Phytoplankton community assembly in an inter-basin water diversion project: Dominance of temporal dynamics over spatial dynamics.

Large-scale water diversion projects play a critical role in the redistribution of essential water resources. Despite their crucial role in maintaining water quality and ecosystem stability in water diversion projects, a deeper understanding of the transport mechanism of phytoplankton in water diversion has yet to be thoroughly explored. A systematic investigation was conducted to better understand the ecological processes related to phytoplankton in the mammoth Middle Route of the South-North Water Diversion Project. Methods including β-diversity partitioning, null model, and generalized linear model were applied to analyze the process and driving mechanism of the assembly of phytoplankton communities. Results indicated that the temporal and spatial variation of phytoplankton in the canal was mainly regulated and controlled by Chlorophyta and Bacillariophyta. The β-diversity of phytoplankton throughout various seasons was mainly driven by species turnover, and was less influenced by dispersal limitation. On a year-long time scale, stochastic processes dominated. However, the relative weights of stochastic and deterministic processes varied with seasons. Phytoplankton communities were primarily influenced by stochastic processes in summer and autumn, and by deterministic processes in winter and spring. Seasonal and season-related factors (e.g., water temperature, dissolved oxygen) had a much greater impact on the β-diversity of phytoplankton than location and location-related factors (e.g., permanganate index). Further analyses showed that phytoplankton communities exhibited characteristics of stochastic assembly in summer and autumn. In winter and spring, however, the composition of phytoplankton communities was significantly regulated by the variations in turbidity, flow velocity, and dissolved organic matter. The above results revealed a unique mechanism: artificial water diversion projects weakened spatial heterogeneity via hydrologic homogeneity, thereby redirecting the main control axis of phytoplankton community assembly towards the temporal dimension. Based on the above findings, we recommend implementing seasonally differentiated real-time algae monitoring and a rapid response mechanism to more effectively address potential ecological risks, optimize water quality management, and ensure ecological safety in long-distance water diversion projects.