Seasonal Changes in the Abundance of Biological Agents Killing Microcystis aeruginosa in a Hypereutrophic Pond

Abstract

Seasonal changes in abundance of the heterotrophic flagellates Polytomella sp., rotifers Cephalodella sp., Brachionus caliciflorous and the testate amoeba Penardochlamys sp. as grazers, cyanophages and algicidal bacteria as infectious agents on mortality of cyanobacterium, Microcysis aeruginosa was intensively studied in a hypereutrophic pond from September to November 2000. Abundance of the rotifers Cephalodella sp., B. caliciflorous and 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', where those of algicidal bacteria were between 0.3 x 12 to 3.7 x 102 PFU ml' and cyanohages were between 0.9 x 103 to 7.1 X 103 PFU ml". Algicidal bacteria were relatively high with fluctuation between 0.3 x 104 to 3.7 X 104 PFU mr' and bacterial peaks were followed with M. aeruginosa peaks. Occasional cyanophages peaks on 2 and 20 October were followed by sudden collapse of M. aeruginosa bloom. 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 flagellate Polytomella sp. was increased to a peak (2.5 x 105 indi. m') from 10 to 17 November with decrease of M. aeruginosa cell density. The percentage of B. caluciflorous individuals ingesting colonies of M. aeruginosa during the bloom period Mal/age was significantly high CHest, P<O.Ol). Percentage of testate amoebae Penardichlamys sp. ingested M. aeruginosa cells was very high (>77%) during the bloom period. The food vacuoles of Penardichlamys sp. contained only Microcystis indicating it's specific preference of Microcystis. The overall results of the study suggest the grazers' protozoa; Polytomella sp., and the testate amoeba Penardochlamys sp., zooplankton; Cephalodella sp. and Brachionus caliciflorous, cyanophages and algicidal bacteria successively involved in suppressing M. aeruginosa bloom in a freshwater environment.