Seasonal changes in the abundance of biological agents killing Microcystis aeruginosa in a hypereutrophic pond

Seasonal changes in abundance of the heterotrophic flagellates Polytomella sp., rotifersCephalodella sp., Brachionus caliciflorous and the testate amoeba Penardochlamyssp. as grazers, cyanophages and algicidal bacteria as infectious agents on mortality ofcyanobacterium, Microcysis aeruginosa was intensively studied in a hypereutrophic pond fromSeptember to November 2000. Abundance of the rotifers Cephalodella sp., B. caliciflorousand the Penardochlamys sp. were relatively high with large fluctuations. The cell density of M.aeruginosa ranged between 3.0 x 105 to 1.9 x 107 cells ml-1, whereas those of algicidal bacteriawere between 0.3 x 12 to 3.7 x 102 PFU ml-1 and cyanophages were between 0.9 x 103 to7.1x 103 PFU ml-1. Algicidal bacteria were relatively high with fluctuation between0.3 x 104 to3.7 x 104 PFU ml-1 and bacterial peaks were followed with M. aeruginosa peaks. Occationalcyanophages peaks on 2 and 20 October were followed by sudden collapse of M.aeruginosabloom. Percentages of M. aeruginosa cells in food vacuole of Penardochlamys sp.was high(77%) during September following increase of the M. aeruginosa cell density. The flagellatePolytomella sp. was increased to a peak (2.5 x 105 indi. M-1) from 10 to 17 November withdecrease of M. aeruginosa cell density. The percentage of B. caliciflorous individuals ingestingcolonies of M. aeruginosa during the bloom period was significantly high (t-test, P Percentageof testate amoebae Penardochlamys sp. ingesting M. aeruginosa cells was very high (?77%)during the bloom period. The food vacuoles of Penardochlamys sp. contained only Microcysisindicating its specific preference of Microcysis. The overall results of the study suggest thegrazers’ protozoa; Polytomella sp., and the testate amoeba Penardochlamys sp., zooplankton;Cephalodella sp. and Brachionus caliciflorous, cyanophages and algicidal bacteria successivelyinvolved in suppressing M. aeruginosa bloom in a freshwater environment.Key words : Microcysis aeruginosa, cyanophages, algicidal bacteria, Penardochlamys sp.,Cephalodella sp. and Brachionus caliciflorou