Carbon Biogeochemistry of the Estuaries Adjoining the Indian Sundarbans Mangrove Ecosystem: A Review

The present study reviewed the carbon-biogeochemistry-related observations concerning CO2 and CH4 dynamics in the estuaries adjoining the Indian Sundarbans mangrove ecosystem. The review focused on the partial pressure of CO2 and CH4 [pCO2(water) and pCH4(water)] and air–water CO2 and CH4 fluxes and their physical, biogeochemical, and hydrological drivers. The riverine-freshwater-rich Hooghly estuary has always exhibited higher CO2 emissions than the marine-water-dominated Sundarbans estuaries. The mangrove sediment porewater and recirculated groundwater were rich in pCO2(water) and pCH4(water), enhancing their load in the adjacent estuaries. Freshwater-seawater admixing, photosynthetically active radiation, primary productivity, and porewater/groundwater input were the principal factors that regulated pCO2(water) and pCH4(water) and their fluxes. Higher chlorophyll-a concentrations, indicating higher primary production, led to the furnishing of more organic substrates that underwent anaerobic degradation to produce CH4 in the water column. The northern Bay of Bengal seawater had a high carbonate buffering capacity that reduced the pCO2(water) and water-to-air CO2 fluxes in the Sundarbans estuaries. Several authors traced the degradation of organic matter to DIC, mainly following the denitrification pathway (and pathways between aerobic respiration and carbonate dissolution). Overall, this review collated the significant findings on the carbon biogeochemistry of Sundarbans estuaries and discussed the areas that require attention in the future.


Introduction
The term "global warming" and its connection with greenhouse gases and ocean circulations came to the limelight in 1975 [1], and the scientific community observed a consistent increase in the global temperature to cross the baseline values since 1976 [2]. In this regard, the Earth Summit in 1992 realized the need for a global partnership for sustainable development, ensuring environmental protection and improved human life. Later, in 2015, the UN Sustainable Development Summit created 17 sustainable development goals (SDGs) to achieve a world with reduced effects from climate change (https://sdgs.un.org/goals, accessed on 30 September 2022). Since the advent of global warming and climate change, countless studies have tried to understand the interactions between climate change and the Earth's environmental systems. Throughout the industrial period, greenhouse gas emissions have increased significantly, and the environment and oceans have received increased amounts of carbon, nitrogen, and phosphorous. This excess carbon and nutrients affected the coastal zones, which include tidal wetlands, estuaries, and continental shelf waters [3]. This anthropogenic loading changes the nutrient cycling and greenhouse gas emissions from coastal systems [4].
The Hooghly and Matla estuaries are the most studied regions in the Indian Sundarbans, a mangrove-dominated estuarine system ( Figure 1). Mukhopadhyay et al. [63] studied the monthly variation of CO 2 fluxes between the surface water of the Hooghly River and the associated atmosphere during 1999 (Table S1). Apart from CO 2 fluxes, they also measured TAlk, salinity, dissolved oxygen (DO), gross primary production (GPP), and community respiration (CR). Later, Padhy et al. [64] developed an algorithm to compute the pCO 2 and CO 2 flux from remotely sensed data on sea surface temperature (SST) and chlorophyll a (Chl-a) concentration and in situ measurements in the Hooghly estuary. Sev-Life 2023, 13, 863 5 of 24 eral studies characterized the spatial and temporal (seasonal and diurnal) variations in CO 2 fluxes in this region [65][66][67]. Akhand et al. [68] and Dutta et al. [69] compared the CO 2 flux dynamics between the Hooghly and the Matla estuaries. The role of groundwater on inorganic carbon dynamics and pCO 2 in the Hooghly and Sundarbans estuarine systems was revealed by Akhand et al. [70] using a coupled automated 222 Rn and pCO 2(water) measurement system. All these studies also measured associated carbon parameters such as dissolved inorganic carbon (DIC) (Table S2), dissolved organic carbon (DOC), and particulate organic carbon (POC) (Table S3), along with pH, salinity, SST, TAlk, wind speed, and Chl-a. Besides the Hooghly and Matla estuaries, the Mooriganga, Saptamukhi, and Thakuran estuaries also received substantial attention regarding carbon biogeochemistry [71][72][73][74][75]. Several researchers also covered parts of Edward Creek, Herobhanga River, and Lothian Island [73,76] (Table S4). Relatively fewer studies are available on CH 4 dynamics in the Indian Sundarbans (Table S5). Biswas et al. [77] studied the CH 4 flux dynamics covering the Sundarbans estuaries (Muriganga, Saptamukhi, and Thakuran Rivers), Lothian Island, and the Hooghly estuary (Diamond Harbour, Kachuberia, Beguakhali) in 2003. This study reported the spatial and temporal changes in the CH 4 fluxes. Neetha [78] compared the CH 4 flux between Jharkhali Island (Sundarbans estuary) and the Hooghly estuary. Later, Dutta et al. [69,[79][80][81][82] studied the CH 4 flux around Lothian Island, the Hooghly estuary, and the Saptamukhi estuary.
