Abstract: The Baishuica Reservoir and the Cangcun Reservoir,are two medium-sized reservoirs designated as sources of drinking water for residents in Shaoguan City,Guangdong Province.The reservoirs were sampled once every month throughout the year of 2011 for comprehensive evaluation of their quality using trophic status index(TSI),for classification of the phytoplanktons into functional groups,and for analysis of characteristics of the succession of dominant functional groups in the reservoirs throughout the year and their key environmental affecting factors.Results show that the Baishuica Reservoir is an oligotrophic aquatic system,while the Cangcun Reservoir an oligo-mesotrophic one.Throughout the year,45 genera of phytoplanktons under 7 phyla were collected and could be sorted into sixteen functional groups.In the Baishuica Reservoir,functional groups tolerant to low nutrient environments were in dominancy all the year around and only during the high water season could some eutrophication-tolerant functional groups be detected,while in the Cangcun Reservoir,eutrophication-tolerant functional groups were detectable all the year round and gained apparent dominancy during the high water season,but they were succeeded by the functional groups tolerant to low nutrient environments in dominancy during the low water and normal water seasons when nutrient contents were low.Variation of nutrient content in the water is the fundamental cause leading to succession of phytoplankton functional groups in dominancy,while changes in water environmental factors caused by changes in the rainy season are innegligible factors triggering the succession in the two reservoirs. 

Key words: eutrophication, phytoplankton, function group, reservoir, succession