Plankton communities form the foundation of marine ecosystems and are highly sensitive to environmental heterogeneity in tropical oligotrophic oceans. Although environmental gradients are known to influence plankton distribution, whether regional variation in community structure translates into broader differences in ecosystem organization remains poorly understood, particularly when total biomass changes little among regions.
Using multidisciplinary observations collected during research cruises in the Eastern Indian Ocean (EIO) in 2022 and 2024, we integrated zooplankton community analyses, stable isotope techniques, and structural equation modeling to investigate how environmental gradients shape plankton communities and their trophic organization.
Total zooplankton biomass showed only limited regional variation, whereas community composition and size structure differed significantly among environmental regions (PERMANOVA, p<0.05). Distinct plankton assemblages were associated with contrasting environmental settings, indicating that environmental gradients primarily reorganized community structure rather than overall biomass. These community differences were further reflected in regional variation in food-web organization. Food-chain length (FCL) ranged from 3.12 in the oligotrophic equatorial region to 4.73 in the seamount region, suggesting substantial differences in trophic complexity despite relatively stable biomass. Structural equation modeling further revealed that plankton nutritional quality explained regional variation in food-chain length more effectively than traditional trophic-level indicators, highlighting food quality as an important mechanism linking environmental heterogeneity with trophic organization.
Our findings demonstrate that environmental gradients in the EIO primarily influence plankton ecosystems through changes in community composition and size structure rather than total biomass. By linking spatial variation in plankton communities with regional differences in trophic organization, this study provides new insights into the ecological responses of tropical oligotrophic plankton ecosystems to environmental heterogeneity.