River and the associated atmosphere during 1999 (Table S1). Apart from CO2 fluxes, they also measured TAlk, salinity, dissolved oxygen (DO), gross primary production (GPP), and community respiration (CR). Later, Padhy et al. [64] developed an algorithm to compute the pCO2 and CO2 flux from remotely sensed data on sea surface temperature (SST) and chlorophyll a (Chl-a) concentration and in situ measurements in the Hooghly estuary. Several studies characterized the spatial and temporal (seasonal and diurnal) variations in CO2 fluxes in this region [65][66][67]. Akhand et al. [68] and Dutta et al. [69] compared the CO2 flux dynamics between the Hooghly and the Matla estuaries. The role of groundwater on inorganic carbon dynamics and pCO2 in the Hooghly and Sundarbans estuarine systems was revealed by Akhand et al. [70] using a coupled automated 222 Rn and pCO2(water) measurement system. All these studies also measured associated carbon parameters such as dissolved inorganic carbon (DIC) (Table S2), dissolved organic carbon (DOC), and particulate organic carbon (POC) (Table S3), along with pH, salinity, SST, TAlk, wind speed, and Chl-a. Besides the Hooghly and Matla estuaries, the Mooriganga, Saptamukhi, and Thakuran estuaries also received substantial attention regarding carbon biogeochemistry [71][72][73][74][75]. Several researchers also covered parts of Edward Creek, Herobhanga River, and Lothian Island [73,76] (Table S4). Relatively fewer studies are available on CH4 dynamics in the Indian Sundarbans (Table S5). Biswas et al. [77] studied the CH4 flux dynamics covering the Sundarbans estuaries (Muriganga, Saptamukhi, and Thakuran Rivers), Lothian Island, and the Hooghly estuary (Diamond Harbour, Kachuberia, Beguakhali) in 2003. This study reported the spatial and temporal changes in the CH4 fluxes. Neetha [78] compared the CH4 flux between Jharkhali Island (Sundarbans estuary) and the Hooghly estuary. Later, Dutta et al. [69,[79][80][81][82] studied the CH4 flux around Lothian Island, the Hooghly estuary, and the Saptamukhi estuary.

Carbon Parameters and Total Alkalinity in Estuarine Water
Carbon parameters (DIC, DOC, and POC concentrations), TAlk, and other physicochemical parameters can regulate CO 2 and CH 4 fluxes. Researchers have extensively used the bulk formula method to estimate air-water CO 2 and CH 4 exchange rates in the Indian Sundarbans [83,84]. Computation of pCO 2(water) requires SST and sea surface salinity data, along with knowledge of DIC and TAlk (expressed in moles per kilogram of seawater), because the concentration of DIC and TAlk remains constant with changes in water temperature and pressure [85]. Different biogeochemical processes can influence the concentration of DIC in an aquatic system. Cycling of DIC is related to the production and utilization of CO 2 through photosynthesis and respiration, i.e., the metabolic state of an estuary [86].
Apart from the heterogenic activities in a system, DIC can originate from riverine and groundwater inputs, inputs from inter-tidal wetlands, and photodegradation of dissolved organic matter [87][88][89][90]. Hence, to understand the net autotrophy (sink) or heterotrophy (source) nature of an estuary or an open water system, analysis of CO 2 saturation is insufficient [91]. Thus, a detailed understanding of the sources, cycling patterns, and fate of DIC in the estuaries is needed to analyze the biogeochemical cycling of carbon and develop carbon budgets for coastal systems.
A significant fraction of the organic matter in estuarine and oceanic water remains dissolved [92][93][94]. Estuaries receive organic matter from adjacent land masses and local sources [95]. In the estuarine mixing processes, the terrestrial organic carbon undergoes rapid removal and decomposition [96]. Biodegradation is one of the primary processes responsible for removing DOC from estuarine and marine waters [97][98][99]. In many estuarine systems, respiration exceeds primary production [35,100], and the available organic matter is wholly or partially mineralized [98,99,101]. Thus, higher DOC may correlate with higher pCO 2 concentrations [102].
Globally, most of the estuaries are net sources of CO 2 , and along with DIC and DOC, land-derived POC plays a vital role in determining that characteristic [103]. Annually, mangroves export 28 Tg C of POC to the sea. Plant debris, phytoplankton, and microphytobenthos generate this POC [104]. This POC in estuarine water causes heterotrophic activity and influences the DIC composition in seawater through mineralization [35,105].
The TAlk represents the inorganic carbon content in the aquatic body. It is the sum of bicarbonate, carbonate, borate, and hydroxide ions [83]. TAlk, when produced more than DIC, is often considered to increase the sinking potential for atmospheric CO 2 since it increases the carbonate buffering capacity of seawater [106]. Rivers are the primary transporters of inorganic carbon [107]; moreover, they are a potent source of the same [48,[108][109][110]. The coastal ocean's source/sink characteristics depend on carbonate buffering capacity. Freshwater discharge, agricultural runoff, and groundwater discharge primarily regulate this buffering capacity [111,112]. Human activities such as deforestation, excessive agricultural practices, and events such as acid rain deposition cause a reduction in the carbonate buffering capacity of coastal systems [113,114]. Hence, along with other carbon parameters, the TAlk dynamics study is crucial in understanding coastal water's CO 2 source/sink behavior.

Spatial Variability
This region exhibited significant spatial variability in the carbon parameters and TAlk. Several studies have compared the DIC content of the anthropogenically disturbed Hooghly estuary with the less influenced Saptamukhi, Thakuran, and Matla estuaries (the Sundarbans estuary). They all reported higher DIC content in the Hooghly estuary than the others (Table S2). Ray et al. [84] observed similar findings from the Hooghly estuary and Lothian Island. Akhand et al. [70], while studying the influence of submarine groundwater discharge (SGD) on the DIC of estuarine water, reported about four times higher DIC content in the groundwater compared to the estuarine surface water. Though DIC has been well sampled throughout the spatial extent of this region, more endeavors are needed to measure the stable isotopic signatures of DIC to develop a better understanding of the DIC sources in this estuarine complex.
Compared to the DIC, fewer studies report DOC dynamics from the Indian Sundarbans region (Table S3). Ray et al. [73,84], Dutta et al. [67,69], and Akhand et al. [66,70]  estuaries. According to the study by Ray et al. [73], rather than the DOC formation and decomposition rates, the lower residence time of water in the mangroves was responsible for the low value of DOC in the Hooghly and Sundarbans estuaries. Akhand et al. [70] reported the surface water and groundwater DOC in the Hooghly and Matla estuaries. In the Matla estuary, DOC in groundwater was significantly lower than the surface water. Thus, groundwater did not prove to be a significant source of DOC in the Sundarbans estuarine system. As mangrove habitats are considered the primary source of DOC in mangrove-adjacent estuaries, studies focusing on DOC variability from the mangrove island periphery to open estuarine channels are required to understand the rate of DOC dilution in these waters.
