2018 Volume 79 Issue 2 Pages 109-117
Harmful algal blooms of certain nuisance cyanobacteria are observed almost every year in Lake Oshima-ohnuma, located in Hokkaido, Japan. However, an unusual milky-white bloom was observed in September 2015. The causative organism of this milky white bloom was an algae-lysing protist, Asterocaelum sp., feeding on Dolichospermum planctonicum, which caused the cyanobacterial bloom. The bloom of the cyanobacterium D. planctonicum was recognized (8.6 × 103 cells mL-1) before the milky-white bloom and the number of Microcystis aeruginosa, a cyanobacterium, increased (1.2 × 104 cells mL-1) after the milky-white bloom. Microscopic observation revealed seasonal fluctuations in the cell numbers of the algae-lysing protist and cyanobacteria. Quantification in this study confirmed the hitherto-known information that members of the genus Asterocaelum feed on D. planctonicum and cause the milky-white bloom. The observation on the number of Asterocaelum sp. as well as that of cyanobacteria is newly proposed to understand the mechanism of harmful algal blooms in freshwater ecosystems better.
