Spatial and Temporal Variations in Phytoplankton Community in Dianchi Lake Using eDNA Metabarcoding

The growth and reproduction of phytoplankton are closely associated with the changes of water environment; thus, phytoplankton have been taken as environmental indicator organisms and provided references for water environment protection. However, the phytoplankton community characteristics of Dianchi Lake (a seriously polluted lake in China) are unclear under the background of the cumulative effects of historical pollutants and current control measures, and environmental DNA (eDNA) metabarcoding monitoring has rarely been applied in phytoplankton research at Dianchi Lake. Therefore, this study investigated the temporal and spatial characteristics of phytoplankton community and the environmental stressors of Dianchi Lake via eDNA metabarcoding monitoring. A total of 10 phyla, 22 classes, 50 orders, 82 families, 108 genera and 108 species of phytoplankton were detected, and distinct temporal and spatial variations in the phytoplankton community (e.g., ASV number, dominant taxon, the relative abundance) were observed in Dianchi Lake. Microcystis dominated the prokaryotic phytoplankton community from the dry period to the wet period, but interestingly, the first dominant cyanobacteria genus was changed from Microcystis (dry period) to Planktothrix (wet period). Cryptophyta dominated in the eukaryotic phytoplankton community from the dry period to the wet period, and eukaryotic-phytoplankton-dominant genera included Cryptomonas, Aulacoseira, Plagioselmis and others. A temporal–spatial heterogeneity of the relationships between the phytoplankton community and environmental factors was shown in Dianchi Lake. Dissolved oxygen was the crucial environmental stressor influencing the phytoplankton community structure in Dianchi Lake during the dry period, while pH was the crucial one during the wet period. The impacts of total phosphorus and nitrogen also showed differences at different periods. This research provides an interesting perspective on phytoplankton diversity monitoring and the health assessment and restoration of Dianchi Lake.