# Characteristics of Carbon Fluxes and Their Environmental Control in Chenhu Wetland, China
## 3 Results and Discussion
### Relationships Between NEE and Some Environmental Parameters
Furthermore, the ecosystem's apparent initial radiation use efficiency (\(\alpha\)), maximum photosynthetic capacity (P\({}_{\text{max}}\)), and daytime ecosystem respiration rate (R\({}_{\text{eco,day}}\)) within the Chenhu wetland exhibited notable seasonal fluctuations. The variable \(\alpha\) reached its peak value in April, followed by a gradual decline. In contrast, P\({}_{\text{max}}\) attained its maximum in August, with subsequent peaks in May and July, and recorded its lowest value in September. R\({}_{\text{eco,day}}\) was observed to be at its highest in both April and May, followed by July and August, while it reached its minimum in September. The mean value of \(\alpha\) during the growing season was determined to be 0.0398 \(\upmu\)molCO\({}_{2}\cdot\upmu\)mol\({}^{-1}\), which is lower than the values reported for need wetlands such as the Yellow River Delta (0.0545 \(\upmu\)molCO\({}_{2}\cdot\upmu\)mol\({}^{-1}\)) [49], Panjin wetland (0.065 \(\upmu\)molCO\({}_{2}\cdot\upmu\)mol\({}^{-1}\)) [50; 5], and Daijuhu peatland (0.0559 \(\upmu\)molCO\({}_{2}\cdot\upmu\)mol\({}^{-1}\)) [19]. The average value of P\({}_{\text{max}}\) was calculated to be 17.30 \(\upmu\)molCO\({}_{2}\cdot\upmu\)\({}^{-2}\cdot\)s\({}^{-1}\), which is comparable to that of the eastern Chongming tidal flat wetland (15.20 \(\upmu\)molCO\({}_{2}\cdot\upmu\)\({}^{-2}\cdot\)s\({}^{-1}\)) [43] and exceeds that of the Daijuhu peatland (11.85 \(\upmu\)molCO\({}_{2}\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\)) [19]. This may be attributed to the fact that the average daily PAR values during the growing season in the Chenhu wetland (ranging from 355.62 to 2215.35 \(\upmu\)mol\({}^{-2}\cdot\)s\({}^{-1}\)) were greater than those observed in the Daijuhu peatland (which ranged from 103.32 to 1253.23 \(\upmu\)mol\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\)) [19]. The fluctuations in the values of P\({}_{\text{max}}\) and \(\alpha\) are associated with vegetation growth and environmental conditions. In 2022, the Chenhu wetland experienced lower precipitation levels compared to previous years, particularly after April. Drought conditions are known to adversely affect the photosynthetic parameters (P\({}_{\text{max}}\) and \(\alpha\)) of plants. Furthermore, it has been observed that the photosynthetic parameters (P\({}_{\text{max}}\) and \(\alpha\)) tend to decline towards the end of the growing season, correlating with reductions in leaf area index and temperature [51]. This trend aligns with the variations in photosynthetic parameters observed in other ecosystems [52]. Figure 7: Scatter plots of daytime NEE against PAR from April to September 2022. The fitting results and correlation coefficients between daytime NEE and PAR are also shown. The response process of CO\({}_{2}\) flux to light in the Chenhu wetland during the growing season, under varying VPD conditions, is illustrated in Figure 8. The results indicate that PAR and daytime NEE demonstrate distinct response patterns contingent upon the levels of VPD. Specifically, when VPD is below 0.5 kPa, PAR levels are low, which hampers the ability of plant leaves to reach light saturation during photosynthesis. A strong linear correlation between NEE and PAR was observed during daylight hours, aligning with the findings of previous studies conducted by [PERSON] [48] and [PERSON] [53]. Conversely, when VPD exceeds 0.5 kPa, NEE during daylight hours progressively transitions to negative values alongside an increase in PAR. This transition signifies an enhancement in carbon dioxide absorption by the wetland ecosystem. A significant rectangular hyperbolic relationship was observed between these two variables, as depicted in Figure 8. Additionally, both the P\({}_{\text{max}}\) and \(\alpha\) were found to be elevated when VPD ranged from 0.5 to 1.5 kPa, in contrast to conditions where VPD surpassed 1.5 kPa. When VPD exceeds 1.5 kPa, an increase in PAR correlates with an enhancement in daytime carbon dioxide absorption.