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.