Comprehensive understanding and quantification of different aspects of estuarine carbon (C) cycle are essential to decipher regional and global changes. Here, premonsoon C biogeochemistry of two contrasting estuarine systems (Hooghly - anthropogenically influenced and Sundarbans - mangrove dominated) located in the deltaic region of Ganges, India, has been investigated. The Hooghly showed sharper salinity gradient (0.48–32.94) and wider pH range (7.56–8.25) compared to the Sundarbans (salinity: 27.22–29.82; pH: 8.16–8.31). The DIC concentration in the Hooghly (2.27 ± 0.22 mmol L⁻¹) was higher than the Sundarbans (1.77 ± 0.06 mmol L⁻¹) with relatively depleted δ¹³C_DIC. The mixing model analysis suggested DIC chemistry in the Hooghly to be principally regulated by respiration of estuarine algae, whereas Sundarbans showed evidences for DIC removal and mangrove-derived DIC contribution. The DOC behaved non-conservatively in both the estuaries. The POC in the Hooghly showed signatures of estuarine algae and marine plankton in the mixing and marine zones, whereas dominance of terrestrial organic matter was found in the freshwater zone of the Hooghly and estuaries of Sundarbans. Both pCO₂ (556–5002 μatm) and CH₄ (15.4–445.7 nmol L⁻¹) varied over a wider range in the Hooghly compared to the Sundarbans (pCO₂: 268–418 μatm; CH₄: 41.6–71.5 nmol L⁻¹). The Sundarbans acted as a net sink for CO₂, whereas the Hooghly was a significant source to the regional atmosphere. Both the estuaries acted as source of CH₄ with comparable exchange fluxes. Overall, the Sundarbans was a net C sink and the Hooghly was a net source to regional atmosphere via cumulative fluxes of CO₂ and CH₄.

全面了解和量化河口碳（C）循环的不同方面对于破译区域和全球变化至关重要。在这里，研究了位于印度恒河三角洲地区的两个相反的河口系统（Hooghly-受人为影响和Sundarbans-红树林为主）的季风C生物地球化学。与Sundarbans（盐度：27.22-29.82； pH：8.16-8.31）相比，Hooghly盐度梯度更大（0.48-32.94），pH范围更广（7.56-8.25）。在胡格利（2.27±0.22毫摩尔L中的DIC浓度^-1）比松达（1.77±0.06毫摩尔L时^-1）具有相对贫δ ¹³ C ^ _DIC。混合模型分析表明，Hooghly地区的DIC化学主要受河口藻类的呼吸作用调节，而Sundarbans则显示了DIC去除和红树林衍生的DIC贡献的证据。在两个河口，DOC的表现都不保守。Hooghly的POC在混合区和海洋区显示出河口藻类和海洋浮游生物的特征，而在Hooghly的淡水区和Sundarbans的河口中发现了陆生有机质的优势。两个p CO ₂（556-5002μatm）和CH ₄（15.4-445.7纳摩尔大号^-1在改变在更宽的范围内）的胡格利相比松达（p CO ₂：268-418μatm; CH ₄：41.6-71.5nmol L ^-1）。Sundarbans充当了CO ₂的净汇，而Hooghly是该地区大气的重要来源。两个河口均作为CH _4的来源，具有可比的交换通量。总体而言，Sundarbans是净C汇，而Hooghly是通过CO ₂和CH _4的累积通量通向区域大气的净源。