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., 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^-1, whereas those of algicidal bacteria were between 0.3 x 12 to 3.7 x 10² PFU ml^-1 and cyanophages were between 0.9 x 10³ to7.1 x 10³ PFU ml^-1. Algicidal bacteria were relatively high with fluctuation between0.3 x 10⁴ to 3.7 x 10⁴ PFU ml^-1 and bacterial peaks were followed with M. aeruginosa peaks. Occational 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^-1) from 10 to 17 November with decrease of M. aeruginosa cell density. The percentage of  B. caliciflorous individuals ingesting colonies of  M. aeruginosa during the bloom period was significantly high (t-test, P Percentage of testate amoebae Penardochlamys sp. ingesting  M. aeruginosa cells was very high (˃77%) during the bloom period. The food vacuoles of  Penardochlamys sp. contained only Microcysis indicating its specific preference of  Microcysis. 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.

Key words : Microcysis aeruginosa; cyanophages; algicidal bacteria; Penardochlamys sp.; Cephalodella sp.; Brachionus caliciflorous

Vidyodaya Journal of Science Vol.14(2) 85-101