COMPETITION BETWEEN CRUSTACEAN ZOOPLANKTON IN CONTINUOUS CULTURES

It has been proposed that cases of diaptomid dominance in oligotrophic lakes are due to lower food requirements for Diaptomus species than Daphnia species. We test this hypothesis by competing Daphnia pulicaria and Diaptomus oregonensis populations in continuous and semicontinuous cultures. The threshold food concentrations for population growth of D. pulicaria and D. oregonensis were statistically indistinguishable. The thresholds of both species were within the range of concentrations characteristic of ultraoligotrophic lakes. The dominant species varied across experiments in a pattern consistent with an effect of temporal variation in food concentration. Neither our data nor previously published studies provide support for the hypothesis that cases of Diaptomus dominance in oligotrophic lakes are due to a simple difference in threshold food concentrations.