Ray et al. [73,84] compared POC between the Hooghly and Saptamukhi estuaries ( Table S3). The studies suggested that POC in the Saptamukhi estuary has an autochthonous origin and that the Hooghly estuary is the primary source of terrestrial organic matter draining into Saptamukhi. The relatively higher POC in the Hooghly estuary compared to the Sundarbans estuaries (Saptamukhi, Matla) was attributed to increased freshwater flow in the estuary from upstream of the Hooghly River [66]. However, such measurements are required in all the major estuaries of this region to generate a more holistic scenario on this issue.
Several researchers studied the buffering capacity of the mangrove-dominated estuarine waters of the Indian Sundarbans (Table S2). Akhand et al. [76] studied the spatial variability of TAlk in the inner (Herobhanga), middle (Thakuran), and outer (Edward) estuaries. They did not observe variations in the TAlk values between the inner and middle estuaries; however, the outer estuary exhibited lower TAlk values. Akhand et al. [68] also reported similar results, where inner estuarine stations showed higher TAlk values than outer estuarine stations. Ray et al. [73] studied TAlk variability along the salinity gradient of the Hooghly estuary, the mangrove-dominated Saptamukhi estuary's adjacent coastal waters of the Bay of Bengal, in 2014. However, they reported a narrow range of TAlk for both regions. Ghosh et al. [83] studied the Hooghly estuary (from the upper to the lower estuary) and the adjacent coastal waters during 2015-2016. They observed TAlk values decreasing from the upper to the lower estuary. Dutta et al. [67,69] studied TAlk variations in the Hooghly and Sundarbans estuaries and reported no spatial differences. However, the Hooghly estuary exhibited wider variations. Akhand et al. [70] compared the TAlk in the surface water and groundwater between the Hooghly and Matla estuaries. Both surface water and groundwater TAlk were marginally higher in the Hooghly estuary. Compared to surface water, TAlk was significantly higher in groundwater for both regions. Acharya et al. [71] studied TAlk in the Saptamukhi, Thakuran, and Matla estuaries, and they did not record any significant variation.
Several researchers have studied pCO 2 in the Indian Sundarbans (Table S4). Sea surface temperature (SST) influences the pCO 2(water) by decreasing the solubility of CO 2 in water [115]. The pCO 2(water) in the estuary decreased with proximity to the open sea [76]. pCO 2(water) in Edward creek (0 km from the shoreline), Thakuran River (31 km from the shoreline), and Herobhanga River (55 km from the shoreline) were 507.98 ± 73.69 µatm, 518.84 ± 71.94 µatm, and 234.34 ± 64.08 µatm, respectively; whereas, pCO 2(air) showed comparatively less variability (386.73 ± 1.97 µatm, 386.80 ± 5.05 µatm, and 376.26 ± 2.75 µatm, respectively). Ray et al. [84] studied the pCO 2 in the riverine (992.5 µatm) and marine parts (372 µatm) of the Hooghly estuary and reported similar findings. Akhand et al. [68] compared the pCO 2(water) in the Hooghly and Matla estuaries during 2013-2014. Being an anthropogenically influenced estuary with a high river discharge, the annual mean pCO 2(water) value for the Hooghly estuary (~2200 µatm) was higher than the comparatively less anthropogenically disturbed Matla estuary (annual mean~530 µatm). Dutta et al. [67] reported similar observations in the Hooghly (267 µatm to 4678 µatm) and Sundarbans estuaries (376 µatm to 561 µatm). pCO 2 for the Hooghly estuary reduces gradually towards the offshore region. However, they did not report any spatial variation for the Sundarbans estuary [67]. Acharya et al. [71] studied the Saptamukhi, Thakuran, and Matla estuaries (Sundarbans estuary) from 2016 to 2020 and reported an average annual value of 993 ± 898 µatm. Though a substantial number of studies have been carried out in this regard, most endeavors indirectly estimated pCO 2(water) , which adds to the uncertainties in flux computation. More endeavors are needed to measure pCO 2(water) directly and continuously through the deployment of automated sensors.
Several researchers characterized the spatial variation in CO 2 flux between air and water (Table S4). Padhy et al. [64] measured the CO 2 flux covering the entire Hooghly estuary. They observed significant variation in CO 2 flux from the seaside to the upstream region due to the estuary's north-south orientation. The estuary acted as a source with variable magnitude (90 mol C m −2 year −1 during summer and 0.5 mol C m −2 year −1 during winter). According to Akhand et al. [76], the inner (Edward Creek) and middle estuary (Thakuran River) acted as net sources (29.69 and 23.62 mg CO 2 m −2 day −1 respectively), while the outer estuary acted as a net sink (−33.37 mg CO 2 m −2 day −1 ). The heterotrophic nature of the water column and the sediments [116,117] added to community respiration in the mangrove estuary, which largely contributed to the CO 2 source behavior of the inner and middle estuaries. At the same time, the dominant autotrophic characteristics of the outer estuary govern its sink behavior. Akhand et al. [68] compared the water-to-air CO 2 emissions from the Hooghly and Matla estuaries. They reported that the annual mean CO 2 emission from the Hooghly estuary was 14 times higher than that of the Matla estuary. The increased river runoff due to high river discharge was primarily responsible for the increased CO 2 efflux from the Hooghly estuary during the last decade [68]. Dutta et al. [67,69,72] reported similar observations on CO 2 efflux while working in the Hooghly and Sundarbans estuaries. Bacterial abundance in the highly nutrient-rich water of the urbanized river stretches triggered higher respiration and rapid mineralization [118], which in turn caused CO 2 supersaturation and higher CO 2 efflux [119,120]. Akhand et al. [65] compared the nature of CO 2 flux between the creeks of Dhanchi Island and the adjacent estuary. According to the study, the creeks acted as net sources of CO 2 , while the estuary was a net sink for CO 2 . Similar to pCO 2(water) , fluxes are estimated in most studies, which leaves a substantial scope of uncertainties. Direct flux measurements should be undertaken by future researchers by deploying chamber techniques to reduce the uncertainty in flux estimations.
