Seasonal changes in densities of cyanophage infectious to Microcystis aeruginosa in a hypereutrophic pond

Seasonal changes in densities of cyanophages infectious to Microcystis aeruginosa were studied in a hypereutrophic pond from March 1997 to January 1998 to elucidate the potential impact of the cyanophage on M. aeruginosa mortality. Densities of M. aeruginosa ranged between 1.8 x 10(4) and 9.4 x 10(5) cells ml(-1), while those of the cyanophages were between 2.0 x 10(2) and 4.2 x 10(4) PFU ml(-1). Sharp decreases in densities of M. aeruginosa were detected on 10 June and 24 September, as densities of the cyanophages increased, suggesting release of the cyanophages due to the lysis of infected M. aeruginosa. Thus, infection by cyanophages may have a substantial effect on cyanobacterial succession in the pond. Densities of cyanophages became undetectable when those of M. aeruginosa were at low levels during winter. We suggest that there is a tight host-pathogen relationship between M. aeruginosa and the cyanophage in the pond.