Effects of Light and Wind Speed on the Vertical Distribution of Microcystis aeruginosa Colonies of Different Sizes during a Summer Bloom

Different dynamics, of the vertical distribution of Microcystis aeruginosa colonies of different sizes, were examined in a large shallow lake (Taihu Lake) during a summer bloom. During the observation the Microcystis colonies, larger than 120 mu m, were mainly concentrated in the upper layer both under calm and windy conditions. Colonies from 36 mu m to 120 mu m showed a clear relationship to wind conditions whereas colonies less than 36 mu m showed a nearly uniform vertical distribution at the different depths. There was no clear relationship between the vertical distribution of colonies larger than 36 mu m and light intensity. However, at solar noon, colonies less than 36 mu m were mainly distributed in the middle layers of the water column. The results indicate that small colonies are easily affected by the wind-induced mixing, and they show an ability for active movement in response to light intensity under calm conditions. In contrast, large colonies tend to show little diurnal repositioning to light intensity and thereby are mainly concentrated in the surface layer.