INSITU DETERMINATIONS OF BACTERIAL SELECTIVITY AND FILTRATION-RATES BY 5 CLADOCERAN ZOOPLANKTERS IN A HYPERTROPHIC SUBTROPICAL RESERVOIR

Cladoceran in situ feeding rates on natural bacteria labelled with [methyl-H-3]thymidine were studied in parallel with feeding rate determinations on C-14-labelled Chlorella in a hypertrophic subtropical reservoir (Lake Hartbeespoort) through spring and summer (1986/87). Community filtration rates (CFRs) on bacteria and algae were similar, but selection for Chlorella (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 pulex/longispina, 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 (L, mm) in all species apart from Bosmina. Species-specific differences in absolute feeding rate (FR, ml animal-1 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 = 5.231L1.42. FR values on bacteria relative to FR values on algae averaged approximately 60% overall for all size classes of Daphnia, with ranges of 70-85, 58-79, 34-63 and 50-117% in various body length classes of Ceriodaphnia, Diaphanosoma, Bosmina and Moina, respectively. Size dependency was evident in certain taxa. Selectivity coefficients (SCs) were approximately 2 on average, indicating algal preferences in all taxa apart from Ceriodaphnia, whose SC was approximately 1 (unselective feeding). Food selectivity was independent of grazer size in Daphnia, Diaphanosoma and Bosmina, but algal preference increased with body size in Ceriodaphnia and particularly Moina. Estimates indicate that bacteria contribute relatively little (less-than-or-equal-to 3%) of the total dietary carbon intake of cladoceran zooplankters in Lake Hartbeespoort, thus challenging earlier views that bacteria are major dietary components of the summer grazer community in eutrophic waters. The findings are discussed in relation to other recent studies of freshwater zooplankton feeding ecology.