Abstract: Located in the Qinghai-Tibetan plateau, the headwaters of the Yangtze River features with specific water environment due to harsh weather conditions. Differences in the characteristics of phytoplankton in the headwaters and their interaction with environmental factors are still unclear. By comparing the characteristics of phytoplankton in the headwaters of the Yangtze River in 2012-2016, we revealed the relations between phytoplankton characteristics and thirteen environmental factors through principal component analysis (PCA) and canonical correspondence analysis (CCA). Our findings unveiled that Cryptophyta only appeared in Dangqu. The southern source of the Yangtze River boasted the largest number of average phytoplankton species (18 species), and the northern source the smallest, only 9 species. In terms of average density of phytoplankton, however, the northern source had the largest, up to 37.70×10⁴ ind/L, while the southern source the least, 28.90×10⁴ ind/L. The average biodiversity index of phytoplankton in the southern source of the Yangtze River was the highest, reaching 3.04, while that of the northern source was the lowest, merely 1.84. In general, the headwaters of the Yangtze River had a relatively low density of phytoplankton, indicating that the water bodies in the headwaters were all in an oligotrophic state. PCA result of various environmental factors showed that the main components of the first axis (71.23%) could be interpreted as total nitrogen and electrical conductivity, which mainly represented the characteristics of river nutrients, while the main components of the second axis (28.77%) were interpreted as altitude, median particle diameter of bed sand, water temperature, sand content and chemical oxygen demand, mainly representing river habitat conditions. In the meantime, the results of CCA illustrated positive relations between the number of Cyanophyta species and total nitrogen, between the number of Cryptophyta species and altitude and median sand diameter, between the number of Chlorophyta species and altitude, as well as between chemical oxygen demand and median sand diameter. The number of Cryptophyta species and Chlorophyta species stayed in negative relation with sand concentration and water temperature.

Key words: headwaters of the Yangtze River, difference in characteristics of phytoplankton community, environmental factors, principal component analysis, canonical correspondence analysis