Compared to other carbon parameters, fewer studies exist on the CH 4 dynamics and air-water flux in Sundarbans estuarine water (Table S5). Biswas et al. [77] studied the spatial dynamics of CH 4 in the Hooghly River estuary; in the stations, Diamond Harbour (inner estuary), Kachuberia (tip of Sagar Island, mid estuary), and Beguakhali (sea end/river mouth/outer estuary). Saptamukhi, Mooriganga, Thakuran, and the adjacent waters of the Lothian Island, were also studied. pCH 4 in the Hooghly estuary was lower than in the Saptamukhi, Mooriganga, and Thakuran river systems, indicating the latter received a higher input of methane-rich waters from the surrounding mangroves. Dutta et al. [80] measured pCH 4 in the surface water and porewater in the Saptamukhi estuary adjacent to Lothian Island. Compared to the groundwater, pCH 4 was marginally higher in the surface water. Dissolved CH 4 in water and CH 4 flux were compared between the Hooghly and Sundarbans estuaries by Dutta et al. [69] during the pre-monsoon of 2016. The CH 4 concentration in the Hooghly estuary varied over a broader range than in the Sundarbans estuary. In both estuaries, air-water CH 4 flux was always positive, suggesting efflux from water to air. In the Hooghly estuary, maximum and minimum CH 4 flux was reported from the freshwater and marine zones, respectively. Table 1 summarizes the range of all the parameters reviewed in this study in the inner, middle, and outer estuaries of the Indian Sundarbans. TAlk and DIC exhibited almost similar spatial variability in these estuaries. The minimum observed concentrations of these two parameters were similar in the inner, middle, and outer estuarine reaches. However, the maximum values decreased from the inner to the outer estuarine regions. The decrease in DIC from the inner to outer regions was higher than the corresponding decrease in TAlk. This observation again indicates the influence of the coastal waters Life 2023, 13, 863 9 of 24 having a high TAlk/DIC ratio, implying a high buffering capacity. The DOC range in the inner estuarine region was significantly higher than the middle and outer reaches, primarily due to the allochthonous anthropogenic inputs from the upper reaches. The POC, on the contrary, showed higher ranges in the outer estuarine reaches. This observation indicates the presence of autochthonous POC derived from the mangrove environment and allochthonous POC outflown, primarily through the Hooghly estuary. pCO 2(water) and air-water CO 2 flux mirrored each other in their spatial variabilities, which indicates that pCO 2(water) is the fundamental parameter that regulates the air-water CO 2 fluxes in these estuaries. The maximum values of pCO 2(water) and air-water CO 2 flux steadily decreased from the inner estuarine reaches to the outer region. This observation again unequivocally indicates that the seawater low in pCO 2(water) having a high buffering capacity leads to lower air-water CO 2 fluxes in the outer regions compared to the inner reaches, where freshwater plays a dominant role. CH 4 concentration in water and air-water CH 4 flux showed a similar spatial trend. The inner estuarine region, having higher organic carbon than the middle and outer reaches, provided ample substrate for methanogenic activities, which led to higher dissolved CH 4 levels in the water and eventually higher air-water CH 4 fluxes.

Temporal Variability
This section discusses the diurnal and seasonal variabilities of the carbon biogeochemistry-related parameters reported by several studies. Padhy et al. [64] studied the DIC in the Hooghly estuary (Table S2) and the adjacent coastal waters following an empirical equation [121]. They reported lower values in May and November 2014 (990 ± 27 µmol kg −1 and 1130 ± 29 µmol kg −1 , respectively) and relatively higher values in January, February, and September 2014 (1310 ± 107 µmol kg −1 , 1711 ± 19 µmol kg −1 , and 1697 ± 77 µmol kg −1 , respectively). The relative riverine water fluxes, which bring terrestrial organic sources into the estuarine system, influenced the lower and higher DIC in the respective months. Dutta et al. [67,69] studied DIC variability in the Hooghly and Sundarbans estuaries during pre-monsoon and post-monsoon. A 6% increase in DIC was observed during pre-monsoon in the Hooghly estuary, while DIC did not vary much in the Sundarbans estuary seasonally. Ray et al. [84] also reported marginally higher values of DIC in the Hooghly estuary during pre-monsoon than post-monsoon. However, Akhand et al. [66] showed that pre-monsoon DIC concentrations were significantly higher than the monsoon and post-monsoon concentrations.
Dutta et al. [72] studied the diurnal dynamics of DOC in the Sundarbans estuary. DOC showed non-conservative behavior during low tide and conservative behavior during high tide (Table S3). The moderately significant relation between DOC and salinity indicated that the DOC in the Sundarbans estuarine system remains regulated by processes other than estuarine mixing. The biogeochemical and physiological processes, along with the photooxidation in the mangrove system, were suggested to regulate the diurnal fluctuations in DOC. Dutta et al. [67,69] reported the seasonal changes in DOC during the pre-monsoon and post-monsoon seasons. In the Hooghly and Sundarbans estuaries, the pre-monsoon DOC concentrations were 48% and 45% higher than the post-monsoon DOC concentrations. However, Akhand et al. [66] did not observe any such trend in the DOC.
Dutta et al. [69,72] reported the seasonal and diel changes in POC, respectively (Table S3). In the Hooghly estuary, POC was higher by 52% during pre-monsoon than post-monsoon, while in the Sundarbans estuary, POC was higher during post-monsoon (by 62%) when compared to pre-monsoon. The diurnal study showed marginally higher POC in the Sundarbans estuary during the daytime. A seasonal study by Ray et al. [84] also reported higher POC in the waters surrounding Lothian Island during post-monsoon (45.4 ± 7.5 µM) than pre-monsoon (28.0 ± 8.6 µM). The diurnal variation study of POC [84] showed an increasing POC trend with increasing water height. Like DIC, Akhand et al. [66] reported significantly higher POC levels in the Matla estuary during the pre-monsoon season compared to the other two seasons.
