In situ determinations of bacterial selectivity and filtration rates by five cladoceran zooplankters in a hypertrophic subtropical reservoir

2Authors listed alphabetically Abstract. Cladoceran in situ feeding rates on natural bacteria labelled with [methyl-3H]thymidine were studied in parallel with feeding rate determinations on "C-labelled Chlortila in a hypertrophic "subtropical reservoir (Lake Hartbeespoort) through spring and summer (1986787). Community filtration rates (CFRs) on bacteria and algae were similar, but selection for Chlortila (relative to natural bacteria) increased from midsummer in association with declining bacterial density and increasing dominance of 'inedible ' components of the natural phytoplankton. Species-specific filtration rates (SSFRs) were determined for Daphnia pulexllongispina, Ceriodaphnia reticulata, Diaphanosoma excisum, Bosmina longirostris and Moina micrura during their respective seasonal occurrence in the study period. SSFRs on algae and bacteria increased with body length (£,, mm) in all species apart from Bosmina. Species-specific differences in absolute feeding rate (FR, ml animal" ' day"1), the slope of the FR-L relationship and bacteria selectivity were evident. The feeding rate of all cladocerans on bacteria is described by the power equation FR = 5.231L ' 42. FR values on bacteria relative to FR values on algae averaged —60 % overall for all size classes o