Inhibition of Daphnia by Oscillatoria in Lake Washington1

The hypothesis of Edmondson and Litt concerning the inhibition of Daphnia by Oscillatoria in Lake Washington was tested in experiments with cultures. Growth and reproduction of Daphnia pulicaria and Daphnia thorata were reduced by increasing concentrations of Oscillatoria agardhii from 0 to 400 filaments ml−1 in the presence of adequate amounts of Cryptomonas as food. Cryptomonas reproduction also was inhibited by Oscillatoria, by an undetermined mechanism. The results are consistent with the idea that the scarcity of Daphnia in Lake Washington during the 10‐year period before 1976 was related to high concentrations of O. agardhii.