The temporal variability of TAlk is a relatively less documented topic in the carbonate chemistry research in the Indian Sundarbans (Table S2). Akhand et al. [68] studied the seasonal variation of TAlk in the Hooghly and Matla estuaries, which showed lower values of TAlk during the monsoon in the inner estuarine stations from both study areas. Ghosh et al. [83] reported similar observations from the Hooghly estuary (from the upper to the lower estuary) and the adjacent coastal waters. The TAlk values were highest during the pre-monsoon and lowest during the monsoon. Dutta et al. [72] studied the diurnal variability of TAlk in the Sundarbans estuary (Saptamukhi, Thakuran, and Matla estuaries), which varied over a narrow range (2.19 to 2.58 m equiv/l). Akhand et al. [66] also reported similar observations from the Matla estuary (2178 to 2273 µmol kg −1 ). They sampled twice during the two consecutive pre-monsoon seasons of 2017 and 2018 and observed significantly higher mean TAlk than in the monsoon and post-monsoon seasons.
Biswas et al. [74] measured the diurnal variation of pCO 2 in the Mooriganga, Saptamukhi, and Thakuran estuaries in 2001 (Table S4). pCO 2(air) varied with the time of day (minimum at 1200 h and maximum at 2100 h) and ranged between 403 µatm and 635 µatm, respectively. The pCO 2(water) was highest at 0600 h (1062 µatm) and lowest at 1500 h (416 µatm). While mangrove plant cover, wind velocity, and turbulence influenced the pCO 2(air) , tide-driven changes in salinity regulated the pCO 2(water) variation. Padhy et al. [64] studied pCO 2 in the Hooghly estuary and the adjacent coastal waters during the winter and summer of 2008. During winter, pCO 2(water) ranged between 320 µatm and 500 µatm in the coastal domain and 340 µatm to 375 µatm in the upstream river. However, during summer, pCO 2(water) was spatially uniform (about 450 µatm). Changes in tidal circulations and physicochemical properties along the estuary (from the upstream river to the offshore estuary) caused spatial variation in pCO 2(water) . Ray et al. [84] studied pCO 2 in the adjacent waters of Lothian Island and throughout the Hooghly River (from the upstream riverine part to the river mouth) during the pre-monsoon and post-monsoon of 2014. The study reported comparatively higher values of pCO 2 during the pre-monsoon season. Akhand et al. [66] reported the highest mean pCO 2(water) during the monsoon and the lowest during the post-monsoon season.
Mukhopadhyay et al. [63] studied the air-water CO 2 flux along the Hooghly estuary in 1999. The study indicated the estuary as a source of CO 2 during the pre-monsoon (the maximum emission rate being 84.4 mmol m −2 d −1 ), and the role reversed during the monsoon (the maximum influx rate being −2.78 mmol m −2 d −1 ). (Table S4). Biswas et al. [74] studied the diurnal variation of air-water CO 2 flux in the Sundarbans estuary (Mooriganga, Saptamukhi, and Thakuran estuaries) in 2001. The study showed a distinct diurnal variation in the CO 2 flux (from −16.2 to 49.9 µmol m −2 h −1 ). Ambient and sea surface temperatures, daylight hours, and the rate of photosynthetic activity influenced the rate of CO 2 flux. Akhand et al. [76] studied the diurnal variation in CO 2 flux in the Edward Creek, Thakuran River, and Herobhanga River estuaries for their distance from the coastal waters. The study provided insight into the changing behavior of the estuarine system in terms of CO 2 flux from the inner to outer estuarine regions. The findings about the Life 2023, 13, 863 11 of 24 diurnal variability were similar to those observed by Biswas et al. [74]; however, apart from the photosynthetic activity and respiration, the semidiurnal tidal cycles were responsible for the variations. Chen et al. [122] also observed that the strong CO 2 source nature in the upper estuaries often turns into a CO 2 sink character in the outer estuarine river plumes. In line with the pCO 2(water) -related observations made by Akhand et al. [66], they reported air-water CO 2 fluxes decreasing from monsoon to pre-monsoon, followed by post-monsoon seasons.
Biswas et al. [77] measured the seasonal changes in CH 4 concentration in water and air-water CH 4 flux in the Hooghly estuary (from the inner riverine zone to the river mouth), the Saptamukhi, Mooriganga, and Thakuran estuaries, and the adjacent waters of Lothian Island. Seasonal variability of CH 4(air) was prominent; however, it spanned over a narrow range (Table S5). The CH 4(water) was maximum during the post-monsoon and minimum during the monsoon season. Increased dissolved CH 4 concentration was during high tide and vice versa. As observed by other researchers, the CH 4 concentration in estuarine rivers generally shows a decreasing trend from freshwater to saltwater regions [123][124][125]. However, in the Hooghly estuary, the CH 4 concentration was observed to increase in the lower stretch of the estuary. Apart from the riverine source, the mangrove ecosystem in the lower stretch of the estuary acted as an additional source of CH 4 ; hence, the CH 4 concentration increased. The primary peak in CH 4 flux was during the post-monsoon, and a secondary peak was during the monsoon. The residence time of CH 4 -rich riverine water in the estuary plays an essential role in CH 4 flux. Dutta et al. [81,82,85,86] recorded similar results from the Saptamukhi estuary. CH 4 concentration was maximum during the postmonsoon and minimum during the pre-monsoon. CH 4 flux was water-to-air throughout the year, and maximum efflux occurred during the monsoon and minimum during the pre-monsoon. Padhy et al. [126] studied the CH 4 emissions from the surrounding waters of degraded mangroves. Based on remote sensing analysis, they considered Sadhupur, Pakhiralaya, and Dayapur as degraded mangrove sites for their study. They measured air-water CH 4 fluxes in stagnant water, during tidewater, and after tidewater. In all the locations, the CH 4 concentration and the CH 4 flux were higher in stagnant water during the monsoon.
