Abstract: In June-August 2008, the cyanobacterial bloom and chlorophycean bloom broke out
 one after the other in the Xiangxi Bay of Three Gorges Reservoir. Based on the
weekly monitoring in summer in the Bay, and by using cluster analysis a
nd discriminant analysis, the spatiotemporal dynamics and related affecting fact
ors of the two blooms were studied. Each of the blooms could be divided into non
-bloom group, transitional group, and bloom group. The two blooms had different
uptake levels of dissolved Si (DSi), NO3--N+NO2--N, and PO43-
-P. Cyanobacterial bloom had lower DSi concentration and lower TN/TP, DSi/TN an
d DSi/TP ratios than chlorophycean bloom. The discriminant factors of cyanobacte
rial bloom were Chl a, TN and PO43--P, while those of chlorophycean blo
om were Chl a and DSi. Better discriminant results were obtained when dividing e
ach bloom into non-bloom group and bloom group. The critical value of Chl a for
 cyanobacterial bloom and chlorophycean bloom was 40 and 20 μg·
L-1, respectively.

Key words: cyanobacterial bloom, chlorophycean bloom, cluster analysis, discriminant analysis, Three Gorges Reservoir, Xiangxi Bay, Dactylis glomerata, soil relative moisture content, drought stress, photosynthesis, root growth.