## 4 Discussion
### Changes in the Sources and Processes of Carbon Components in Rivers  
The composition of the basin affects the DIC content of river water, as carbonates and silicates dissolve at different rates [43; 44]. In this study, HCO\({}_{3}\)\({}^{-}\) at Q1 mainly originates from carbonate rock weathering, as silicate rock weathering has a negligible impact [45]. DIC mainly includes HCO\({}_{3}\)\({}^{-}\), CO\({}_{3}\)\({}^{2-}\), and dissolved CO\({}_{2}\)[46], with HCO\({}_{3}\)\({}^{-}\) accounting for approximately 90% of the DIC composition in water. Carbonate rocks are the main source of river DIC in this study area. The precipitation of carbonate minerals does not always result in the release of CO2 into the atmosphere, as DIC can be utilized by aquatic microorganisms to form endogenous organic carbon sinks. Aquatic organisms can transform DIC, POC, DOC, and PIC into each other within the water body [5; 26; 47]. Photosynthesis increases DO and reduces DIC and HCO3-, while respiration releases CO2, increasing DIC and decreasing nighttime DO. There was a positive correlation between DO and HCO3- at H2 (r = 0.73, \(p\) < 0.01). Changes in DIC are related to pH (r = \(-\)0.67, \(p\) = 0.01) and are not significantly correlated with HCO3- or DO in water. Therefore, changes in DIC are not only related to the photosynthesis of submerged plants but also involve other biological processes. The organic carbon C/N ratios of the Heilongtan Basin in this study ranged from 6.24 to 10.39 with an average of 8.01. The average organic carbon C/N ratio at the Q1 was 10.39, that at H1 was 7.40, and that at H2 was 6.24. The C/N ratio decreases in the clockwise direction, and the source of TOC is calculated based on the C/N ratio. Previous studies have shown that the C/N ratio of exogenous organic carbon is greater than 15 [48]. According to the typical product of photosynthesis formed by algae, C5.7H9.8O2.3N should have a C/N of 5.7; fungi (C10H17O6N) should have a C/N of 10; and bacteria (C5H7O2N) should have a C/N of 5. Therefore, the C/N ratio of endogenous organic matter should be between 5 and 10. With a C/N ratio of 5.7 as the endogenous endmember and 20.69 as the exogenous endmember [49], the proportion of endogenous organic carbon to total organic carbon can be calculated using a mixed endmember model. The equation is as follows:  
\[\text{C/N}=\text{f}_{\text{au}}\times\text{f}_{\text{B}}+\text{f}_{\text{al}} \times(1-\text{f}_{\text{B}}) \tag{2}\]  
where C/N is the C/N ratio of the sample; fau is the endogenous terminal element of the C/N ratio, taken as 5.7; fal is the external terminal element with a C/N ratio, taken as 20.69; and fB is the exogenous organic carbon fraction relative to the total organic carbon. By combining the concentration values of TOC (POC + DOC) in the Heilongtan Basin, the concentration of endogenous carbon in total organic carbon (TOCau) can be calculated, and the equation is as follows:  
\[\text{TOC}_{\text{au}}=\text{TOC}\times\text{f}_{\text{B}} \tag{3}\]  
In the formula, TOC represents the sample's total organic carbon content; fB is the endogenous organic carbon fraction relative to the total organic carbon; and TOCau is the percentage of total organic carbon that is endogenous carbon. From Q1 to H1 and H2, the C/N ratio gradually drops as the water flows. The contribution ratios of endogenous organic carbon to TOC are 68.71%, 88.66%, and 96.40%, with an average of 84.59%. The contribution of endogenous organic carbon at Q1 is the least, and the water retention time at Q1 is long. Although there is an absence of submerged plants, the contribution of plankton cannot be ignored. Combining the previous CO2 flux analysis, it is clear that at Q1, the correlation between the photosynthesis of phytoplankton and CO2 carbon flux is low, and it may even promote carbon emissions. Moreover, it also indicates that apart from the organic carbon produced by the water's biological carbon pump effect, there is also a 31.29% contribution of exogenous organic carbon at Q1.