Abstract:

Community structure of the phytoplankton in the mainstream of the Yangtze River in Hubei was investigated during May to June, 2014. A total of 57 sampling sites were laid out between the Sanjiang Bridge of Yichang and Xinzhou of Huangmei for sampling of phytoplankton. Results show that 11 genera of 4 families of Cyanophyta, 17 genera of 8 families of Chlorophyta and 140 species 30 genera of 10 families of Bacillariophyta were gathered and identified. Bacillariophyta, dominated with Cyclotella, was the most abundant in the phytoplankton community and followed by Cryptophyta, Cyanophyta and Chlorophyta. With the water flowing from upstream to downstream, the proportion of Bacillariophyta reduced gradually, while the proportion of Cryptophyta increased and the proportions of Cyanophyta and Chlorophyta are remained almost unchanged. The average cell density was the highest at the sampling sites between the estuary of the Songzi River and the Zhongzhou River bend (Section B) (about 7.4×10⁶  L^-1), lower between Shanjiangkou and the Zhicheng Bridge (Section A) (3.0×10⁶ L^-1) and the lowest between the Dongting Lake Bridge and Xinzhou (Section C). Composition of the phytoplankton varied significantly with velocity of the flow. In the water moderate in flowing velocity, Bacillariophyta dominated, accounting for as high as 70.0%, in the water low in flowing velocity, Cryptophyta and Bacillariophyta did, being quite close in proportion, 47.2% and 37.5%, respectively, while in the water, rapid in flowing velocity, Cryptophyta became the dominant one. Taking into account comprehensively the cell density，the ShannonWeaver biodiversity index and the Pielou evenness index of the phytoplankton, it can be concluded that this section of the Yangtze River is poorly or moderately eutrophied in water quality.

Key words: the mainstream of Yangtze River, Hubei Section, phytoplankton, cell density, diversity index