**5. Conclusions**

Using a mass balance model, we calculated the NEP, FCO2bio, and FCO2 at eight time-points per day in the near-shore, front, and o ffshore regions of the Changjiang River estuary plume in summer. Then, we calculated the contribution of biological processes to FCO2 in the three regions. In the mixed layer, both FCO2 and FCO2bio significantly varied at di fferent times within the 24-h period. The near-shore region was found to be a source of atmospheric CO2, and the o ffshore region is a sink for atmospheric CO2. The front region is a sink for atmospheric CO2 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 e ffect on the variation in the air–sea CO2 exchange flux. Within the 24 hour period, the mean values of FCO2 and FCO2bio were both positive in the near-shore region, indicating that CO2 was being released into the atmosphere, and microbial degradation of organic matter accounted for a large part of this. In the front and o ffshore regions, the daily mean values of FCO2 and FCO2bio were both negative, indicating that these areas absorb CO2 from the atmosphere and that phytoplankton also fixes CO2 from the atmosphere into the ocean. The daily averages of Cont of stations from west to east were 32% (±40%), 34% (±216%), and 9% (±13%), respectively. Cont reached 360% in the front region. Under the mixed layer, the near-shore, front, and o ffshore regions could be potential carbon sources for the atmosphere. Therefore, the CO2 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 CO2 exchange flux were 34% (±43%), 8% (±13%), and 19% (±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 o ffshore regions are both autotrophic systems. Conversely, in all three regions, under the mixed layer is heterotrophic. However, at di fferent 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 CO2 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 CO2 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.

**Author Contributions:** 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.

**Funding:** This study was jointly supported by the National Natural Science Foundation of China (U1609201, U1709201, 91128212, 41203085, and 41206085), the Public Science and Technology Research Funds Projects of Ocean (201105014 and 201205015), the Scientific Research Fund of the Second Institute of Oceanography, State Oceanic Administration, China (JT1603), the Natural Science Project of Zhejiang Province (Y5110171,LQ17D060006), and the Project of State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography (SOEDZZ1402 and SOEDZZ1521).

**Acknowledgments:** The authors would like to thank the crew of R/V "China Marine Surveillance 49" for their supports in sampling and logistics. We also thank Wei-jun Cai (University of Delaware), Zhaohui Aleck Wang (Woods Hole Oceanographic Institution), Quanzhen Chen and his group, Daji Huang and his group, Jianfang Chen, Bin Wang and Tianzhen Zhang (Second Institute of Oceanography, Ministry of Natural Resources) for their technical support and helpful comments.

**Conflicts of Interest:** The authors declare no conflict of interest.
