Seasonal and diurnal changes of pCO2 in the lower Brahmaputra River, Bangladesh

Monthly 24-h real-time high-resolution monitoring was conducted for 1 year to investigate the carbon dioxide (CO₂) dynamics in the lower Brahmaputra River. The partial pressure of CO₂ (pCO₂) was observed between 136 and 1213 µatm. The pCO₂ was almost 1.5 times higher during the wet season (May-October) than during the dry season (November-April). Diurnal variation in pCO₂ was pronounced during the dry season. In contrast, no clear diurnal pattern was observed during the wet season. The combined measurements of O₂-CO₂ in river water provide insights into net chemical and microbial activity. In January and March, there was a bidirectional exchange of CO₂, occurring from air to water (sink) and from water to air (source). However, in April, the CO₂ exchange was unidirectional, taking place solely from air to water, whereas in all other months, it occurred from water to air. The carbon dioxide flux (FCO₂) in the river ranged from - 31.12 to 137.74 mmol m^-2 day^-1, with an average of 25.26 mmol m^-2 day^-1 during the dry season and 78.56 mmol m^-2 day^-1 in the wet season. This flux significantly contributes to the regional net carbon budget of the world's largest delta.