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.