Tidal forcing enhances ecological connectivity while preserving spatial heterogeneity in plankton communities across a subtropical lagoon-sea continuum.

Coastal lagoons are dynamic ecosystems where tidal forcing regulates environmental gradients and mediates biological exchange across the lagoon-offshore continuum However, how short-term tidal dynamics influence plankton community organization and assembly remains poorly understood. In this study, we investigated environmental variability, plankton dynamics, ecological connectivity, and community assembly patterns across six sequential stages of a flood-tide cycle in Xincun Lagoon, South China. Flood tide increased salinity and water depth while concurrently decreasing nutrients concentrations and chlorophyll-a levels, resulting in enhanced, but incomplete environmental homogenization. Plankton communities exhibited clear tidal responses, species richness increased progressively along the tidal sequence, whereas total abundance generally declined. Coastal taxa remained dominant throughout the cycle; however, offshore-associated taxa, particularly among zooplankton, increased markedly during tidal intrusion, suggesting increased offshore influence on lagoon communities. Bray-Curtis dissimilarity decreased progressively, and SourceTracker analyses revealed a concomitant increase in the proportional contribution of offshore-associated assemblages, collectively indicating enhanced ecological connectivity and partial community convergence. Co-occurrence network analyses showed higher overall connectivity, increased taxon associations, and reduced modularity during mid-to-late tidal stages, indicating a more integrated community structure. Yet, despite this enhanced apparent connectivity, spatial heterogeneity was maintained. Neutral modeling and redundancy analysis further revealed trophic-level-specific differences in community assembly mechanisms. Phytoplankton communities showed stronger alignment with neutral expectations, suggesting that stochastic dispersal processes play a relatively dominant role in their assembly. In contrast, zooplankton communities exhibited stronger associations with niche-based processes, including environmental filtering and hydrodynamic conditions. Collectively, our results demonstrate that tidal forcing regulates plankton community organization through the interplay of constrained ecological connectivity, species responses to tidal transport, and trophic-level-dependent environmental filtering. Rather than driving complete homogenization, tidal exchange promotes transient and partial community convergence while maintaining both spatial heterogeneity and trophic differentiation.