Morphology, flow regimes, and filtering rates of Daphnia, Ceriodaphnia, and Bosmina fed natural bacteria

Body size is the best overall indicator of the abilities of the cladocerans Daphnia magna, D. parvula, Ceriodaphnia lacustris and Bosmina longirostris to filter natural bacteria (<1.0 μm). However, species differences exist which cannot be inferred from differences in size, behavior, or morphology alone. The relationship between filtering rate (FR in ml animal^-1h^-1) and body length (L in mm) for the cladocerans studied can be described by the power function: $$\begin{gathered} FR = 0.538 L^{1.545} \hfill \\ (r^2 = 0.88, F = 168.54, P < 0.001). \hfill \\ \end{gathered}$$ In D. parvula, algal filtering rates are higher and increase more rapidly with increasing body size than do bacterial filtering rates which are 26 to 33% of algal rates. This suggests that different processes may be involved in the capture of these ultrafine particles and that ultrafine particle capture efficiency decreases with increasing body size within a species. Weight specific filtering rates (in μl μg dry wt^-1h^-1) have a strong negative relationship to body size and show species specific differences. Appendage beat rates intersetular distances, setule diameter, appendage, area, % open space on the filtering appendage, Reynolds number, and boundary layer thickness do not provide simple predictions of bacterial filtering rates for the cladocerans studied. Filtering rates on cultured laboratory bacteria and algae may not indicate filtering rates on natural bacterioplankton because of differences in bacterial size, motility, and surface properties. Uptake of ultrafine particles may be enhanced by the presence of larger, more readily filtered particles through a “piggybacking” phenomenon.