The Eastern Indian Ocean is a climatically sensitive region characterized by strong monsoon forcing, persistent upper ocean stratification, complex circulation, and extensive oxygen deficient waters. Despite relatively low primary productivity, this region can exhibit high carbon export efficiency, highlighting the importance of plankton community structure, particle formation, and sinking processes in regulating ocean carbon sequestration. Based on multidisciplinary observations collected during a series of Indian Ocean expeditions from 2010 to 2024, we investigated major plankton mediated carbon fixation and export pathways, including primary production, nitrogen fixation, transparent exopolymer particles, phytoplankton sinking, zooplankton fecal pellet production, calcification, silicification, and greenhouse gas production.
Depth integrated primary production in the euphotic zone ranged from 113 to 817 mg C m^-2 d^-1, with an average of 522 mg C m^-2 d^-1. Nitrogen fixation contributed approximately 0.3 to 1.3 percent of primary production, with small diazotrophs contributing most strongly throughout the water column. Phytoplankton sinking rates ranged from minus 0.291 to 2.188 m d^-1 and were jointly regulated by community composition, nutrient availability, temperature, and water column stratification. Transparent exopolymer particles represented an important component of particle aggregation, with an integrated carbon inventory of 2136.36 ± 929.94 mg C m^-2 and sinking rates ranging from minus 0.3 to 1.9 m d⁻¹. Zooplankton fecal pellets provided an additional pathway for rapid carbon export, with estimated production rates of 4.033 to 68.19 mg C m^-2 d^-1.
Regional synthesis suggests an annual primary production of approximately 718 Tg C, of which transparent exopolymer particles, fecal pellets, particulate organic carbon export, and calcification account for substantial fractions. However, microbial degradation of sinking organic matter also produces CO₂, CH₄, and N₂O, partially offsetting the climatic benefits of biological carbon sequestration. These findings emphasize that assessments of the Indian Ocean biological carbon pump should integrate carbon fixation, particle export, remineralization, and associated greenhouse gas emissions within a unified framework.
 

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