Mechanisms of the impact of hydropower development on phytoplankton diversity and community assembly in the lower Jinsha River

<p indent="0mm">Hydropower development in the lower Jinsha River has drastically altered the natural torrent landscape of this mountain river. This has led to fragmented, slow-flowing, and even stagnant reservoir habitats, resulting in significant changes in aquatic ecosystems. Although previous studies have provided foundational insights into the diversity and community structure of phytoplankton in this reach based on traditional sampling surveys and morphological identification methods, there is a lack of research on the changes in diversity, community assembly, and the influencing factors under the impact of hydropower development. In this study, phytoplankton were sampled along the 238-km Xiangjiaba section using the environmental DNA method. The diversity, community structure, and functional characteristics of phytoplankton were analyzed, and the diversity and community construction mechanism of phytoplankton under the impact of the Xiangjiaba Hydropower Station were explored. The results revealed that hydrodynamic conditions were the most significant factor influencing α diversity. Phytoplankton α diversity showed a spatial gradient following “mainstream downstream of the dam &gt; tributaries upstream of the dam &gt; mainstream upstream of the dam”. Along this gradient, the structure, interspecies interactions, and robustness of the co-occurrence network increased, demonstrating the enhanced resistance of the phytoplankton community in the face of environmental changes and disturbances. Notably, there was a significant shift in α diversity at the dam location. In particular, the Chao1 and PD indices showed a significant downward trend from upstream to the dam, shifting to a decreasing pattern downstream when moving away from the dam. Changes in hydrodynamics and water quality were the primary drivers of shifts in β diversity. At the Xiangjiaba reach scale, phytoplankton β diversity was primarily driven by the turnover component, showing stochastic processes as the main mechanism of community assembly. Tributary species influenced the mainstream community through dispersal and environmental filtering. β diversity at the dam location was significantly higher than upstream and downstream and was dominated by the nestedness component, demonstrating deterministic processes such as environmental screening as playing a key role in this local reach. This study contributes to the understanding of the evolution and adaptation mechanisms of river ecosystems under the impact of hydropower development and offers a scientific basis for coordinated and sustainable development of mountain river conservation and development.