Vertical distribution of Microcystis colony size in Lake Taihu: Its role in algal blooms

This study investigated the vertical distributions of Microcystis cell density and colony size in Lake Taihu where algal blooms occur frequently. Measurements were made from April 2011 to January 2012 to gain a seasonal outlook on the role of such distributions in the blooms. It was found that large colonies tended to accumulate on the water surface, but the cell density fluctuated widely. The cell density in the water column increased continuously from spring to summer (i.e., April to October) and decreased after late autumn, showing apparent seasonal variations. The abrupt occurrence and disappearance of Microcystis blooms over short periods of time were not caused by the rapid growth of Microcystis but by the rise and accumulation of large Microcystis colonies at the water surface, both of which are affected by colony size. The ascent velocity of large colonies was higher than that of small ones, which enables large colonies to more readily overcome the stirring effects of water flows, waves and perturbations to rise to the surface. The results of canonical correspondence analysis (CCA) of Microcystis vertical distribution in relation to environmental factors suggested that nutrient concentrations and temperature were the main influencing factors related to bloom formation by Microcystis in Lake Taihu during our investigation.