Interconnected River–Lake Project Decreased CO2 and CH4 Emission from Urban Rivers

Urban riverine networks are hotspots of CO2 and CH4 emissions, due to river impoundment and pollution. The river&ndash:lake connection is considered to be an important way to improve the ecological environment of urban rivers: however, its impact on CO2 and CH4 emissions from urban rivers and regulatory mechanisms are still unclear. Rivers and lakes have been studied separately by lots of traditional studies. In this study, we investigated the concentration and emission of CO2 and CH4 from March 2021 to December 2021 in an interconnected river&ndash:lake system in Central China. We found that the urban river&ndash:lake system was a hotspot of CO2 and CH4 emissions. CO2 and CH4 emissions from urban rivers were much higher than those from the lakes, which are 2.7 times and 11.9 times that of lakes, respectively. The correlation analysis indicated that the spatial variation of CO2 and CH4 emissions was determined by nutrient content. The abundant nutrients promoted microbial growth and consumed dissolved oxygen (DO), thus resulting in high emissions of CO2 and CH4 in the isolated urban rivers (UR). The average CO2 and CH4 emissions of urban rivers are 991.56 and 14.82 mmol m&minus:2 d&minus:1, respectively. The river&ndash:lake connection decreased the nutrients of urban rivers connected to lakes (LUR). The moderate nutrients wreaked in situ respiration, exhibiting moderate CO2 emission in the LUR. The average CO2 emission of LUR is 543.49 mmol m&minus:2 d&minus:1. The river&ndash:lake connection increased the DO concentrations in the LUR, inhibited methanogenesis, and enhanced CH4 oxidation, reducing CH4 emission from LUR sharply. The average CH4 emission of LUR is 1.26 mmol m&minus:2 d&minus:1. A correlation analysis showed that the seasonal variations of CO2 and CH4 emissions were controlled by DO and T. Hence, the highest emissions of CO2 were observed in the spring and the lowest in the winter, and the CO2 emissions in spring were 10.7 times that in winter. The highest emissions of CH4 were observed in the summer and the lowest in the winter, and the CH4 emissions in summer were 6.6 times those in winter. The connection of urban rivers and lakes changes the environmental factors, thereby varying the production and emission of greenhouse gases. This study advanced the knowledge of the greenhouse gas emission response to the river&ndash:lake connection, providing the theoretical basis for greenhouse gas emission reduction from urban rivers.