Spatial Disparities Among Oceanic Protists' Ecological Dynamics in the Bay of Bengal.

Protists are the dominant eukaryotes in the ocean biosphere, and oceanic protists (OP) serve as a critical link among bacterial production, primary producers, and higher trophic levels. This study examined the community structure and vertical organization of OP assemblages in the Bay of Bengal (BoB) across three depth layers: surface (5 m), mid-depth (150 m), and bottom (300 m). Multivariate ordination, diversity metrics, and network analyses consistently demonstrated that vertical depth is the primary factor structuring OP communities, whereas regional (north-south) variability was minimal. Alpha diversity analyses showed that Shannon and Simpson diversity differed significantly among depth layers, whereas Chao1 richness did not, indicating that depth-related changes were reflected mainly in community evenness and dominance structure rather than estimated richness. Beta diversity analyses (PERMANOVA) further confirmed strong depth-wise separation of communities and no significant regional effect. Surface OP communities were characterized mainly by Syndiniales, with additional contributions from Polycystinea, Dinophyceae, Diatomea, Intramacronucleata, and Prymnesiales, whereas mid-depth and bottom assemblages showed stronger contributions from Polycystinea and Diplonemea. Network analyses revealed depth-specific differences in modularity, connectivity, and robustness, with the mid-depth network showing the highest modularity and the bottom network showing the strongest local connectivity but greater sensitivity to node removal. RDA highlighted clear depth-related associations between OP communities and environmental gradients, with surface assemblages aligned with temperature and Chl-a, and deeper assemblages associated with salinity, phosphate, and silicic acid gradients. Overall, these findings reveal a vertically stratified organization of OP communities in the BoB, associated with depth-dependent environmental gradients and potential biotic associations within a stratified tropical ocean system.