Environmental heterogeneity shapes divergent assembly processes and network stability of plankton communities along a tropical island coast: Evidence from Hainan Island

Understanding how plankton communities respond to environmental heterogeneity is essential for assessing tropical island coastal ecosystems and supporting region-specific management. Based on data from 59 environmental monitoring stations, including 16 stations with plankton samples, this study investigated the spatial differentiation, assembly processes, network characteristics, and environmental response pathways of phytoplankton and zooplankton communities along the coast of Hainan Island. We integrated analyses of community diversity, β-diversity, community assembly, co-occurrence networks, and piecewise structural equation modelling (piecewise SEM). The results showed marked heterogeneity in salinity, nutrients, chlorophyll a , suspended solids, and dissolved oxygen among coastal sectors. Regional differences in both communities were weak at higher taxonomic levels. In contrast, genus-level β-diversity showed significant sectoral effects in both groups, with stronger spatial differentiation in phytoplankton than in zooplankton (PERMANOVA: R 2 = 0.65, P = 0.002; R 2 = 0.58, P = 0.001). At the α-diversity level, zooplankton showed clearer regional differentiation, whereas phytoplankton α-diversity did not differ significantly among sectors. Community assembly analysis suggested that phytoplankton deviated more strongly from neutral expectations and was jointly influenced by environmental filtering and spatial processes, whereas zooplankton showed a greater contribution of stochastic processes, with dispersal limitation and undominated processes playing more important roles. Co-occurrence network analysis and piecewise SEM further identified region-specific network organization and variation in network-derived stability metrics, with statistical associations between environmental gradients and network properties being predominantly indirect and involving community composition and diversity. These findings reveal hierarchical responses of coastal plankton communities to environmental heterogeneity along a tropical island coast and provide ecological evidence for coastal ecosystem assessment, biodiversity evaluation, and region-specific management.