# Seasonal Variations of Carbon Dioxide, Water Vapor and Energy Fluxes in Tropical Indian Mangroves
## 4 Results and Discussions
### NEE and Photosynthetic Active Radiation (PAR)  
Since the photosynthetic activity is limited to the availability of photons rather than other limiting factors, daily variations in GPP were also correlated with daily averaged photosynthetic active radiation (PAR). GPP shows a positive non-linear relation with daily averaged PAR, indicating that GPP increases with PAR up to a certain value and saturates further (Figure 12). Though Sundarbans mangroves exhibit less phenological changes, a relative increase in NEE sequestration was observed during September 2012 to December 2012 compared to other months (Figure 6). Accordingly, relationships of monthly diurnal averaged NEE with monthly diurnal averaged PAR were analyzed in two phases. During September 2012 to December 2012, NEE increased incrementally with PAR until reaching 900 \(\upmu\)mol\(\cdot\)m\({}^{-2}\)\(\cdot\)s\({}^{-1}\), thereafter leveling off at -8 \(\upmu\)mol\(\cdot\)m\({}^{-2}\)\(\cdot\)s\({}^{-1}\) incrementally with PAR (Figure 13). During the remaining period, NEE showed a similar trend in response with PAR, but saturated at lower levels of sequestration around 5.5 \(\upmu\)mol\(\cdot\)m\({}^{-2}\)\(\cdot\)s\({}^{-1}\) (Figure 13). \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Estimated Values** & **\% Gap-Filled** \\ \hline Bias Error (CO\({}_{2}\) flux) & 0.13 \(\pm\) 0.1 \(\upmu\)mol\(\cdot\)m\({}^{-2}\)\(\cdot\)s\({}^{-1}\) & \\ RMSE (CO\({}_{2}\) flux) & 1.81 \(\pm\) 0.24 \(\upmu\)mol\(\cdot\)m\({}^{-2}\)\(\cdot\)s\({}^{-1}\) & \\ Net Ecosystem Productivity (NEP) & 249 \(\pm\) 20 g C m\({}^{-2}\)\(\cdot\)year\({}^{-1}\) & 35\% \\ Gross Primary Productivity (GPP) & 1271 g C m\({}^{-2}\)\(\cdot\)year\({}^{-1}\) & 35\% \\ Ecosystem Respiration (Reco) & 1022 g C m\({}^{-2}\)\(\cdot\)year\({}^{-1}\) & 35\% \\ RMSE (H\({}_{2}\)O flux) & 0.73 \(\pm\) 0.12 \(\upmu\)mol\(\cdot\)m\({}^{-2}\)\(\cdot\)s\({}^{-1}\) & \\ Evapotranspiration (ET) & 715 \(\pm\) 120 mm\(\cdot\)year\({}^{-1}\) & 20\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary table of annual estimates of CO\({}_{2}\) fluxes and evapotranspiration rates (ET) over the study period. Figure 12: Relationship between daily GPP and daily average photosynthetic absorbed radiation (PAR) during daytime. Apparent quantum yield (\(\alpha\)), described as the number of molecules of CO\({}_{2}\) fixation per unit of photons irradiated on the leaf, plays an important role in understanding the tree photosynthetic response, and estimating \"\(\alpha\)\" would provide useful information for ecosystem models over larger areas [50]. Apparent quantum yield is often calculated by fitting a rectangular hyperbolic function between NEE and PAR, where there is a near-linear response during low light conditions and saturation to maximum productivity at higher light conditions. The ecosystem photosynthetic parameters in the present study are estimated using the Michaelis-Menten equation as given below and as described by [PERSON] _et al._[51]. \[NEE=\frac{\alpha*PAR*P_{max}}{\alpha*PAR+P_{max}}-R_{e}\]  
where \(\alpha\) (\(\mu\)mol CO\({}_{2}\)\(\mu\)mol\(\cdot\)photon\({}^{-1}\)) is the ecosystem apparent quantum yield, \(P_{max}\) (\(\mu\)mol CO\({}_{2}\) m\({}^{-2}\). s\({}^{-1}\)) is the maximum photosynthetic capacity and \(Re\) (\(\mu\)mol CO\({}_{2}\) m\({}^{-2}\). s\({}^{-1}\)) is the daytime ecosystem respiration. The month-wise estimated coefficients are shown in Table 2. Estimated quantum yield varied from a minimum of 0.0205 \(\pm\) 0.0019 (95% confidence) in May 2012 to a maximum of 0.0398 \(\pm\) 0.0038 in September 2012. Higher quantum yield in September 2012 is observed to be in accordance with favorable growing conditions at the study site, and lower quantum yield during May 2012 could be mainly attributed to high temperatures recorded at the study site.