1. Based on both field data and laboratory studies, the summer population of Daphnia longispina living in a stratified eutrophic lake was examined in relation to the abundance of algae, nanoflagellates and picocyanobacteria.2. In early July, the Daphnia population replaced Bosmina and remained the dominant zooplankter during summer 1994. Its development in July was concomitant with an increase of edible algae but, despite the apparent abundance in available food, the Daphnia population decreased throughout August suggesting poor food conditions.3. From mid‐August to the beginning of September, the biomass of inedible phytoplankton was greater than that of the smaller, edible fraction. Consequently the average rate of increase, birth rates and fecundity of Daphnia remained low. Although the biomass of heterotrophic nanoflagellates was consistently low, the demographic parameters of Daphnia were correlated throughout this period with these protozoans.4. Life table experiments run in the laboratory showed that epilimnetic food supported both the growth in length of individual Daphnia and an increase in fecundity, but metalimnetic food supported only individual growth. D. longispina probably failed to reproduce because of the abundance of detritus mixed with the heterotrophic nanoflagellates in the metalimnetic water at that period of the year. The vertical migration of Daphnia into these deeper layers could be caused by a predator avoidance mechanism.