic McDaniel Lake, Missouri U.S.A., cyanobacteria (primarily Oscillatoria) first appeared at 6‐m depth and then became abundant throughout the epilimnion. 3. During the May field experiment, D. lumholtzi, D. parvula and D. mendotae all consumed a similar diet of algae, showing positive selection for small greens (chlorophytes and cryptophytes <20 μm). During the July experiment, when the epilimnion exceeded 25 °C and cyanobacteria were common in the lake, D. lumholtzi consumed significantly more total algae and more cyanobacteria than the two native species. Although the Daphnia selected against cyanobacteria, all three species consumed about 25% of this food in their diet. 4. A life table experiment compared the responses of D. lumholtzi and D. parvula with variation in density of high‐quality food (Ankistrodesmus) and concentration of a toxic strain of cyanobacteria (Anabaena flos‐aquae). Both Daphnia species showed reduced survivorship, fertility and intrinsic rates of increase in response to elevated concentrations of cyanobacteria, particularly at the higher food level. 5. The results suggest that D. lumholtzi shows similar inhibition from cyanobacteria as does the native Daphnia. However, their continued high in situ feeding rates imply that D. lumholtzi is less affected by midsummer conditions in warm‐water reservoirs than are native Daphnia.

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