Abstract: In order to understand the ecological integrity and diversity in the old course of Yellow River, we investigated the phytoplankton community in the old course of Yellow River at Anhui Section in 2018. A total of 94 phytoplankton species, which belonged to 7 phyla and 46 genera, were recorded. Chlorophyta species (20 genera, 51 species) and Bacillariophyta species (11 genera, 17 species) accounted for 54.26% and 18.09% of the total species number, respectively. The abundance and biomass of phytoplankton were in the range of 0.507×10⁵-8.406×10⁵ cell·L^-1 and 0.039-0.926 mg·L^-1, with average values of 2.875×10⁵ cell·L^-1 and 0.254 mg·L^-1, respectively. Cyanophyta and Chlorophyta dominated the abundance of phytoplankton, while Chlorophyta and Dinophyta dominated the biomass. Biological evaluation of water quality indicated that the water of the old course of Yellow River at Anhui Section was at β secondary to light pollution level. Results of similarities and non-metric multidimensional scaling analysis showed that phytoplankton community structure significantly differed among sampling seasons (r=0.625,P<0.05). The results of redundancy analysis showed the environmental factors affecting phytoplankton community composition and distribution varied across different seasons, and ammonium nitrogen, water temperature, water depth, and dissolved oxygen were the major environmental factors affecting phytoplankton community composition. Our results could provide basis for water resources protection and ecological restoration in the old course of Yellow River.

Key words: the old course of Yellow River at Anhui Section, phytoplankton, environmental factor, redundancy analysis.