Abstract: To understand the phytoplankton community structure and its relation to the environmental conditions, two surveys were conducted in the Anning River and its tributaries in July and August 2015 (wet season), and January and February 2016 (dry season). Sampling was also performed monthly from July 2015 to June 2016 in the tributaries of the Anning River. A total of 95 species of phytoplankton belonging to 53 genera of 7 phyla were identified, with Bacillariophyta (64 species) as dominant taxa. There was no significant difference in phytoplankton biomass between the mainstream and the tributaries during the wet season. The density and biomass of phytoplankton in the mainstream were significantly higher than those in the tributaries during the dry season. Across the whole year, the dominant species in the tributaries were Achnanthes sp.,Cymbella ventricosa, Cymbella sp. andCocconeis placentula. The density and biomass of phytoplankton were the highest in March and the lowest in January. The phytoplankton density was significantly correlated to pH and altitude in the wet season, while it was correlated with the channel width and pH in the dry season. In addition, monthly variations of phytoplankton structure in the tributaries were correlated to pH, conductivity and water temperature. Dissolved oxygen was not associated with phytoplankton growth. This study provides a scientific basis for river biodiversity conservation in high altitude areas.

Key words: Bacillariophyta, diversity, redundancy analysis, annual variation.