Filtering rates on natural bacteria by Daphnia longispina and Eodiaptomus japonicus in Lake Biwa

Filtering rates on ³H]thymidine-labelled natural unattachedbacteria and that on ¹⁴C]bicarbonate-labelled natural planktonwhich pass through the 25 µm-mesh-size screen were measuredfor Daphnia longispina and Eodiaptomus japonicus in Lake Biwa.Errors associated with the radioisotope technique, i.e the lossof labels after feeding trials and the self-absorption of thebeta emittance of ³H, were checked and corrected for the calculationof the filtering rates. It was suggested that Daphnia collectsbacteria efficiently, although the efficiency is somewhat variabledepending on food particle composition (i.e. presence and absenceof larger particles) and feeding condition (i.e. animal densityand physical disturbance). By contrast, copepodites of Eodiaptomuswere suggested to be less efficient bacteria feeders. Food resourceexploitation strategies of these two co-existing zooplanktersare discussed.