## 3 Results and Discussion
### Seasonal and Interannual Variations in Carbon Fluxes  
The daily mean NEE strongly fluctuated between \(-3.51\) g C m\({}^{-2}\) d\({}^{-1}\) and \(6.57\) g C m\({}^{-2}\) d\({}^{-1}\), with negative values (CO\({}_{2}\) uptake) accounting for approximately half (Figure 3a). The monthly mean NEE values were positive (CO\({}_{2}\) emissions) most of the time in 2019 (from January to September and December) and 2020 (from January to June and September) (Figure 3b). However, the positive monthly mean NEE values only occurred from April to July in 2021, as well as June and November in 2022 (Figure 3b). The highest positive NEE values often occurred in the spring (March in 2019 and 2020, May in 2021) and the summer (June in 2022), with monthly means of 33.93, 43.06, 14.27, and 4.73 g C m\({}^{-2}\) mon\({}^{-1}\) from 2019 to 2022, respectively. In contrast, the measurements indicated the strongest net CO\({}_{2}\) uptake in autumn, with the highest monthly mean values of \(-18.54\) g C m\({}^{-2}\) mon\({}^{-1}\) in November 2019, \(-27.56\) g C m\({}^{-2}\) mon\({}^{-1}\) in October 2020, \(-35.58\) g C m\({}^{-2}\) mon\({}^{-1}\) in November 2021, and \(-43.65\) g C m\({}^{-2}\) mon\({}^{-1}\) in September 2022 (Figure 3b). Figure 2: Temporal variations in daily (**a**) air and soil temperatures (Ta, Ts), (**b**) precipitation (P) and salinity (s), (**c**) vapor pressure deficit (VPD), (**d**) relative humidity (RH), (**e**) solar radiation (Ra), (**f**) leaf area index (LAI), (**g**) wind speed (WS), and (**h**) proportion of the land surface area (Pl). Figure 3: The seasonal variations in daily and monthly (**a,b**) NEE and (**c,d**) CH\({}_{4}\) fluxes. The long- and short-term variations in the NEE values were determined by photosynthesis and respiration. The daily GPP and Re displayed strong seasonal variabilities during the last 4 years, with relatively higher and lower values during summertime and wintertime (Figure 3a), respectively. The strongest daily GPP ranged between 4.37 g C m\({}^{-2}\) d\({}^{-1}\) and 9.28 g C m\({}^{-2}\) d\({}^{-1}\). Although the GPP had a strong carbon uptake (49.28-185.72 g C m\({}^{-2}\) mon\({}^{-1}\)) in the spring and summer, Re (88.55-196.27 g C m\({}^{-2}\) mon\({}^{-1}\)) offset or surpassed it most of the time in the first 3 years and finally resulted in a net carbon release. However, the Re values (84.94-167.93 g C m\({}^{-2}\) mon\({}^{-1}\)) were weaker than the GPP values (89.45-167.97 g C m\({}^{-2}\) mon\({}^{-1}\)) most of the time in autumn, which led to a net carbon sink in this season (Figure 3b). The source areas of daily GPP from mangrove photosynthesis accounted for 55% (19-78%) of the footprint area. Except for mangrove respiration, the source areas of daily Re also consisted of 35% (12-62%) open water area and 10% (1-60%) waste fishpond area. On average, the Re values surpassed the GPP values by 12% and 7% in 2019 and 2020, respectively, while they offset 92% and 86% of GPP values in 2021 and 2022, respectively (Table 1). As a result, the source area in the mangrove park displayed net CO\({}_{2}\) sources of 175.70 g C m\({}^{-2}\) a\({}^{-1}\) and 81.92 g C m\({}^{-2}\) a\({}^{-1}\) in 2019 and 2020, and net CO\({}_{2}\) sinks of \(-102.89\) g C m\({}^{-2}\) a\({}^{-1}\) and \(-194.53\) g C m\({}^{-2}\) a\({}^{-1}\) (Table 1) in 2021 and 2022, respectively. After the tower replacement, the proportion of land surface (PI) decreased in the last 2 years (Table 1). Our results imply that Re from open water may be weaker than that from mangroves, which resulted in a decrease in the total Re and finally converted the CO\({}_{2}\) sources to sinks (Table 1). CH\({}_{4}\) emissions cannot be overlooked in mangrove parks. Strong CH\({}_{4}\) sources of 23.18-26.34 g C m\({}^{-2}\) a\({}^{-1}\) were observed during the last 4 years (Table 1). These measurements had no significant interannual trend but had strong seasonal variations (Table 1), with higher and lower CH\({}_{4}\) emissions in the summer and winter (Figure 3c).