Though an adequate number of studies characterized the seasonal variability of the carbon biogeochemistry parameters discussed above, long-term temporal monitoring over the course of decades at a fixed location is absent from this region. Systematic monitoring of these parameters in high and low-tide sessions during spring tide and neap tide phases over multiple years would enable us to characterize the changes in the concentrations of these parameters over time. Such prolonged monitoring would allow us to correlate the changes in concentrations of these parameters with climate change indicators and/or anthropogenic activities. In the absence of such long-term monitoring, the role of climate change and/or anthropogenic activities in governing the carbon biogeochemistry in these regions remains unanswered. Table 2 summarizes the range of all the parameters reviewed in this study in the pre-monsoon, monsoon, and post-monsoon seasons. The maximum concentrations of TAlk and DIC occurred during the pre-monsoon season, followed by the post-monsoon and the monsoon seasons. This observation indicates that these two parameters remained high during the dry seasons (pre-monsoon and post-monsoon) and underwent dilution in the monsoon season. DOC and POC also showed similar trends, which indicates that during the dry season, when evaporation rates remain high with minimal freshwater discharge from the upper reaches, the estuarine water mass remains enriched with both of these forms of organic carbon. However, during the monsoon, these values remain lower primarily due to the dilution effect in the presence of voluminous freshwater flow. pCO 2(water) , on the contrary, exhibited higher concentration ranges during the monsoon season. The dominance of riverine freshwater and catchment flow triggered by the monsoon-induced rainfall reduces the pH of the estuarine water column, which facilitates a higher pCO 2(water) than that observed in the dry seasons. Air-water CO 2 fluxes reciprocated accordingly, and the highest flux ranges occurred during the monsoon, followed by the pre-monsoon and post-monsoon seasons. The pre-monsoon season, which coincides with the summer months, experiences enhanced temperatures that reduce the solubility of CO 2 in the estuarine water and give rise to substantially higher magnitudes of air-water CO 2 flux. However, the effect of enhanced temperature (during the pre-monsoon season) could not overrule the effect of lowered pH levels (during the monsoon season) in regulating pCO 2(water) . In the case of CH 4 concentrations, however, the pre-monsoon season exhibited higher ranges, as did the air-water CH 4 fluxes. This observation showed that higher temperatures activated methanogenic activities more in the pre-monsoon season than in the other two seasons. Table 2. The range (minimum to maximum) of TAlk, DIC, DOC, POC, pCO 2 (water), air-water CO 2 flux, CH 4 concentration in water, and air-water CH 4 flux observed in the pre-monsoon, monsoon, and post-monsoon seasons in the Indian Sundarbans.

Parameters
Pre

Factors Regulating Air-Water CO 2 and CH 4 Flux
Various factors governed the air-water CO 2 and CH 4 flux in the Indian Sundarbans mangrove system. For ease of discussion, we categorized them into three major types: physical, biogeochemical, and hydrological.

Role of Physical Factors
Water temperature, wind speed, tidal activity, rainfall, and photosynthetically active radiation (PAR) are critical in regulating CO 2 and CH 4 concentrations in water and, subsequently, the CO 2 and CH 4 fluxes. Water temperature is known to influence CO 2 solubility in seawater [127] and, subsequently, the variations of pCO 2(water) [128]. Almost all the aforementioned studies that monitored the seasonal variability of pCO 2(water) and air-water CO 2 fluxes observed higher magnitudes of both these parameters during the pre-monsoon season compared to the post-monsoon season [66,68]. However, attributing only temperature as the sole factor in regulating pCO 2(water) would be erroneous, as several other interlinked physical factors play crucial roles.
The availability of PAR regulates autotrophic activities in estuarine water [129]. Reduced PAR causes dominance of heterotrophy and induces CO 2 efflux [130]. Increased turbidity or total suspended matter in the seawater reduces PAR penetration in the water column [131], increasing the CO 2 source potential of seawater. Akhand et al. [68] measured PAR in the Hooghly and Matla estuaries. They reported a significant difference in PAR between the two estuaries, with substantially lower PAR in the Hooghly compared to the Matla estuary. This low PAR was mainly due to the high turbidity levels in Hooghly compared to Matla. Sadhuram et al. [132] reported that the estuarine water column of the Hooghly remains highly turbid almost throughout the year due to strong tidal and wave actions, which leads to the bottom-churning of sediments. Tidal cycles also play a crucial role in regulating pCO 2(water) as they cause the physical mixing of the seawater with the riverine freshwater. Akhand et al. [65,68] observed that freshwater from upstream had a higher pCO 2(water) , whereas the seawater encroaching during the high tide had a low pCO 2(water) . Thus, it is an essential driving factor for CO 2 flux. During high tide phases, due to the dominance of seawater in the estuary, CO 2 efflux remains low. The CH 4 concentration in estuarine water also varies with tidal cycles. During high tides, flushing the mangrove swamp brings large quantities of CH 4 into the estuary, causing an increased CH 4 concentration in the water [77]. Wind speed is another physical forcing factor that regulates flux magnitude. Higher wind speed facilitates CO 2 /CH 4 exchange at the air-water interface, increasing flux magnitude [76]. Rainfall also plays a vital role in regulating flux in the estuarine region. Increased rainfall leads to increased river runoff and more drainage of mangrove porewater into the estuary. Thus, higher CO 2 efflux generally prevailed during the monsoon seasons [66,68]. During the dry seasons, high levels of air pollution from the urban sectors are advected into the coastal regions, causing a gradual decrease in CH 4 concentration in the air due to atmospheric oxidation [77].

Role of Biogeochemical Factors
Salinity is one of the significant factors that govern CO 2 flux. Higher salinity means the dominance of seawater, where pCO 2 is low [133]. Thus, CO 2 efflux from the seawater also remains low. Decreased salinity in a coastal region refers to an increased river runoff or pore water discharge, rich in pCO 2 , promoting CO 2 efflux [134]. The DIC in water mainly remains in carbonate and bicarbonate forms. Lower pH induces excess DIC to transform into a gaseous form, which eventually releases into the surrounding atmosphere through efflux. Akhand et al. [66] and Dutta et al. [72] observed a significant negative correlation between pCO 2(water) and pH.
