Feeding and assimilation rates of Daphnia puZex fed Aphanixomenon jlos-aquael

Feeding and filtering rates of Dophnicl pulex on the blue-green alga Aphanizomenon jlos-aqzme were measured in laboratory experiments. Single filaments (broken from colonies) and small colonies of A. jlos-nqucle cl.5 mm long could be gmzcd by D. pulex at rnaximurn rates of 15,000-50,000 cclls*L)ccphnia- ’ ah-‘. The filtering r;ltc increased as a function of the cube of the body length of D. pulex. Colonies>1.5 nm long were not grazed appreciably by 1). pulex. Daphnia pulex continued to feed on A. jlos-clqu~e in the presence of another alga, Ankistr-odesmus fbZcatus. Colonies ofA.fEo.r-nquae> 1.5 mm long tended to lower the feeding and filtering rates on A. fkkatus. The net assimilation efficient? (uncorrected for respiration) of D. pulex was about 11 % for A. flos-clqucle and about 35 % for A. jdcdus. The prcsencc in lakes of Aphanixomenon as the large colony type (A. flos-m/nm) or as single filament types may be determined by the grazing ability of D. puZcx. Aphanixomenonflos-aqutle (IL.) Ralfs is a blue-green alga which dominates sum-mcr algal blooms in many temperate freshwater lakes. It usually occurs in col-onies or flakes of hundreds of filaments aligned in parallel. There are also reports of naturally occurring single filament types of Aphanixomenon (Schwabe an