## 4 Discussion
### The Influence of Factors on CO\({}_{2}\) Exchange Flux
There was a significant negative correlation between the \(p\)CO\({}_{2}\) concentration and DO concentration (Figure 6d), which reflects the photosynthesis and respiration of submerged plants. An increase in \(p\)CO\({}_{2}\) raises the CO\({}_{2}\) gradient between the air and water during respiration, leading to increased CO\({}_{2}\) degassing [27]. Usually, CO\({}_{2}\) in river water bodies is supersaturated, where the \(p\)CO\({}_{2}\) in rivers is greater than that in the atmosphere [23]. During the day, photosynthesis may exceed respiration, and CO\({}_{2}\) consumption may cause CO\({}_{2}\) in the atmosphere to dissolve in river water. The diurnal variation in \(p\)CO\({}_{2}\) at H2 reflects this difference. The \(p\)CO\({}_{2}\) concentration in rivers is mainly controlled by soil CO\({}_{2}\) input, the respiration of aquatic microorganisms, and organic matter degradation, which increase \(p\)CO\({}_{2}\), while the breakdown of carbonate rocks and submerged plant photosynthesis consume CO\({}_{2}\), reducing \(p\)CO\({}_{2}\)[38; 39; 40; 41]. At Q1, the water had low fluidity, and the lowest \(p\)CO\({}_{2}\) value occurred during the day due to the biological action of microorganisms on the water surface. At H2, the minimum \(p\)CO\({}_{2}\) value during the day was lower than H1, indicating the biological role of submerged plants during water flow. The influence of carbonate minerals on the concentration of dissolved CO\({}_{2}\) can be controlled based on the water Slc value [27; 42]. A positive Slc value indicates calcite precipitation, which facilitates CO\({}_{2}\) degassing. Conversely, a negative Slc value indicates calcite dissolution, which restricts CO\({}_{2}\) escape. During the entire sampling period, the Slc values at all points were \(<\)0, and the H1 (r = \(-\)0.91, \(p\)\(<\) 0.001) and H2 (r = \(-\)0.96, \(p\)\(<\) 0.001) and DIC (Figure 6e) showed a substantial negative association, indicating that the dissolution of calcite in the Heilongtan River transformed into HCO\({}_{3}\)\({}^{-}\) in the water, limiting the escape of CO\({}_{2}\). However, the various indicator factors of calcite dissolution at Q1 are not correlated, indicating that calcite dissolution does not have a significant impact on the CO\({}_{2}\) flux changes at the outlet point. There are also other factors that affect the source of HCO\({}_{3}\)\({}^{-}\): for instance, \(p\)CO\({}_{2}\) (r = 0.68, \(p\) = 0.01). Surprisingly, the Slc values of the H1 (r = \(-\)0.34, \(p\) = 0.27) and H2 (r = \(-\)0.76, \(p\)\(<\) 0.01) show a negative correlation with CO\({}_{2}\) flux (Figure 6f), which contradicts our theoretical expectations. This indicates that there are other processes that have greater impacts than precipitation or the dissolution of carbonate rocks. As the conductivity of the water body continuously changes, the CO\({}_{2}\) flux at Q1, H1, and H2 shows different daily variation patterns. The conductivity is proportional to the total dissolved solids. The conductivity change in section H1 began to show the same change as that of Q1 on the second day, while the conductivity of Q1 was positively correlated with the change in CO\({}_{2}\) flux (r = 0.56, \(p\) = 0.05) (Figure 6g). The changes in solutes affect the metabolic function of microorganisms in water, which may mainly determine the changes in CO\({}_{2}\) flux at the H1 section during the flow process. However, during the process from H1 to H2, the dense distribution of submerged plants led to changes in CO\({}_{2}\) flux at the H2 section. DO (\(p\)\(<\) 0.001) is the primary driver of changes in CO\({}_{2}\) flux (Figure 6h), while EC (\(p\) = 0.003) has a significant positive effect (Figure 6g). CO\({}_{2}\) flux exhibits a diurnal pattern and generally decreases with increasing DO concentration. Therefore, DO is an important variable that may regulate variations in CO\({}_{2}\) flux.