Compounding impacts of riparian hardening and navigation erode phytoplankton community resilience in a large river...

Phytoplankton communities are foundational to the functioning and stability of river ecosystems, yet how they respond to the combined effects of multiple anthropogenic stressors remains unclear. This study addresses the impact of riparian hardening and navigation disturbance on phytoplankton community structure, diversity, and stability in the middle and lower reaches of the Yangtze River. Water quality and phytoplankton samples were collected during the wet and dry seasons across gradients of riparian hardening and navigation intensity. We used Random Forest modeling, Co-occurrence network analysis, and Partial Least Squares Structural Equation Modeling (PLS-SEM) to analyze the complex relationships between environmental factors and community responses. Our results show that while nutrients and temperature remain key drivers, their influence is strongly mediated by physical habitat structure and disturbance. Specifically, riparian hardening significantly shifted beta diversity through habitat homogenization without inducing a significant decline in alpha diversity. In the wet season, Cyanobacteria dominated the community (64.9%), while navigation disturbance further amplified their dominance. Notably, a Two-way ANOVA revealed a significant interaction effect between riparian hardening and navigation on alpha diversity specifically during the wet season (F = 4.35, P < 0.01). Critically, co-occurrence network analysis revealed a severe erosion of phytoplankton community resilience. Under the combined pressure of these stressors, the phytoplankton community became pathologically fragmented, characterized by a sparsely connected yet highly modular network (average degree dropped by 42.5% from 2.32 in the wet season to 1.33 in the dry season, while modularity increased to 0.85). This structure indicates a loss of functional redundancy and response diversity, rendering the ecosystem more vulnerable to future perturbations. PLS-SEM confirmed that riparian hardening indirectly impacts phytoplankton assembly by altering key water quality parameters. This study reveals the compounding impacts of human activities on riverine ecosystems. It demonstrates that resilience restoration requires an integrated management approach that moves beyond nutrient control to include restoring physical habitat complexity and mitigating chronic disturbances.