## 4 Discussion
### 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 [PERSON] 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 agreement with [PERSON] 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  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Regions** & **00:00** & **03:00** & **06:00** & **09:00** & **12:00** & **15:00** & **18:00** & **21:00** & **Mean** \\ \hline Near-shore & 96\% & 42\% & \(-\)16\% & \(-\)6\% & 90\% & 39\% & 5\% & 4\% & 32\% \\ Front & 360\% & 63\% & 79\% & \(-\)269\% & \(-\)341\% & 126\% & 78\% & 175\% & 34\% \\ Offshore & 19\% & 13\% & 15\% & 25\% & 1\% & \(-\)20\% & 1\% & 19\% & 9\% \\ \hline \hline \end{tabular}
\end{table}
Table 5: The contribution of CO\({}_{2}\) flux variation caused by biological processes to F\({}_{\rm{CO2}}\) (Cont) in the mixed layer.  
Figure 7: Twenty-four-hour variations in potential carbon flux (\"F\({}_{\rm{CO2}}\)) and the biological contribution to carbon flux (\"F\({}_{\rm{CO2 Bio}}\)) in the near-shore (**a**), front (**b**), and offshore (**c**) regions in summer.  
of kilometers). In 24 hours, eddies and LSW have little effect on the variation in water properties. Therefore, we suggest 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 offshore regions show that trophic status can be used as an index of the contribution of biological process to the air-sea CO\({}_{2}\) flux. Cont in the near-shore region has a significantly negative correlation (\(r^{2}\) = 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 F\({}_{\rm CO2}\). When there is no biological contribution to F\({}_{\rm CO2}\) (Cont = 0), the NEP background value is \(-\)0.003 mmol C m\({}^{-3}\) 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 offshore region was significantly and positively correlated with Cont (\(r^{2}\) = 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 F\({}_{\rm CO2}\). Assuming that there are no biological processes in the offshore region (Cont = 0), the background value of NEP was also 0.03 mmol C m\({}^{-3}\) day\({}^{-1}\) (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 CO\({}_{2}\) exchange flux in the near-shore region than that in the offshore region when the two systems have an equal trophic status level.