| Summary: | Urban riverine networks are hotspots of CO<sub>2</sub> and CH<sub>4</sub> emissions, due to river impoundment and pollution. The river–lake connection is considered to be an important way to improve the ecological environment of urban rivers; however, its impact on CO<sub>2</sub> and CH<sub>4</sub> 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 CO<sub>2</sub> and CH<sub>4</sub> from March 2021 to December 2021 in an interconnected river–lake system in Central China. We found that the urban river–lake system was a hotspot of CO<sub>2</sub> and CH<sub>4</sub> emissions. CO<sub>2</sub> and CH<sub>4</sub> 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 CO<sub>2</sub> and CH<sub>4</sub> emissions was determined by nutrient content. The abundant nutrients promoted microbial growth and consumed dissolved oxygen (DO), thus resulting in high emissions of CO<sub>2</sub> and CH<sub>4</sub> in the isolated urban rivers (UR). The average CO<sub>2</sub> and CH<sub>4</sub> emissions of urban rivers are 991.56 and 14.82 mmol m<sup>−2</sup> d<sup>−1</sup>, respectively. The river–lake connection decreased the nutrients of urban rivers connected to lakes (LUR). The moderate nutrients wreaked in situ respiration, exhibiting moderate CO<sub>2</sub> emission in the LUR. The average CO<sub>2</sub> emission of LUR is 543.49 mmol m<sup>−2</sup> d<sup>−1</sup>. The river–lake connection increased the DO concentrations in the LUR, inhibited methanogenesis, and enhanced CH<sub>4</sub> oxidation, reducing CH<sub>4</sub> emission from LUR sharply. The average CH<sub>4</sub> emission of LUR is 1.26 mmol m<sup>−2</sup> d<sup>−1</sup>. A correlation analysis showed that the seasonal variations of CO<sub>2</sub> and CH<sub>4</sub> emissions were controlled by DO and T. Hence, the highest emissions of CO<sub>2</sub> were observed in the spring and the lowest in the winter, and the CO<sub>2</sub> emissions in spring were 10.7 times that in winter. The highest emissions of CH<sub>4</sub> were observed in the summer and the lowest in the winter, and the CH<sub>4</sub> 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–lake connection, providing the theoretical basis for greenhouse gas emission reduction from urban rivers.
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