Abstract: Jieshou Bay National Wetland Park is an important wetland park of the north Huaihe. Water samplings were carried out every two month from October of 2015 to August of 2016 at the 8 sampling sites(The main river has 5 sampling sites, and the ancient river has 3 sampling sites)．The results showed that a total of 172 species of phytoplankton were identified, which belonged to 8 phyla and 69 genera. Chlorophyta was the dominant group, followed by Bacillariophyta and Cyanophyta. The analysis of biodiversity index indicated that the complex and stability of phytoplankton community in main river was higher than ancient river. The water quality of main river was of slightly polluted, but slightly or moderately polluted in the ancient river, at the same time. The study suggested that the biological diversity of phytoplankton in Huaihe River is higher than that in its tributaries. The mainstream of Huaihe River belongs to eutrophic water body, and its tributaries belong to oligotrophic water body. The effects of climate on the species diversity of phytoplankton was reduction trend from the subtropical monsoon climate to the north warm temperate semi-humid monsoon climate and to the marine (semi-humid) temperate monsoon climate.

Key words: Jieshou Bay National Wetland Park, phytoplankton, community characteristics, mainstream of Huaihe River, regional climate