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Because of warm climte, cyanobacteria benefit from high nutrient concentrations and easily bloom in waters in Guangdong Province located at the north edge of tropic region. A huge number of small and middle-sized reservoirs function as main water source supplying for domestic use. Due to small storage capacity, however, these reservoirs have a limited self-purification ability, and they are sensitive to external loadings of nutrients and show an increasing eutrophication. Reservoir eutrophication and the following water blooms of toxic cyanobacterial have directly challenged the safety of supply water for drinking. Regarding the regional climate and hydrological conditions of Guangdong Province, the present thesis investigated eutrophication, dynamics of cyanobacterial population and distribution of microcystin in ten selected small and medium sized reservoirs. The study aims to provide useful data to support water quality management of the small or middle size reservoirs which mostly are near or in towns and countryside.Among the ten reservoirs, two are oligo-mesotrophic, two are mesotrophic, three are meso-eutrophic, and the others are eutrophic. The reservoirs located in mountain or hill regions are less polluted by agricultural, industrial and domestic sewages, the trophic state index (Carson index) is low, while those near cities and countryside have a high tophic state index with an increasing trend due to the strong impacts of high population, industry and agriculture.In total, we detected 121 phytoplankton species in the ten reservoirs, including 26 species in Cyanobacteria,71 species in Chlorophyta,14 species in Bacillariophyta, 3 species in Dinophyta, one species in Chrysophyta,5 species in Euglenophyta and one species in Xanthophyta. The common cyanobacterial species included Dactylococcopsis acicularis, Chroococcus sp., Pseudanabaena limnetica, Limnothrix sp., Merismopedia tenuissima, Merismopedia minima, Microcystis aeruginosa, Microcystis flos-aquae. Except Cangcun reservoir, Chishijing reservoir and Qiyeshi reservoir, the phytoplankton in the other seven reservoirs were dominated by cyanobacteria. In Cangcun reservoir and Chishijing reservoir, the phytoplankton were dominated by bacillariophyta and dinophyta, the seasonal variation of phytoplankton was regulated by the low concentration of nutrients and hydrodynamic stability condition of the water bodies. In Qi-ye-shi reservoir, the phytoplankton was dominated by Chlorophyta, and regulated mainly by water temperature and water inflow from Dongjiang river in December. Muxi reservoir, Henggang reservoir, Shuilianshan reservoir, Dongfeng reservoir, Gangcun reservoir and Heliu reservoir are located near cities and countryside, cyanobacteria was dominant in phytoplankton community, its variation in abundance was contributed mainly by Pseudanabaena limnetic in different seasons.The abundance dynamics of cyanobacteria was mainly explained by the light limitation in August, the high phosphorus limitation in March, the nitrogen and phosphorus limitation in December. The Minghu reservoir is a diversion reservoir, Anabaena circinalis was the dominant species, and the phytoplankton abundance was reduced by the loss with water supply.Microcystin was detected to be higher than diction limit in seven reservoirs. The concentration of microcystin was higher than the safe standards of drinking water in Minghu reservoir and Heliu reservoir. Microcystis sp. and Anabaena circinalis were suggested the toxin-producing stains.
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