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

Seasonal changes in densities of cyanophages infectious to Microcystis aeru ginosa were studied in a hypereutrophic pond from March 1997 to January 199 8 to elucidate the potential impact of the cyanophage on M. aeruginosa mort ality. 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 incr eased, 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 u ndetectable when those of M. aeruginosa were at low levels during winter. W e suggest that there is a tight host-pathogen relationship between M. aerug inosa and the cyanophage in the pond.