**4. Discussion**

### *4.1. Variations in FCO2bio and FCO2 in the Mixed Layer*

FCO2 is strongly positive in each timeslot (Figure 6a), indicating that this region acts as a source of atmospheric CO2 [43,44] because the near-shore region is affected by the CDW [45] which has abundant *p*CO2 [25,26]. FCO2bio is strongly positive most of the time (Figure 6a), meaning that heterotrophic respiration releases CO2 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*CO2 value in the near-shore region.

**Figure 6.** Twenty-four hour variations in FCO2, FCO2bio, and FCO2non-bio in the near-shore (**a**), front (**b**), and offshore (**c**) regions in summer.

The 24-h FCO2 in the front region is almost negative (Figure 6b), indicating that the front region acts as a sink for atmospheric CO2. The 24-h FCO2bio in the front region is also almost negative which indicates the biological processes absorb CO2 (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 grea<sup>t</sup> capacity for biological productivity and that a large amount of CO2 is fixed in the surface water by phytoplankton.

Most FCO2 values in the offshore region are mostly slightly less than 0 (Figure 6c), indicating that the offshore region acts as a sink for atmospheric CO2. This finding is in agreemen<sup>t</sup> with the study by Song et al. [2]. FCO2bio in the offshore region was mostly slightly less than 0 (Figure 6c), indicating that the photosynthesis rate of fixed CO2 by phytoplankton is higher than degradation rates of organic matter releasing CO2 by microbial action in the offshore region.

### *4.2. The Contribution of Biological Processes to the Air–Sea CO2 Exchange Flux in the Mixed Layer*

FCO2 in the mixed layer of the three regions shows that the near-shore region acts as a strong source of atmospheric CO2 (Figure 6a) and that the front and offshore regions act as sinks for atmospheric CO2 (Figure 6b,c), similar to the results of other studies [1,48,49]. The daily average Cont in the mixed layer shows that the biological processes have a positive feedback effect on air–sea CO2 exchange in the near-shore, front, and offshore regions (Table 5). This agrees with the conclusion of Borges et al. [15]. The air–sea CO2 flux is inversely proportional to the NEP in the mixed layer, indicating that the contribution to the variation in air–sea CO2 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 CO2 flux.


**Table 5.** The contribution of CO2 flux variation caused by biological processes to FCO2 (Cont) in the mixed layer.

### *4.3. Potential Carbon Sources under the Mixed Layer*

Under the mixed layer (Table 4), the water column is determined to be a potential carbon source of atmospheric CO2 in the three regions (Figure 7a–c). The variations in \*FCO2 and \*FCO2bio show that the near-shore, front, and offshore regions could be potential atmospheric carbon sources, and a large amount of CO2 produced by biological processes (e.g., respiration) is stored under the mixed layer (Figure 7). Although the surface water in the front and offshore regions acts as a sink for atmospheric CO2, respiration under the mixed layer will result in the degradation of organic matter with substantial CO2 release, which could be observed when vertical mixing occurred [27]. Hence, the CO2 sink region in the Changjiang River estuary plume will become a source region when there is a tropical storm or an upwelling process. In a relevant study of the East China Sea, Chen et al. [4] also proposed that phytoplankton and planktonic bacteria could store dissolved inorganic carbon in the subsurface and might affect the surface air–sea CO2 flux. Further, the daily means (standard deviations in brackets) of the potential contribution of biological processes to air–sea CO2 exchange flux in the near-shore, front, and offshore regions are 34% (±43%), 8% (±13%), and 19% (±24%) in 24 hours, respectively, indicating that local respiration accounts for a large part of the total potential CO2 release under the mixed layer. Other factors probably include KSSW intrusion, temperature elevation, and so on, which need further exploration.

**Figure 7.** Twenty-four-hour variations in potential carbon flux (\*FCO2) and the biological contribution to carbon flux (\*FCO2bio) in the near-shore (**a**), front (**b**), and offshore (**c**) regions in summer.

### *4.4. Trophic Status Assessments and the Relationship between Cont and NEP*

The mixed layer in the front and offshore regions is an autotrophic system (Table 3), but that in the near-shore region is a heterotrophic system. On the whole, we consider the Changjiang River estuary plume to be an autotrophic ecosystem in summer, similar to the conclusion of Li et al. [11], in August 2006. The daily mean NEP values of the study region are negative under the mixed layer, indicating that they are heterotrophic systems, which is in agreemen<sup>t</sup> with Chou et al. [27]. However, the positivity or negativity of the NEP values changes throughout a 24-hour period, and the trophic status of the same region varies as well. The Changjiang River estuary plume has a complex current structure featuring multiple eddies [50] or low salinity water detachment (LSW) [16]; however, eddies and LSW are on the mesoscale in terms of time and space (e.g., a couple of weeks and hundreds

of kilometers). In 24 hours, eddies and LSW have little e ffect on the variation in water properties. Therefore, we sugges<sup>t</sup> that trophic statuses in a day are regulated by the tide.

In order to explore the influences of trophic status on Cont, we compared the Cont and NEP in the mixed layer in the region (Figure 8). The significant correlations between Cont and NEP in the mixed layer in the near-shore and o ffshore regions show that trophic status can be used as an index of the contribution of biological process to the air–sea CO2 flux. Cont in the near-shore region has a significantly negative correlation (*r2* = 0.94, *p* < 0.05) with NEP, indicating that the more heterotrophic the system, the greater the influence on the contribution of biological processes (e.g., organic matter degradation by microorganisms) to FCO2. When there is no biological contribution to FCO2 (Cont = 0), the NEP background value is −0.003 mmol C m<sup>−</sup><sup>3</sup> day−1. In the front region, the correlation between Cont and NEP is not significant (Figure 8c). This could be because there are opposing processes causing the trophic status on the east and west sides of the front region, the west side of the front region is dominated by the degradation of organic matter, while the east side is dominated by the absorption of dissolved inorganic carbon. When the tide has a continuous impact on the front region, the NEP in the front region would present a large fluctuation. The NEP of o ffshore region was significantly and positively correlated with Cont (*r2* = 0.94, *p* < 0.05), indicating that the more autotrophic the system, the greater the contribution of the biological processes (e.g., primary production) to FCO2. Assuming that there are no biological processes in the o ffshore region (Cont = 0), the background value of NEP was also 0.03 mmol C m<sup>−</sup><sup>3</sup> day−<sup>1</sup> (Figure 8c). In addition, the slopes of NEP and Cont show that the biological processes have a stronger influence on the variation in the air–sea CO2 exchange flux in the near-shore region than that in the o ffshore region when the two systems have an equal trophic status level.

**Figure 8.** Correlations between Cont and NEP in the near-shore (**a**), front (**b**), and o ffshore (**c**) regions in the mixed layer in summer.
