Abstract: To investigate the distribution of phytoplankton as well as their relationship with environmental factors, water samples from different layers were collected in the river and center regions of Xin’anjiang Reservoir. Temporal and vertical variations of phytoplankton were detected using microscopy and Illumina sequencing technique based on eDNA. The results showed that the phytoplankton in the reservoir were mainly composed of Bacillariophyta, Chlorophyta, Cryptota, and Cyanobacteria. The dominant genera at Jiekou section site in the river area of the reservoir were Cyanobium (8.3%-37.5%) and Synechococcus (7.6%-30.3%) in winter, while Cyclotella (29.3%) and Cryptomonas (16.2%) were dominant in spring. The dominant genera of the section in front of dam in winter and spring were Cyanobium (16.0%-60.6%) and Synechococcus (7.8%-33.0%). At different depths, the relative abundance of Chlorella at 5 m depth was 15.2%, which was significantly lower than that in other water layers. The relative abundance of phytoplankton Cyanobium at the 5 m depth (50.7%) was significantly higher than that in the other two layers (25.3% and 43.2%, respectively). Chlorophyll a, water temperature, chemical oxygen demand (COD_Mn) and total nitrogen were the environmental factors affecting the composition and distribution of planktonic communities. Our findings suggested that eDNA-based technique has great potential in analyzing phytoplankton biodiversity in reservoirs, including prokaryotic algae and eukaryotic algae.

Key words: Xin’anjiang Reservoir, phytoplankton, community structure, eDNA