Ecological Restoration Effectively Mitigated pCO₂ and CO₂ Evasions From Severely Polluted Urban Rivers

Urban rivers are significant hotspots of CO2 emissions into the atmosphere, playing important roles in global carbon emission inventories. However, little is known about the effect of ecological restoration on CO2 dynamics in severely polluted urban rivers, and this strongly hindering our understanding of the positive effects of human activities on CO2 emissions in urbanized aquatic ecosystems. We measured CO2 partial pressure (pCO(2) ) and fluxes in nine severely polluted urban rivers with varying degrees of ecological restoration, and assessed the relationships with water physicochemical parameters, pollution levels and basin environmental investment. Our results indicate that urban rivers with high pollution loadings were indeed hotspots of CO2 evasion. However, fully restored rivers had generally lower pCO(2) and CO2 emissions than partially restored rivers and were observably lower than unrestored rivers, suggesting that watershed eco-restoration could effectively reduce CO2 evasion. In particular, environmental investment per unit basin area had a significant negative correlation with CO2 evasion and could explain 51%-85% and 51% of total variability in pCO(2) and CO2 fluxes among nine urban rivers respectively, emphasizing the potential benefits of carbon emission regulation resulting from positive environmental management. Nutrient removal and sewage interception during watershed eco-restoration were key processes reducing pCO(2) and CO2 fluxes in polluted urban rivers. pH alteration kept close correlations with pCO(2) and acted as a sensitive regulator of CO2 evasion in the nine rivers. We highlight the importance of considering the coupling effects of pollution and restoration on the spatiotemporal variability of CO2 evasion and the uncertainty of related monitoring methods in urban watersheds.