Seasonal shifts in phytoplankton community assembly mediated by hydrological connectivity in a sluice-regulated distributary estuarine network—as a case in the Pearl River Estuary

Estuarine deltas are global biodiversity hotspots where hydrological connectivity plays a pivotal role in shaping ecosystem structure and functioning. However, hydrological connectivity in many estuaries has been profoundly altered by freshwater regulation and tidal intrusion, and its effects on community assembly remain poorly understood. Using the Pearl River Estuary as a model system, this study integrates flow-based functional and path-based structural connectivity to investigate how hydrological connectivity mediates phytoplankton community assembly under contrasting hydrographic regimes. Hydrological connectivity exhibited pronounced spatiotemporal heterogeneity, with functional connectivity showing clear spatial clustering during the wet season but declining sharply and becoming fragmented in the dry season. Structural connectivity presented a distinct spatial pattern, concentrated in the central and eastern delta. Phytoplankton communities exhibited marked seasonal contrasts: during the wet season, river-dominated conditions enhanced environmental filtering and species turnover, resulting in high spatial heterogeneity (βsor: 0.65 ± 0.11), while in the dry season, tidal intrusion and reduced freshwater inflow intensified environmental constraints and dispersal limitation, leading to stress-tolerant assemblages and elevated total beta diversity (βsor: 0.70 ± 0.15). Hydrological connectivity significantly modulated the balance between environmental and spatial processes, displaying season-dependent, nonlinear threshold-like responses along connectivity gradients. By explicitly distinguishing functional and structural connectivity, these findings advance a process-based framework for interpreting hydrological–ecological interactions in sluice-regulated distributary estuarine networks, with broader relevance for understanding biodiversity dynamics under anthropogenic pressures.