eDNA biomonitoring revealed the ecological effects of water diversion projects between Yangtze River and Tai Lake.

Water diversion has been widely used to address water shortages and security issues. However, its long-term ecological impacts, particularly on the biodiversity and structure of the local community, have often been neglected due to limitations of conventional biomonitoring. Taking the water diversion projects from Yangtze River to Tai Lake (WDYT) as examples, environmental DNA (eDNA) metabarcoding was used to investigate the potential ecological impact of water diversion on the connected basins. Firstly, 136 phytoplankton genera/species, including 31 cyanobacteria and 105 eukaryotic phytoplankton (Euk-phytoplankton), were identified from 26 sites by metabarcoding of 16S rDNA V3 and 18S rDNA V9 regions. eDNA metabarcoding showed an obvious advantage in detecting nano/pico-plankton (< 20 μm in size) compared with the morphological approach. Secondly, more shared taxa and higher similarity of community composition were observed in Gonghu Bay/Zhushan Bay with its connected river than with the center of Tai Lake, indicating that water diversions were accelerating the biotic homogenization between different waterbodies. Skeletonema potamos, the native species of Yangtze River (4.04% of the total Euk-phytoplankton reads) was detected in different connecting regions of Tai Lake (0.03%-0.54% of the total Euk-phytoplankton reads), where its relative abundance was consistent with the influence of water diversion from Yangtze River. Furthermore, the introduction of S. potamos significantly affected the local community compositions of phytoplankton in Tai Lake. Finally, the ecological effect (e.g., taxa richness, community composition and species invasion) of the WDYT on phytoplankton in the west of Tai Lake was more significant than that in the east, which was consistent with the scale (volume and duration) of the water diversion projects. Overall, this study highlights the value of eDNA biomonitoring in the ecological impact assessment of water transfer projects.