Variation of phytoplankton communities and their driving factors along a disturbed temperate river-to-sea ecosystem

Phytoplankton, as an important primary producer in rivers, greatly affects an aquatic ecosystem's function and biodiversity. However, the effect of human activities on phytoplankton community composition remains largely unstudied in rivers. Herein a typical temperate river, Xiaoqing River in the north of China, was the target to investigate the phytoplankton distribution and regime along stretches subject to different human disturbances. Other than in the wetland located in a tributary of the river, which had Cyanophyta as the dominant phylum, Bacillariophyta overwhelmingly predominated across the sampling sites. Notably, the main genus was Fragilaria in the source region of the river, while Cyclotella dominated along the trunk of the river, and the main species shifted from Cyclotella gamma Sovereign to Cyclotella meneghiniana Kutzing when the river entered the sea. Phytoplankton community structures in the source region were affected to some extent by DO and pH, and in the human activity areas were mainly influenced by nitrogen, phosphorus, and CODMn. Human interference led to a decrease in diatom density by increasing the concentration of nutrients and organic matter. Furthermore, algal diversity was higher and density was lower in the disturbed areas than in the catchment areas, in accordance with the intermediate disturbance hypothesis. It was also found that human activities more obviously affected the abundant than rare phytoplankton communities.