Zooplankton feeding on size fractionated Microcystis colonies and Chlorella in a hypertrophic lake (Hartbeespoort Dam, South Africa): implications to resource utilization and zooplankton succession

Abstract

Experiments measuring zooplankton filtration rates on radiolabelled Chlorella and on Microcystis colonies were carried out in situ during both the annual spring Daphnia phase and following the shift to the summer Ceriodaphnia phase in Hartbeespoort Dam. Cladoceran filtration rates on natural Microcystis of discrete colony size-classes (5–20, 20–40, 40–60 and 60–100 μm) were compared with rates measured on cultured Chlorella (∼ 8.5 μm cell diameter). The small bodied cladoceran species Ceriodaphnia reticulata, Moina micrura and Diaphanosoma exasum filtered Microcystis colonies at very low rates compared to Chlorella . Only the largest herbivore Daphnia pulex fed significantly on Microcystis colonies of up to 60–100 μm. Colonies of 100–150 μm were not ingested by any zooplankter tested. Both particle size limitation and low filtration rates on small Microcystis colonies and unicells led to low Daphnia filtration efficiencies and reduced grazing pressure on Microcystis . Suppression of Daphnia filtration rates in situ on Chlorella was evident during the mid-summer increase in Microcystis abundance. Daphnia filtration rates during this period when inedible phytoplankton were abundant were only 25–36% of the maximum filtration, rates measured during the spring clear water phase when the phytoplankton was largely composed of edible species. The aforementioned small cladoceran species exhibited high filtration rates on Chlorella when Microcystis was abundant, suggesting that filtration efficiency of the summer cladoceran community was not reduced by interference from Microcystis colonies. However, their low filtration rate on Microcystis suggests that this is not an important food resource. Concomitant reductions in bacterial numbers and edible phytoplankton food resources available in summer imply that the small body-sized cladoceran community rather relies on the smallest size fraction of the summer food resource.