Wastewater-boosted biodegradation amplifying seasonal variations of pCO2 in the Mekong-Tonle Sap river system

Water pollution disrupts the ecological integrity of urbanized river systems, but its impacts on riverine metabolic processes and carbon fluxes are poorly studied in developing countries. Three seasonal field surveys were combined with two high-resolution measurements and an in situ incubation experiment to investigate the effects of untreated wastewater on organic matter biodegradation and the partial pressure of CO2 (pCO(2)) along the Mekong-Tonle Sap network around Phnom Penh. High-resolution measurements during the dry-season survey exhibited large downstream increases in pCO(2) along the Mekong reaches receiving Tonle Sap inflows carrying urban sewage, contrasting with little spatial variation during a monsoon survey when the Mekong floodwater reversed the Tonle Sap flow. The monsoonal and dry-season surveys revealed flooding-induced homogenization and large spatial divergences in dissolved organic carbon (DOC) concentration and its delta C-13 and Delta C-14 between the Tonle Sap and connected Mekong reaches. During the 3-day incubation of Mekong waters, alone or mixed with sewage, a large initial nocturnal increase in pCO(2) in sewage-supplemented river water exceeded the subsequent daytime CO2 uptake by phytoplankton photosynthesis varying with light exposure. This, combined with the preferential consumption of labile DOC components displaying protein-like fluorescence, implies sewage-enhanced biodegradation of riverine organic matter. These results suggest that neglecting wastewater-enhanced CO2 production in urbanized river basins during long dry periods can result in a significant underestimation of riverine CO2 emissions.

Automated summary

Water pollution from untreated wastewater significantly impacts organic matter biodegradation and CO2 pressure along the Mekong-Tonle Sap network near Phnom Penh. High-resolution measurements in the dry season show a large increase in pCO2 downstream in areas receiving urban sewage, while the monsoon survey reveals little spatial variation due to the reversal of water flow. This highlights the importance of considering wastewater-enhanced CO2 production in urbanized river basins to accurately estimate CO2 emissions.