Feeding and assimilation rates of Daphnia pulex fed Aphanizomenon flos‐aquae1

Feeding and filtering rates of Daphnia pulex on the blue‐green alga Aphanizomenonflosaquae were measured in laboratory experiments. Single filaments (broken from colonies) and small colonies of A. flos‐aquae <1.5 mm long could be grazed by D. pulex at maximum rates of 15,000–50,000 cells·Daphnia−1·h−1. The filtering rate increased as a function of the cube of the body length of D. pulex. Colonies >1.5 mm long were not grazed appreciably by D. pulex. Daphnia pulex continued to feed on A. flos‐aquae in the presence of another alga, Ankistrodlesmus falcatus. Colonies ofA. flos‐aquae > 1.5 mm long tended to lower the feeding and filtering rates on A. falcatus. The net assimilation efficiency (uncorrected for respiration) of D. pulex was about 11% for A. flos‐aquae and about 35% for A. falcatus. The presence in lakes of Aphanizomenon as the large colony type (A. flos‐aquae) or as single filament types may be determined by the grazing ability of D. pulex.