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Chlorophyll a concentration [ρ(Chla)] in phytoplankton is used to understand algal biomass and the primary productivity of a water body. To investigate the seasonal dynamics of [ρ(Chla)] and their relationship with environmental factors Tiegang Reservoir at Guangdong China, continuous observation and analysis was carried out from April 2013 to June 2014. The distribution of algae had an obvious spatial heterogeneity, and the ρ(Chla) gradual increased from the southeast to the northwest of the reservoir. From April to early December 2013 and from late April to June 2014, the dominant algae was cyanobacteria. In other months, bacillariophyta were dominant. Chlorophyta were at a low level throughout the year. Multivariate analysis showed that water temperature (WT) was the primary driving factor of the spatio-temporal distribution of algae. The main environmental factors of cyanobacteria, bacillariophyta and chlorophyta were greatly different. Single environmental factors such as WT, total organic carbon (TOC), nitrate nitrogen (NO3--N), and chemical oxygen demand (COD) were significantly related to the competitiveness of cyanobacteria and bacillariophyta. And they can be used as early warning indexs for cyanobacteria and diatom blooms. The dual synergistic effects of WT and other environmental factors on the competitiveness of cyanobacteria and bacillariophyta were analyzed. A combination of environmental factors at high risk for cyanobacteria or diatoms were found. [ABSTRACT FROM AUTHOR]