## 5 Conclusions  
Using a mass balance model, we calculated the NEP, F\({}_{\rm CO2 bio}\), and F\({}_{\rm CO2}\) at eight time-points per day in the near-shore, front, and offshore regions of the Changjiang River estuary plume in summer. Then, we calculated the contribution of biological processes to F\({}_{\rm CO2}\) in the three regions. In the mixed layer, both F\({}_{\rm CO2}\) and F\({}_{\rm CO2 bio}\) significantly varied at different times within the 24-h period. The near-shore region was found to be a source of atmospheric CO\({}_{2}\), and the offshore region is a sink for atmospheric CO\({}_{2}\). The front region is a sink for atmospheric CO\({}_{2}\) on the whole, but it transforms between a source and a sink from time to time. The biological processes in the mixed layer in the three regions were shown to have an overall positive feedback effect on the variation in the air-sea CO\({}_{2}\) exchange flux. Within the 24 hour period, the mean values of F\({}_{\rm CO2}\) and F\({}_{\rm CO2 bio}\) were both positive in the near-shore region, indicating that CO\({}_{2}\) was being released into the atmosphere, and microbial degradation of organic matter accounted for a large part of this. In the front and offshore regions, the daily mean values of F\({}_{\rm CO2}\) and F\({}_{\rm CO2 bio}\) were both negative, indicating that these areas absorb CO\({}_{2}\) from the atmosphere and that phytoplankton also fixes CO\({}_{2}\) from the atmosphere into the ocean. The daily averages of Cont  
Figure 8: Correlations between Cont and NEP in the near-shore (**a**), front (**b**), and offshore (**c**) regions in the mixed layer in summer. of stations from west to east were 32% (\(\pm\)40%), 34% (\(\pm\)216%), and 9% (\(\pm\)13%), respectively. Cont reached 360% in the front region. Under the mixed layer, the near-shore, front, and offshore regions could be potential carbon sources for the atmosphere. Therefore, the CO\({}_{2}\) sink region might become a source when there is a tropical storm or upwelling process that overturns the water from the deep. Under the mixed layer, the daily means of the potential contribution of biological processes to air-sea CO\({}_{2}\) exchange flux were 34% (\(\pm\)43%), 8% (\(\pm\)13%), and 19% (\(\pm\)24%) within the 24-h period, respectively. In addition, in the mixed layer, the near-shore region was shown to be a typical heterotrophic system, while the front and offshore regions are both autotrophic systems. Conversely, in all three regions, under the mixed layer is heterotrophic. However, at different time points, the trophic statuses change, even in the same region. At a short timescale or in a steady-state environment, these conclusions can accurately represent the influence of biological processes on the variation in air-sea CO\({}_{2}\) exchange flux and can be used to assess the trophic statuses in the Changjiang River estuary plume in summer. Nevertheless, the biochemical and hydrological conditions in coastal regions constantly change at high frequency; thus, the use of data with high spatial and temporal resolutions is necessary to study the contribution of biological processes to the air-sea CO\({}_{2}\) exchange flux and to more accurately quantify the potential carbon stock of deep water bodies. Further, variations in long-term trophic statuses require additional exploration, especially in coastal waters, given the intensity of human activities and quickly progressing climate change. Conceptualization, Y.Z. and K.W. ; Formal analysis, Y.Z. and D.L. ; Investigation, K.W. and B.X. ; Writing-original draft, Y.Z. ; Writing-review & editing, D.L. and K.W.