# Response of Mangrove Carbon Fluxes to Drought Stress Detected by Photochemical Reflectance Index
## 3 Results  
### Temporal Variations of Environmental Factors and Carbon Fluxes  
Significant seasonal patterns of PAR were observed with higher and lower mean values in summer and winter, respectively (Figure 2a). On an annual scale, the mean values of PAR in 2020 were higher than in previous years, especially in summer when the mean value of 2020 reached 1.21 mmol m\({}^{-2}\) s\({}^{-1}\) with other years only about 1.00 mmol m\({}^{-2}\) s\({}^{-1}\) (Table 1). The air temperature shared a similar seasonal pattern with PAR, and the seasonal mean value of summer in 2020 was 1 \({}^{\circ}\)C higher than previous summers. The seasonal patterns of VPD were similar with air temperature, presenting a slight difference among four years with higher VPD in summer and autumn, especially from late 2019 to late 2020 (Figure 2b, Table 1). Furthermore, the mean value of VPD for each season in 2020 was higher than in previous years, with the mean value of summer (up to 1.25 kPa) significantly higher than other seasons (Table 1). This can be explained by less rainfall from autumn in 2019 to summer in 2020, leading to drought stress over this period (Figure 2c). Over this period, the cumulative rainfall only reached 778.9 mm, roughly equivalent to the rainfall of summer (774.8 mm) in 2018 and even less than that of summer (814.1 mm) in 2017 (Table 1). We also compared the amount of rainfall over this period with the multi-year (2011-2019) rainfall measurements from a meteorological station of the China Meteorological Administration (~10 km away from the study area). The rainfall over this period only accounted for approximately half of the multi-year mean annual rainfall. The daily maximum water level tended to be higher in autumn and showed no obvious difference among years (Figure 2b). The salinity was higher in winter among seasons and the average salinity was relatively higher in 2020 than in previous years (Figure 2c).  
The seasonal LUE varied from 0.013 to 0.028 mol mol\({}^{-1}\) and tended to show a U-shaped pattern with higher LUE in spring and winter and lower LUE in summer and autumn (Figure 3a). Compared with annual mean values in previous years, the mean values in 2020 decreased by 12%. GPP and NEE shared similar seasonal patterns, showing higher values in spring and summer (Figure 3b). Compared with the annual mean values of GPP in previous years (16.71, 15.88, and 16.51 umol m\({}^{-2}\) s\({}^{-1}\)), the annual mean values in 2020 decreased by 9.0%. Similarly, the mean values of NEE in 2020 decreased by 10.7% in comparison with previous years (13.22, 12.53, and 12.98 umol m\({}^{-2}\) s\({}^{-1}\)) (Table 1). The decrease in the carbon sink appeared to lag behind the increasing VPD and decreasing rainfall (Figures 2 and 3).  
Figure 2: Seasonal and annual variations in daytime mean values of environmental variables ((**a**): photosynthetically active radiation and air temperature, **(**b**): vapor pressure deficit and maximum surface water level, **(**c**): rainfall and surface water salinity) from January 2017 to December 2020. Lines and bars indicate monthly mean values. PAR = photosynthetically active radiation; Tair = air temperature; VPD = vapor pressure deficit.