The carbonate buffering capacity of estuaries depends on the TAlk/DIC ratio [135]. The higher this ratio, the lower the potential of CO 2 emissions from the estuaries. Akhand et al. [65] observed that the northern Bay of Bengal waters has high TAlk/DIC, which enhances the carbonate buffering capacity within the estuaries during the high tide. This phenomenon prevents the conversion of carbonate and bicarbonate in water into excess DIC, reducing CO 2 efflux from water to air. Chen et al. [136] also reported that mixing with seawater is the primary cause for the pCO 2 to decrease near the river mouths. Akhand et al. [65] also noted that DOC and POC degradation to DIC occurs in the Sundarbans estuaries primarily through denitrification. The mangrove pedosphere offers a platform where organic matter degradation eventually enhances the DIC levels in the adjacent estuary, reducing carbonate buffering capacity [116].
Salinity shows a strong negative correlation with CH 4 concentration in water, indicating riverine freshwater is the primary source of CH 4 [137]. In several instances, higher chlorophyll concentrations also lead to higher levels of dissolved CH 4 in water. Higher chlorophyll ensures increased productivity (provided all the other parameters such as water surface temperature, PAR, salinity, and pH are optimum), which eventually increases the organic matter supply to the sediments, and this causes increased methane production by methanogens [138].

Role of Hydrological Factors (Pore Water, Groundwater, and Freshwater Discharge)
Compared to the surface water, pCO 2 and pCH 4 in the pore water remain several folds higher [119]. Pore water and groundwater are often responsible for the high pCO 2 and pCH 4 in mangrove estuaries during low tide [139]. Microbial and root respiration in the rhizosphere and anaerobic methane emission driven by methanogens enhance the CO 2 and CH 4 levels in the porewater [9,79]. Das et al. [140] reported that porewater from the mangrove sediments of the Sundarbans has substantial potential to enhance the pCO 2 and pCH 4 in the adjacent estuaries. Studies indicate that pore water outflux from the sediments caused pCO 2 to be high in the creeks during low tide [65]. However, this effect of pore water is not visible in the estuaries, a) due to the higher volume of water in the estuaries, which dilutes the pore water, and b) because the higher residence time of water in the creeks causes organic matter degradation, which further increases pCO 2 . Higher CH 4 concentration in the estuarine water during post-monsoon is caused by the flux of pore water from the mangrove swamp sediments after monsoon rainfall and higher litter fall during post-monsoon [77]. This pore water laden with CH 4 increases the CH 4 concentration in estuarine water, causing maximum CH 4 flux in the post-monsoon season.
Groundwater discharge is another process that influences the pCO 2 in estuarine and oceanic waters [141]. Groundwaters are generally rich in CO 2 [142]. Thus, it can significantly increase the pCO 2(water) and CO 2 efflux in the discharged area. Groundwaters are also laden with nutrients and organic matter, which further increase the organic load in the water, increasing pCO 2 . However, Akhand et al. [70] did not observe any significant difference in DOC between groundwater and surface water in the Hooghly and Sundarbans estuaries. Groundwaters are also high in TAlk and DIC content, adding to CO 2 flux. Akhand et al. [70] found TAlk and DIC in recirculated and fresh groundwaters four times higher than the surface waters in the Hooghly and Matla estuaries. They also observed that low-tide sessions had higher Radon-222 signatures, indicating higher groundwater contribution, which coincided with higher pCO 2(water) . Akhand et al. [70] could not quantify the submarine groundwater discharge in the Sundarbans estuaries and, hence, could not assess the exact contribution of groundwater in enhancing estuarine pCO 2(water) . However, their findings strongly indicate that groundwater seepage in these estuaries could play a pivotal role in regulating its pCO 2(water) and air-water CO 2 fluxes, as groundwater samples had a significantly higher TAlk/DIC ratio than surface waters, which can significantly increase the estuarine water's buffering capacity as well. Freshwater discharge from the upper reaches is crucial in regulating the pCO 2 dynamics in estuarine regions worldwide [143]. In the Indian part of the Sundarbans (where carbon-biogeochemistry-related studies mostly concentrate), freshwater mainly comes from the Hooghly estuary on the western margin and the Raimangal in the east. The estuaries that flow through the central part of this mangrove forest receive minimal freshwater and retain their estuarine character mainly through monsoon-induced rainfall and runoff from upper catchment areas [61]. Akhand et al. [68] explicitly portrayed that higher riverine discharge into the Hooghly estuary led to four times higher pCO 2(water) and fourteen times higher CO 2 emission in the Hooghly compared to the Matla estuary, which does not receive much freshwater and flows through the central part of the Sundarbans. Akhand et al. [68] also correlated the observed monthly mean pCO 2(water) and air-water CO 2 flux in the Hooghly estuary with the monthly mean freshwater discharge reported by Rudra [144]. They observed that both mean pCO 2(water) and air-water CO 2 flux increased with freshwater discharge. The highest CO 2 effluxes were during the monsoon season, when the discharge peaked, and vice versa. Akhand et al. [68] concluded an undeniable role of riverine allochthonous DIC and organic matter input that enhanced pCO 2(water) with increasing riverine freshwater discharge. The same holds for Sundarbans estuaries, as well; however, riverine freshwater input data for the Sundarbans estuaries are unavailable, which barred several researchers from quantifying the role of riverine freshwater in governing the carbon dynamics in these estuaries.
The existing set of studies was, to a large extent, successful in characterizing the role of physical, biogeochemical, and hydrological factors in regulating CO 2 and CH 4 fluxes; however, future research should strive to acquire more data, as sampling number and frequency are the two major constraints at present. Given the vast expanse of this estuarine complex, the sampling stations should be randomly distributed throughout the estuarine system, and future research should focus on high-temporal-resolution sampling from each station to increase the number of observations that can reduce the uncertainties in the observed relationships.

