• Chydorus;
  • ciliates;
  • cyanobacterial bloom;
  • microbial loop;
  • small-bodied zooplankton

SUMMARY 1. We studied the effect of the small crustacean zooplankton on heterotrophic micro-organisms and edible phytoplankton in a eutrophic lake during a cyanobacterial bloom.

2. Small (15 L) enclosures were filled with natural or screened (100 μm) lake water and incubated for 5 days in the lake. Screening removed crustacean zooplankton but the initial density of rotifers and phytoplankton remained the same in control and removal treatments. Changes in the abundance and biomass of bacteria, autotrophic picoplankton (APP), heterotrophic nanoflagellates (HNF) and ciliates were measured daily.

3. The crustacean zooplankton, dominated by the small cladoceran Chydorus sphaericus, did not affect cyanobacteria, the main phytoplankton group during the experiment.

4. The removal of the crustacean zooplankton induced a higher abundance of ciliates and reduced that of the HNF, indicating the importance of ciliates in controlling HNF in this system.