Effects of cascade reservoir constructions in the upper reaches of the Yangtze River on the assembly mechanism and stability of phytoplankton communities

ABSTRACT

Cascade reservoirs reshape phytoplankton interaction networks through enhanced environmental filtering, with partial downstream recovery along physicochemical and habitat gradients.

Phytoplankton are key primary producers in river ecosystems, yet their responses to cascade reservoir development remain insufficiently understood. In this study, phytoplankton and water samples were collected from 2021 to 2023 along the Zhimenda–Zhutuo reach in the upper Yangtze River to investigate spatial variations in community structure and assembly mechanisms. Phytoplankton communities exhibited pronounced spatial heterogeneity, with cell abundance, species richness, and diversity following the pattern S3 (downstream recovery section) > S2 (cascade reservoir–impacted section) > S1 (natural river section). Community assembly mechanisms also differed among river sections: stochastic processes dominated in S1 and S3, whereas deterministic processes prevailed in S2. Co-occurrence network analysis revealed a relatively simple community structure in S1, while stronger intra- and interspecific interactions were observed in S2. In S3, community stability gradually increased with distance from the final reservoir but remained slightly lower than that in S1. Partial least squares path modeling further indicated that flow velocity in S2 played a key role in regulating phytoplankton community stability. Overall, these findings provide new insights into how cascade reservoir development reshapes phytoplankton community structure, assembly processes, and stability in large river systems.