# Seasonal Variations of Carbon Dioxide, Water Vapor and Energy Fluxes in Tropical Indian Mangroves
## 4 Results and Discussions
### Net Carbon Sequestered  
Figure 11 shows the daily 30-min average of the NEP partitioned into GPP and respiration (Re). The bold lines represent the weekly average for the partitioned fluxes. Positive fluxes of NEP and GPP represent net sequestration (sink) by the forest ecosystem whereas negative values indicate a net loss (source) of CO\({}_{2}\) to the atmosphere. Respiration represents the net release of CO\({}_{2}\) from the ecosystem to the atmosphere. During the study period, daily GPP estimates ranged from 3 to 5.5 g C m\({}^{-2}\cdot\)day\({}^{-1}\) during October and November 2012, and between 2.5 to 4 g C m\({}^{-2}\cdot\)day\({}^{-1}\) during January and February 2013. Greater fluctuations (1 to 4.5 g C m\({}^{-2}\cdot\)day\({}^{-1}\)) were recorded during monsoon periods (July and August 2012), which may be attributed to less availability of photosynthetic radiation during cloudy/rainy days. The annual variation of the day-wise ecosystem respiration was also similar to GPP, with maximum values of around 5.5 g C m\({}^{-2}\cdot\)day\({}^{-1}\) during October and November 2012 and minimum values of 2 g C m\({}^{-2}\cdot\)d\({}^{-1}\) during January and February 2013. Figure 11 also indicates that the daily NEPs are positive throughout the majority of days during the study period, suggesting that the ecosystem  
Figure 10: Surface energy closure in daytime in the Sundarbans mangroves during April 2012 to March 2013 with half-hourly fluxes. Soil heat flux includes energy storage between the surface and the 5 cm soil. is a sink for CO\({}_{2}\). During September to December 2012, the values of NEP were measured to be nearly 2.5 g C m\({}^{-2}\). day \({}^{-1}\) and they decreased to around 0.5 g C m\({}^{-2}\). day \({}^{-1}\) during January to March 2013. Over one year (April 2012 to March 2013), the estimated NEP was 249 \(\pm\) 20 g C m\({}^{-2}\). year\({}^{-1}\). The estimated GPP and Re were 1271 g C m\({}^{-2}\). year\({}^{-1}\) and 1022 g C m\({}^{-2}\). year\({}^{-1}\), respectively (Table 1). The estimated NEP values (249 g C m\({}^{-2}\). year\({}^{-1}\)) during April 2012 to March 2013 fall in the reported range for terrestrial ecosystems [45; 46; 47]. In comparison with the mangroves of the Florida Everglades (NEP = 1170 \(\pm\) 145 g C m\({}^{-2}\). year\({}^{-1}\)), the estimated NEP of Sundarbans is quite low and is attributed to the fact that the Everglades mangroves are dense with trees of greater heights, have a high leaf litter rate and also have a greater wood production [17]. Also, the species composition which contributes to biomass and, in turn, NEP is different in the wet evergreen system of Sundarbans (predominant species _viz. Avicennia alba_, _Bruguiera gymnorrhiza_ and _Rhizophora_) in comparison with the mangroves of the Everglades (predominant species _viz. Rhizopara mangle_, _Avicennia germinans_ and _Laguncularia racemosa_). The presence of significantly less biomass (10-50 t\(\cdot\) ha\({}^{-1}\)) in the Sundarbans mangrove forest [48] in comparison with the high biomass of around 150-200 t\(\cdot\) ha\({}^{-1}\) in the Everglades mangroves [49] explains the huge difference in the NEP. Figure 11: Daily average of partitioned flux into gross primary productivity (GPP), respiration (Re) and net ecosystem productivity (NEP) over the study period. Bold lines represent seven-day running mean. The measured respiration rates (2 to 4 \(\upmu\)mol\(\cdot\)m\({}^{-2}\)\(\cdot\)s\({}^{-1}\)) were low throughout the year in comparison with other forest types. It was reported by the measurements across terrestrial AmeriFlux and EuroFlux sites that the respiration rates range from 3.72 \(\pm\) 2.20 to 5.92 \(\pm\) 4.40 \(\upmu\)mol\(\cdot\)m\({}^{-2}\)\(\cdot\)s\({}^{-1}\) at different soil temperatures of 15 \({}^{\circ}\)C and 20 \({}^{\circ}\)C,respectively [38]. In the study area, though high soil temperatures (20 \({}^{\circ}\)C to 30 \({}^{\circ}\)C) were recorded, the observed respiration rates were less than half of the globally reported values.