摘要：Variation of Microcystis spp. (Cyanobacteria) colony size and photosynthetic quantum yield were examined during a summer bloom in a large shallow lake (Lake Taihu). After exposure to direct solar radiation for four hours, about 60% of the colonies larger than 36 μm had changed into smaller colonies and single cells. Meanwhile, the photosynthetic quantum yields of Microcystis spp. decreased rapidly. Under a treatment of 30% of the light intensity ofthat at the surface, only about 10% of the large colonies changed into smaller colonies and single cells, and the photosynthetic quantum yields showed only a slight decrease. When the samples were incubated at the surface layer and 0.5 m depth from 0800 h to 1800 h, the number of small colonies and single cells increased more markedly than that at 1.0 m depth. During this incubation, the photosynthetic quantum yields of the Microcystis spp. at the surface water were significantly lower than that at the 0.5 m and 1.0 m depths from 1000 h to 1800 h. Direct exposure to excess light severely reduced the photosynthetic quantum efficiency of the Microcystis spp. cells but also induced large Microcystis spp. colonies to change into smaller ones and single cells.
