Drivers of inorganic carbon dynamics and air–water <scp>CO<sub>2</sub></scp> fluxes in two large tropical estuaries: Insights from coupled radon (<scp><sup>222</sup>Rn</scp>) and <scp><i>p</i>CO<sub>2</sub></scp> surveys
Anirban Akhand, Abhra Chanda, Kenta Watanabe, Sourav Das, Tatsuki Tokoro, Sugata Hazra, Tomohiro Kuwae
Abstract
Abstract Estuaries and coastal waters are generally significant emitters of CO 2 to the atmosphere. Globally, submarine groundwater discharge (SGD) is a significant driver of inorganic carbon dynamics and the partial pressure of CO 2 in water [ p CO 2 (water)] in estuaries and coastal waters. However, there are few studies of CO 2 emission and SGD in large tropical estuaries. To investigate the drivers of p CO 2 (water) dynamics in two large tropical estuaries in India, the Hooghly and Matla estuaries, we conducted coupled radon and p CO 2 (water) surveys of groundwater and surface waters. We also collected high‐temporal‐resolution (1 min) data of related biogeochemical parameters. Using radon‐222 ( 222 Rn) as a proxy for SGD, we found significant regulation of p CO 2 (water) by SGD in both studied estuaries. This observation can be considered direct evidence of the influence of SGD on p CO 2 (water) and consequent air–water CO 2 fluxes in these two previously poorly studied estuaries. Our finding that total alkalinity (TAlk) and dissolved inorganic carbon (DIC) were also significantly higher in the groundwater than in the estuarine surface waters also supports a substantial contribution of SGD to the inorganic carbon dynamics of these estuaries. However, high groundwater TAlk to DIC ratios (> 1) indicate that SGD also enhances the carbonate buffering capacity in these estuaries. By contributing to an improved understanding of the role of SGD in inorganic carbon cycling in tropical estuaries and coastal oceans, this study will contribute to efforts to upscale regional‐scale coastal carbon budgets to a global scale.