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The fugacity of CO2 (fCO2 (water)) and air‐water CO2 flux were compared between a river‐dominated anthropogenically disturbed open estuary, the Hugli, and a comparatively pristine mangrove‐dominated semiclosed marine estuary, the Matla, on the east coast of India. Annual mean salinity of the Hugli Estuary (≈7.1) was much less compared to the Matla Estuary (≈20.0). All the stations of the Hugli Estuary were highly supersaturated with CO2 (annual mean ~ 2200 µatm), whereas the Matla was marginally oversaturated (annual mean ~ 530 µatm). During the postmonsoon season, the outer station of the Matla Estuary was under saturated with respect to CO2 and acted as a sink. The annual mean CO2 emission from the Hugli Estuary (32.4 mol C m−2 yr−1) was 14 times higher than the Matla Estuary (2.3 mol C m−2 yr−1). CO2 efflux rate from the Hugli Estuary has increased drastically in the last decade, which is attributed to increased runoff from the river‐dominated estuary.

Upwelling enhances pCO 2 levels due to injection of carbon-rich water to the surface despite the removal of carbon due to increase in primary production supported by enhanced nutrients. It is hypothesized that in the Bay of Bengal, upwelling may decrease pCO 2 due to existence of low saline and pCO 2 -poor waters in the subsurface layer. In order to test this hypothesis, a high-resolution state-of-the-art ocean biogeochemical model (Regional Ocean Modeling System) runs are examined at the sea east of Sri Lanka (SESL) where intense upwelling occurs during summer monsoon (May to August). Upwelling enhances pCO 2 by 34 μatm, whereas decrease in surface temperature and increase in surface salinity reduce pCO 2 by 24 μatm. The estimated net effect of upwelling is an increase in pCO 2 by 10 μatm. In contrast, soft and hard tissues together contribute to a decrease in pCO 2 by 21 μatm suggesting that the biological effect dominates over upwelling, resulting in a net decrease of pCO 2 by 11 μatm in the SESL. This striking contrast between the increase in pCO 2 due to physical dynamics (upwelling) and the removal of pCO 2 due to biological processes is caused by shallow (deep) nitracline (dissolved inorganic carbon-isoline) in the SESL.

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