Abstract: Current research on plankton in the Lancang River is mostly limited to phytoplankton;discussion from the perspective of the whole eukaryotic community is in lack.To fill the blank in the research area,the eukaryotic plankton diversity and its response to environmental factors were explored by using 18S rRNA metabarcoding technology based on water samples collected from 36 locations covering an area of nearly 2 000 kilometers of Lancang River in October 2020.Overall,1 269 OTUs were obtained from the samples;the eukaryotic plankton species detected in these samples cover 45 phyla and 284 genera,reflecting the majority of the diversity.Based on NMDS and ANOSIM analysis,the eukaryotic community can be divided into three spatial groups:the natural channel in the upstream area,the midstream area between Wunonglong hydropower station and Gongguoqiao hydropower station,and the downstream area of Gongguoqiao hydropower station.According to the diversity and community composition data,the upstream area sees the most evenly distributed eukaryotic community,with Chrysophyte and Bacillariophyta broadly distributed;the midstream and the downstream features similar diversity,both lower than that in the upstream,with Cryptophyta most broadly distributed.Water temperature and alkalinity are dominant environmental factors that affect the structural change of eukaryotic plankton community in the Lancang River.Driving factors vary among the upstream,midstream,and downstream:electrical conductivity is the key environmental factor affecting the community structure of eukaryotic plankton in the upstream area,while water temperature in the midstream,and total phosphorus concentration in the downstream.

Key words: Lancang River, eukaryotic plankton, 18S rRNA, spatial distribution, environmental factors