## 4 Discussion
### CO\({}_{2}\) Exchange Flux Comparation of Study Area and the World Rivers  
At Q1, H1, and H2, the average CO\({}_{2}\) flux is 14.86 mmol/(m\({}^{2}\cdot\)d), 12.62 mmol/(m\({}^{2}\cdot\)d), and \(-\)0.68 mmol/(m\({}^{2}\cdot\)d), respectively. The research results show that the Heilongtan  
Figure 7: Differences in organic carbon changes between H2 and H1. H1 represents the river sections without submerged plants, and H2 represents the river sections with submerged plants. The gray portion represents the period of night during the sampling period in the study area, while the white portion represents the daytime period. Basin is an important net flux of CO\({}_{2}\) from rivers to the atmosphere, similar to other major rivers in the world (Table 1). However, in the Heilongtan Basin, CO\({}_{2}\) flux exhibits significant temporal and spatial changes, especially in terms of CO\({}_{2}\) sinks appearing at H2. The average value from H1 to H2 in the Heilongtan Basin is 5.97 mmol/(m\({}^{2}\)-d), which is equivalent to the CO\({}_{2}\) flux of Longchuan (6.87 mmol/(m\({}^{2}\)-d) [51]) but slightly higher than the observed values of other subtropical and tropical rivers [26; 52; 53; 54]. It is worth noting that the CO\({}_{2}\) flux in the Heilongtan Basin is even lower than in some temperate and northern rivers [55; 56; 57]. Research suggests that in Arctic tundra vegetation (moss), metabolic activity exceeds photosynthesis, leading to a significant release of CO\({}_{2}\)[58]. However, this depends on the study area, temperature, and the amount of bicarbonate in the water. In this study area, the release of CO\({}_{2}\) in the open-flow section of karst underground rivers has been reduced due to the biological effects of submerged plants. In inland waters, it has been found that the biological effects of phytoplankton can lead to the release of more CO\({}_{2}\) from the water [59]. Phytoplankton are mostly found in lakes, reservoirs, or slow-moving water bodies, and the CO\({}_{2}\) released from these water bodies is often much greater than that from rivers [32; 60]. The CO\({}_{2}\) flux at Q1 is similar to that in reservoirs and may be influenced by phytoplankton. Therefore, the different distribution and geological conditions of submerged plants in water bodies may have a more important impact on CO\({}_{2}\) flux than the climate conditions in the Heilongtan Basin. This further emphasizes the important role of aquatic ecosystems dominated by submerged plants in suppressing CO\({}_{2}\) emissions in karst areas. The impact of CO\({}_{2}\) balance systems and submerged plant photosynthesis on CO\({}_{2}\) flux are the first two categories of influencing factors for research and analysis. The third category includes the effects of meteorological environmental factors and water-soluble organic and inorganic carbon on CO\({}_{2}\) flux. DO is an important variable that may regulate variations in CO\({}_{2}\) flux. By analyzing the CO\({}_{2}\) flux of the widely distributed river section (from H1 to H2) of submerged plants in the Heilongtan Basin, it was found that the photosynthesis of submerged plants significantly inhibits the carbon flux at H2 with metabolism inhibiting carbon emissions of approximately 0.131 Tg C/yr from H1 to H2. At the same time, it indicates that high-intensity pCO\({}_{2}\) is easily absorbed by submerged plants, indicating a process from release to absorption. Therefore, this study emphasizes the process change in CO\({}_{2}\) flux in the open-flow section release to absorption under the metabolism of submerged plants, providing an effective way for carbon sequestration and