2009 Volume 4 Issue 3 Pages 95-103
The initial stage of bloom formation of cyanobacteria was investigated in Hirosawa-no-ike Pond, Kyoto, Japan, from February to May 2006. This pond is prone to periodic heavey blooms of cyanobacteria from spring to autumn, despite the complete draining of water for more than two months every winter. The dominant types of cyanobacteria in the bloom were Aphanizomenon flos-aquae from February to March, followed by Microcystis spp. in April. The planktonic populations of Microcystis spp. increased continuously during April when the water temperature was in the range of 12.0–17.3°C, which was almost below the growth threshold for most Microcystis species. An incubation experiment revealed that the recruitment of Microcystis colonies from the pond sediment increased with temperature, especially at and above 15°C. Recruitment of Anabaena trichomes was also observed, but there was no clear relationship between water temperature and trichome densities of Anabaena spp. The M. aeruginosa colonies in the sediment in winter were likely to be dormant, as implied by the invariable frequencies of the division of cells throughout a day. These results suggest that once water temperature exceeded the growth threshold of Microcystis species, the recruitment of the colonies is enhanced and a bloom is formed by both conspicuous recruitment and active growth.