2010 年 33 巻 9 号 p. 123-129
We carried out research in Lake Suwa from 2004 to 2008 to study how retention time affects the biomass and species composition of blue-green algae. The retention time of the lake ranged from 2.3 (rainy season in July 2006) to 120 (dry season in June 2005) days. Microcystis was predominant in summer, but Aphanizomenon was predominant in 2005 and 2006. Owing to nitrogen deficiency, the Microcystis population increase was suppressed when retention time was kept at more than 80 days. The increase in the population of Aphanizomenon flos-aquae was suppressed probably owing to the dilution and discharge of cells by precipitation, and the change of the predominant species caused by nutrient inflow. In this paper, we suggest an appropriate retention time for respective blue-green algae on the basis of changes in the predominant species caused by changes in retention time.