Zooplankton community grazing in a hypertrophic lake (Hartbeespoort Dam, South Africa)

Seasonal abundance as total biomass and specific densities of the main herbivorous zooplankton (>60 μm) in hypertrophic Hartbeespoort Dam from 1981 to 1983 are described. After spring zooplankton biomass maxima each year there followed a mid-summer decline in the Daphnia population and a shift to a smaller bodied cladoceran community dominated by Ceriodaphnia concomitant with a change from largely edible phytoplankton species to abundant colonial Microcystis. In situ community grazing rates were measured throughout 1983 using a 14 C-labelled unicellular alga. Integrated community grazing rates measured in the aerobic water column were highest in December (260.2%/day) when Daphnia was abundant and the edible component of the phytoplankton was diminishing. Lowest integrated community grazing rates occurred in January–February (19.8–35.3%/day) and July (28.3%/day) when the phytoplankton was composed almost entirely of Microcysris , and Ceriodaphnia dominated the zooplankton community whilst food availability was low. Feeding in Ceriodaphnia was not hindered by abundant large Microcysris colonies; total biomass specific grazing rate was high when Ceriodaphnia dominated and low when Daphnia dominated the zoo-plankton community. Results indicate that in hypertrophic conditions it is unlikely that large filter-feeders such as Daphnia are able to retard or limit the development of Cyanophyceae blooms by high grazing pressure.