Estuaries have been under sampled to establish them as sources or sinks of the atmospheric carbon dioxide. Such poor coverage is well known for tropical, particularly monsoon driven, estuaries. In an attempt to study the variability in CO2 in a tropical monsoon estuary we made systematic time‐series observations in the Gautami Godavari estuarine system in the east coast of India. Our 18 month‐long extensive monitoring in the tropical Godavari estuarine system revealed pH >7.8 during dry period that decreased by 1.5 ± 0.01 during peak discharge period. The decrease in pH was associated with high nutrients and bacterial activities suggesting significant organic carbon decomposition. High bacterial respiration (20.6 ± 7.2 μMC l−1 d−1) in the estuary resulted in very high pCO2 of ∼30,000 μatm during peak discharge period, which otherwise were <500 μatm during dry period. Such high pCO2 levels were unknown to occur in any aquatic region. Several major and minor estuaries flow into the northern Indian Ocean from the Indian subcontinent and the monsoon associated processes make these systems chimney for emitting CO2 to atmosphere unrealized hitherto.

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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Intense oxygen minimum zone (OMZ) occurs in the middepth of the Eastern Tropical Pacific (ETP), Arabian Sea (AS), and Bay of Bengal (BoB). However, the occurrence of anammox/denitrification was reported only in the ETP and AS and its absence in the BoB is attributed to presence of traces of dissolved oxygen (DO). Anticyclonic eddies (ACEs) supply high-nutrient, organic-rich, and oxygen-poor waters from the coastal upwelling regions leading to strengthening of OMZ in the offshore of AS and ETP. In the absence of western boundary upwelling, we hypothesize that the ACEs supply DO-rich water leading to weakening of OMZ in the BoB. Six ACEs were sampled by bio-argo floats and measured in situ hydrography and DO. All sampled eddies were formed in the eastern BoB, close to Andaman Sea, and moved toward west. Warm and DO-rich waters were observed in the core of OMZ (150-300 m), influenced by ACEs, by 0.5-1.46°C and 3.2-6.5 μM, respectively, than no eddy region in the BoB. Based on the mean lifetime of ACE, the rate of ventilation in the 100-300 m was estimated to be 0.07 ± 0.02 μmol/L/day, which is 3 to 4 times higher than bacterial respiration rates (0.02 μmol/L/day). Concerns of OMZ intensification leading to denitrification/anammox conditions in the BoB have been expressed, but with several ACEs forming in time and space in the BoB the denitrification occurrence is unlikely.SARMA AND UDAYA BHASKAR 2145