The relative importance of algae and bacteria as food for Daphnia longispina (Cladocera) in a polyhumic lake

SUMMARY. 1. Grazing on algae and bacteria by the planktonic cladoceran, Daphnia longispina , was studied in a small polyhumic lake with low phytoplankton primary production in southern Finland.
2. D. longispina filtered algae at average rates of 0.09-0.82 ml ind^?1 h^?1. The filtering rates on bacteria were 26-92% of those on algae in parallel experiments.
3. From June to August algae, including mixotrophic and heterotrophic forms, comprised 56-93% of the food ingested by D. longispina. In mid-September and early October, when the Daphnia population was declining and the algal biomass was low, bacteria comprised 73% and 55% respectively of the food of Daphnia.
4. For D. longispina , an energy pathway via bacteria and bactivorous flagellates is probably a more important link to allochthonous organic matter than direct utilization of epilimnetic bacteria.