Size-dependent responses of zooplankton communities to environmental gradients in the eastern Indian Ocean

Zooplankton size structure mediates energy transfer and carbon export, yet its environmental controls remain poorly resolved in the eastern Indian Ocean (EIO). Based on a 2024 cruise across 19 stations, we examined zooplankton community composition, size-fractionated abundance, and their relationships with hydrographic and biogeochemical gradients. The study area was generally warm, stratified, and oligotrophic, with total zooplankton abundance ranging from 267.60 to 1770.50 ind. m^-3 (742.14 ± 347.28 ind. m^-3). Copepods dominated all stations, and the 200-1000 μm fractions accounted for approximately 76.06% of total zooplankton abundance, whereas organisms ≥2000 μm were much less abundant. Across the four environmental groups representing the seamount, Bay of Bengal, equatorial, and Sumatra regimes, community composition differed significantly, whereas total zooplankton abundance did not. Mantel tests showed size-dependent environmental responses: the 200-500 μm fraction was associated with silicate and temperature, the 500-1000 μm fraction with temperature and salinity, and the >4000 μm fraction with dissolved oxygen. Generalized additive models further showed that copepods were positively related to chlorophyll a and phosphate, euphausiids were positively related to phosphate, whereas gelatinous zooplankton and pelagic larvae were negatively related to dissolved oxygen. Overall, environmental heterogeneity in the EIO was expressed primarily through changes in community composition, size structure, and taxon-specific responses rather than total abundance. This pattern improves our understanding of zooplankton responses in tropical oligotrophic oceans, where community reorganization may occur despite relatively stable total abundance.