Drivers of inorganic carbon dynamics and air–water CO₂ fluxes in two large tropical estuaries: Insights from coupled radon (²²²Rn) and pCO₂ surveys

Abstract

Estuaries and coastal waters are generally significant emitters of CO2 to the atmosphere. Globally, submarine groundwater discharge (SGD) is a significant driver of inorganic carbon dynamics and the partial pressure of CO2 in water [pCO2(water)] in estuaries and coastal waters. However, there are few studies of CO2 emission and SGD in large tropical estuaries. To investigate the drivers of pCO2(water) dynamics in two large tropical estuaries in India, the Hooghly and Matla estuaries, we conducted coupled radon and pCO2(water) surveys of groundwater and surface waters. We also collected high‐temporal‐resolution (1 min) data of related biogeochemical parameters. Using radon‐222 (222Rn) as a proxy for SGD, we found significant regulation of pCO2(water) by SGD in both studied estuaries. This observation can be considered direct evidence of the influence of SGD on pCO2(water) and consequent air–water CO2 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.

Seagrass blue carbon stocks and air-sea CO2 fluxes in seagrass meadows Jepara, Indonesia

“…In the meanwhile, there is still a lack of studies connecting Indonesia's CO2 fluxes with the organic carbon stocks within seagrass and sediment (OCS-Sg and OCS-S). According to previous study shows that the waters of seagrass meadows can absorb and then release CO2 back into the atmosphere or CO2 source [26][27][28][29][30]. Even though seagrass waters act as a CO2 source, the CO2 flux value of waters containing seagrass ecosystems (vegetated) was lower than those without seagrass (unvegetated) [30] and increase if the seagrass ecosystem contains additional sources of carbonate from algae [29; 30] and mangroves [26; 30; 31].…”