CO₂ partial pressure and CO₂ emission along the lower Red River (Vietnam)

Abstract. The Red River (Vietnam) is representative of a south-east Asian river system, strongly affected by climate and human activities. This study aims to quantify the spatial and seasonal variability of CO₂ partial pressure and CO₂ emissions of the lower Red River system. Water quality monitoring and riverine pCO₂ measurements were carried out for 24 h at five stations distributed along the lower Red River system during the dry and the wet seasons. The riverine pCO₂ was supersaturated relative to the atmospheric equilibrium (400 ppm), averaging about 1589±43 ppm and resulting in a water–air CO₂ flux of 530.3±16.9 mmol m⁻² d⁻¹ for the lower Red River. pCO₂ and CO₂ outgassing rates were characterized by significant spatial variation along this system, with the highest values measured at Hoa Binh station, located downstream of the Hoa Binh Dam, on the Da River. Seasonal pCO₂ and CO₂ outgassing rate variations were also observed, with higher values measured during the wet season at almost all sites. The higher river discharges, enhanced external inputs of organic matter from watersheds and direct inputs of CO₂ from soils or wetland were responsible for higher pCO₂ and CO₂ outgassing rates. The difference in pCO₂ between the daytime and the night-time was not significant, suggesting weak photosynthesis processes in the water column of the Red River due to its high sediment load.