Community interactions drive phytoplankton regime shifts in interconnected water bodies: Insights from space for time...

Biological communities exhibit nonlinear responses to environmental pressures, with interconnected water bodies significantly influencing both multi-trophic biota and environmental conditions. Phytoplankton assemblages respond rapidly to concurrent environmental and biological fluctuations, serving as sensitive bioindicators of aquatic ecological dynamics. However, the drivers of phytoplankton regime shifts remain unclear when environmental factors and multi-trophic interactions co-vary. Based on alternative stable states, food web, and size spectrum theory, it collected 66 biological samples to evaluate how external and internal changes impact phytoplankton states. Results showed that interconnected zones enhanced flow velocity, mitigated carbon-to-nitrogen ratios, and fostered greater biodiversity within multi-trophic communities. Combining trophic structure and energy flow analysis revealed low synchrony (approximately 0.4) and high grazing pressure (approximately 1.3) between predator-prey relationships, highlighting top-down regulation in shaping biotic interactions. Bimodal distributions and potential energy analysis identified carbon-to-nitrogen ratio and flow velocity as key environmental drivers of phytoplankton regime shifts. Partial least squares path modeling (PLS-PM) further demonstrated that biotic community relationships primarily governed phytoplankton community transitions. This study elucidates how multi-trophic interactions drive phytoplankton regime shifts, providing valuable insights for ecosystem regulation and management in interconnected aquatic environments.