| Phytoplankton community is closely related to the water quality and thus to be an important biological indicator for water quality assessment. Nansi Lake has been employed in the east route of the South-to-North Water Diversion Project for accommodating water storage and part of the water channel. Investigations of phytoplankton community are an important aspect in studying water quality, preventing and controlling eutrophication of lakes, and ensuring diversion water quality. This paper focused on the phytoplankton community characteristics and the assessment of water trophic status in Nansi Lake. The main researches included as follows:The composition characteristics and seasonal variations of phytoplankton in Nansi Lake were studied from June 2008 to May 2009.8 phyla,59 genera and 117 species have been identified from 5 monitoring stations based on a monthly sampling. The dominated Chlorophyta has 57 species, accounting for 48.7% of the total phytoplankton species, and Bacillariophyta has 28 species, accounting for 23.9% of the total phytoplankton speices. There were much more species in summer and autumn than that in winter and spring. The phytoplankton densities and chlorophyll a concentrations of each sampling station ranged from 5×104 to 5500×104 cells/L, and 2.14 to 158.36mg/m3, respectively. The quantity of phytoplankton in the order from large to small was:summer>autumn>spring>winter. The predominant indices were not high, ranging from 0.04 to 0.35, and there were 23 predominant species including Chlorella vulgaris, Scenedesmus dimorphus, Aulacoseira granulate, etc. The phytoplankton diversity and evenness indices varied from 1.56 to 2.36, and 0.59 to 0.84, respectively. It can be concluded that the phytoplankton community structure in Nansi Lake was complex, and the stability of the phytoplankton community composition and quantitative distribution showed a high degree in Nansi Lake.The species composition of cyanobacteria community revealed 10 genera,19 species, which accounted for 16.2% of the total number of phytoplankton species observed in Nansi Lake. The cyanobacteria densities of each monitoring station ranged from 0 to 1120×104 cells/L, with an average of 92.9×104 cells/L. The cyanobacteria species number and cell densities had the same variation trends during the year, with higher values in summer and autumn and lower values in spring and winter. The dominant species of cyanobaceria community were the species of Merismopedia, not the bloom-forming algae such as Microcystis, Anabaena, etc. The dominant species of phytoplankton community were not the species of cyanobacteria causing water bloom, and environmental conditions such as climate, hydrology were not suitable for the excessive propagation of cyanobacteria, which are the important reasons for that the water bloom has never occurred in Nansi Lake.The trophic state of Nansi Lake was investigated using the methods of modified trophic state index (TSIM), index defined by the "Evaluation of surface water quality technical specification (SL395-2007)" and biological indicators. According to the majority of the investigated parameters and indices derived from water quality and phytoplankton, Nansi Lake was eutrophic. Most of the detected phytoplankton speices were common genus in freshwater lakes and some species indicated eutrophication.The construction of the east route of South-to-North Water Diversion Project promoted the environmental protection in Nansi Lake watershed and the lake water quality has been meliorated significantly. To prevent water bloom occurring in Nansi Lake and achieve the required water quality for water diversion, it is still need to improve water environmental monitoring, control, emergency system and implement the water pollution control measures such as constructing the wetlands system, wastewater reuse, non point source pollution control, etc.
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