| Summary: | The significance of CO<sub>2</sub> emissions at the water–air interface from inland water bodies in the global carbon cycle has been recognized and is being studied more and more. Although it is important to accurately assess CO<sub>2</sub> emission flux in a catchment, little research has been carried out to investigate the spatio-temporal variations in CO<sub>2</sub> emissions in view of a water continuum. Here, we systematically compared the differences and control factors of CO<sub>2</sub> degassing across the water–air interface of a spring–river–lake continuum in the discharge area of Baotuquan Spring in July 2017, which is a typical temperate karst spring area in Jinan city, northern China, using hydrogeochemical parameters, stable carbon isotope values, and CO<sub>2</sub> degassing flux. Affected by the <i>p</i>CO<sub>2</sub> concentration gradient between the water and ambient air, the spring water showed a high CO<sub>2</sub> degassing flux (166.19 ± 91.91 mmol/(m<sup>2</sup> d)). After the spring outlet, the CO<sub>2</sub> degassing flux in the spring-fed river showed a slight increase (181.05 ± 155.61 mmol/(m<sup>2</sup> d)) due to river flow rate disturbance. The river flow rate was significantly reduced by the “blockage” of the lake, which promoted the survival and reproduction of phytoplankton and provided favorable conditions for aquatic plant photosynthesis, increasing the plankton biomass in the lake to 3383.79 × 10<sup>4</sup>/L. In addition, the significant decrease in the dissolved inorganic carbon (DIC) concentration and the increase in the δ<sup>13</sup>C<sub>DIC</sub> values in the lake also indicated that the photosynthesis of the lake’s aquatic plants resulted in a significant decrease in the <i>p</i>CO<sub>2</sub> concentration, thus limiting the amount of CO<sub>2</sub> off-gassing (90.56 ± 55.03 mmol/(m<sup>2</sup> d)).
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