## 4 Discussion  
### Variations in \(F_{\text{CO2 bio}}\) and \(F_{\text{CO2}}\) in the Mixed Layer  
F\({}_{\text{CO2}}\) is strongly positive in each timeslot (Figure 6a), indicating that this region acts as a source of atmospheric CO\({}_{2}\)[43; 44] because the near-shore region is affected by the CDW [45] which has abundant \(p\)CO\({}_{2}\)[25; 26]. F\({}_{\text{CO2 bio}}\) is strongly positive most of the time (Figure 6a), meaning that heterotrophic respiration releases CO\({}_{2}\) to the atmosphere for most of the day in the near-shore region. Because this study region is located in the largest turbid zone of the Changjiang River estuary plume [46; 47], we infer that the mixing effect and extremely limited light may reduce the primary production by phytoplankton photosynthesis and that planktonic community respiration may dominate the biological processes, which maintain a high \(p\)CO\({}_{2}\) value in the near-shore region.  
The 24-h F\({}_{\text{CO2}}\) in the front region is almost negative (Figure 6b), indicating that the front region acts as a sink for atmospheric CO\({}_{2}\). The 24-h F\({}_{\text{CO2 bio}}\) in the front region is also almost negative which indicates the biological processes absorb CO\({}_{2}\) (Figure 6b). Because of the high values of NEP (Figure 5b, Table 3) and Chl \(a\)[6] in this region, we infer that the front region has a great capacity for biological productivity and that a large amount of CO\({}_{2}\) is fixed in the surface water by phytoplankton.  
Most F\({}_{\text{CO2}}\) values in the offshore region are mostly slightly less than 0 (Figure 6c), indicating that the offshore region acts as a sink for atmospheric CO\({}_{2}\). This finding is in agreement with the study by [PERSON] et al. [2]. F\({}_{\text{CO2 bio}}\) in the offshore region was mostly slightly less than 0 (Figure 6c), indicating that the photosynthesis rate of fixed CO\({}_{2}\) by phytoplankton is higher than degradation rates of organic matter releasing CO\({}_{2}\) by microbial action in the offshore region.  
F\({}_{\text{CO2}}\) in the mixed layer of the three regions shows that the near-shore region acts as a strong source of atmospheric CO\({}_{2}\) (Figure 6a) and that the front and offshore regions act as sinks for atmospheric CO\({}_{2}\) (Figure 6b,c), similar to the results of other studies [48; 49; 1]. The daily average Cont in the mixed layer shows that the biological processes have a positive feedback effect on air-sea CO\({}_{2}\) exchange in the near-shore, front, and offshore regions (Table 5). This agrees with the conclusion of [PERSON] et al. [15]. The air-sea CO\({}_{2}\) flux is inversely proportional to the NEP in the mixed layer, indicating that the contribution to the variation in air-sea CO\({}_{2}\) flux in these coastal waters is dominated by biological processes during a diel cycle. However, the average Cont in the offshore region is lower than that in the near-shore and front regions. This could be related to the fact that primary production in the offshore region is very low, even in summer, and other effects such as wind, temperature, and water mixing may play more important roles in controlling air-sea CO\({}_{2}\) flux.  
Figure 6: Twenty-four hour variations in F\({}_{\text{CO2}}\), F\({}_{\text{CO2 bio}}\), and F\({}_{\text{CO2 non-bio}}\) in the near-shore (**a**), front (**b**), and offshore (**c**) regions in summer.