The oceans absorb anthropogenic CO₂ and store carbon in deep water through the marine biological pump, playing an irreplaceable role in global climate change. The spatiotemporal distributions of seawater CO₂ system parameters were investigated in the Eastern Indian Ocean during autumn 2020 and spring 2021. Significant seasonal variations were observed in the study area. Overall, the surface pH was lower and dissolved inorganic carbon (DIC), alkalinity (Alk), and pressure of carbon dioxide (pCO₂) were higher during spring 2021 than during autumn 2020. The pH generally decreased from east to west during autumn 2020, whereas it decreased from north to south during spring 2021. The low values of DIC and Alk that detected in the Bay of Bengal (BoB) in these two seasons were mainly attributed to the influence of river inputs. Coastal upwelling during monsoon periods led to higher pCO₂ and DIC values near Sumatra and Sri Lanka during spring 2021. In vertical profiles, pH decreased initially and then increased with depth, while pCO₂ exhibited an opposite trend to pH during both seasons. DIC and Alk concentrations in deep layers were higher than those in upper layers. Overall, DIC was higher and dissolved organic carbon (DOC) was lower in the water column during spring 2021 than autumn 2020. pH elevated and pCO₂ lowered in the BoB and the equator zone, while pH decreased and pCO₂ increased in the upwelling zone and the south hemisphere during spring 2021. The average sea-air flux of CO₂ was 10.00 mmol m⁻² d⁻¹ during autumn 2020 and was 16.00 mmol m⁻² d⁻¹ during spring 2021, which revealed that the EIO served as a CO₂ source during the study period. Moreover, the effects of biological processes on regulating the CO₂ system parameters were explored through incubation experiments. The DIC absorption rates in surface seawater of autumn 2020 were 52, 28 and 33 µmol L^-1 d^-1 at stations E87-36, EQ-12 and E87-11, respectively, dominated by light, phytoplankton composition and abundance. The DIC production rates measured by community respiration experiments declined with depth during spring 2021.The integrated-average rates in the water column were 1.10, 1.48, 0.54 and 1.77 mmol m^-3 d^-1 at stations E87-30, EI-02, E87-13 and E80-01, respectively, heavily relying on microorganism species and abundance. Additionally, respiration processes in the aphotic layer also played important roles in regulating DIC variations and the carbon cycle in the ocean.

Seawater CO2 species,Eastern Indian Ocean,biological process,carbon cycle,sea-air CO2 flux