Role of the Microbiomes in Carbon Dynamics of Sundarban
Microbiomes play a crucial role in carbon dynamics and biogeochemistry through the degradation of organic matter and through mineralization pathways. The CO 2 and CH 4 evasion caused by the microbiome-dependent processes mainly facilitates offsetting the carbon sequestration ability in the mangrove ecosystem, especially in the adjacent soil and water. In the Sundarbans, the microbiomes concerning carbon dynamics and sequestration have been relatively poorly studied compared to other counterparts of the mangrove ecosystem. Ghosh and Bhadury [145] studied coastal bacterioplankton diversity concerning biogeochemical cycling on the largest island of the Indian Sundarban delta, Sagar Island, using a 16S rRNA clone library and Illumina MiSeq techniques. They found several sequences belonging to Sphingomonadales, Chromatiales, Alteromonadales, Oceanospirillales, and Bacteroidetes, which might have a regulatory role in coastal carbon cycling. Among the bacterioplankton, an abundance of Synechococcus sp. during the monsoon period signified the role of oxygenic photoautotrophs in the coastal carbon cycling of the Sundarbans. Bhadury and Singh [146] further confirmed the dominance of Synechococcus-like 16S rRNA sequences and the influence of small-sized picocyanobacterial cells in regulating carbon export in the Sundarbans' mangrove ecosystems while working in the Mooriganga estuary.
They also reported other cyanobacterial sequences showing taxonomic affiliation with members of the Chroococcales, Pleurocapsales, Oscillatoriales, and Stigonematales. Das et al. [147,148] reported that organic carbon was the most significant factor that regulated the total microbial population in the mangrove-adjacent sediment. They also depicted that cellulose-degrading bacteria dominated throughout the year in the sediment, which might have a decisive role in the mineralization and decomposition processes, majorly affecting the carbon biogeochemistry of the Sundarbans. Mukherjee et al. [149] conducted a post-monsoonal study in the Saptamukhi and Thakuran estuaries of the Sundarbans on bacterioplankton concerning inorganic nutrients and carbonate variables. They stated that Proteobacteria dominated the bacterioplankton, with a contribution from Bacteroidetes in the Saptamukhi and Cyanobacteria and Actinobacteria in the Thakuran; the interactions between physicochemical parameters, nutrient levels, and estuarine carbonate chemistry controlled post-monsoonal bacterioplankton abundance. Dhal et al. [150] revealed microbial communities in the waters of the "Island of Sundarban Mangroves" and "Open Marine Water" of the Thakuran-Matla River estuarine complex adjacent to the Maipith coastal area. They reported dominance of marine hydrocarbon-degrading bacteria under families Oceanospirillaceae and Spongiibacteraceae, where the most abundant bacterial family Rhodobacteracea almost equally dominated in both study sites. Their investigation further reported Actinobacteria, which is known to be associated with carbon cycling to decompose the plant biomass via degrading the cellulose and hemicellulose materials (a dominant resource material in mangrove plants).

The Lateral Flux of Carbon to the Bay of Bengal
The last two decades witnessed several studies that characterized the lateral fluxes of carbon from the terrestrial sectors to the adjacent ocean through the estuaries [8,151,152]. Recent developments in understanding estuarine carbon biogeochemistry indicate that long-term carbon sinks should encompass the laterally exported DIC from the estuaries to the nearshore coastal oceans [153]. The lateral export of organic matter and nutrients from the mangrove sediments to the estuarine waters plays a vital role in regulating the carbon flux. In this regard, Mukhopadhyay et al. [87] were the first to quantify the lateral fluxes of nutrients and DIC from the Hooghly estuary to the adjacent northern Bay of Bengal by deploying a biogeochemical mass balance model. They reported a lateral flux of DIC of 2.3 × 10 11 mol per year while working from 1999 to 2001. Mukhopadhyay et al. [87] observed that 7.5% of the DIC was removed from the system as air-water CO 2 flux during this lateral transport. Ghosh et al. [83], during 2015-16, observed that annually, the Hooghly estuary outfluxes around 4.45 ± 1.90 × 10 11 mol and 4.59 ± 1.70 × 10 11 mol of TAlk and DIC, respectively, to the adjacent coastal ocean. They concluded that the enhanced discharge load of inorganic and organic matter in the upper reaches of the Hooghly River led to an increased lateral flux of DIC and DOC, compared to the observations of Mukhopadhyay et al. [87]. Ghosh et al. [83] also inferred that the lateral DIC fluxes to the coastal ocean were 30 to 60 times higher than the air-water CO 2 fluxes from the Hooghly estuary towards the atmosphere. Ray et al. [84] quantified the lateral fluxes of DIC, DOC, and POC from the Hooghly estuary and the Sundarbans estuaries as baseline data. They reported a lateral DIC, DOC, and POC flux of 3.45 × 10 11 mol, 0.28 × 10 11 mol, and 0.05 × 10 11 mol per year, respectively, from the Hooghly estuary. In contrast, the Sundarbans estuaries outwelled 3.07 × 10 11 mol, 2.52 × 10 11 mol, and 0.48 × 10 11 of DIC, DOC, and POC mol per year, respectively, with a total of 7.3 Tg C yr −1 . Thus, Ray et al. [84] concluded that the mangroveadjacent waters of the Sundarbans estuaries outwelled slightly less DIC but nine times more DOC and 9.6 times more POC than the Hooghly estuary. One of the constraints of this study [84] was the use of river discharge data from the Hooghly estuary to compute the lateral fluxes from the Sundarbans estuaries, which might lead to substantial uncertainties. Thus, further studies are of the utmost necessity to characterize the overall carbon budget of the Sundarbans estuaries.

Summary and Conclusions
Collating all the observations considered in this review, we can conclude that the pCO 2(water) and pCH 4(water) in the Sundarbans estuaries remain supersaturated compared to the atmospheric concentrations. Hence, these estuaries act as sources of CO 2 and CH 4 . Figure 2 summarizes the entire review in a nutshell. The inner estuarine regions emit more CO 2 and CH 4 than the outer reaches close to the northern Bay of Bengal. Some studies indicated that the outer reaches could sometimes act as a sink for CO 2 , which is unusual for mangrove-adjacent waters. Analyzing the reasons behind this observation, several authors noted that the water from the adjacent Bay of Bengal is primarily responsible for enhancing the carbonate buffering capacity of these estuaries, especially in regions with an adequate quantity of riverine freshwater that does not end up in the upper reaches. Among the physical drivers, water temperature and photosynthetically active radiation play a pivotal role in governing CO 2 emissions. Some significant biogeochemical and hydrological factors that governed the CO 2 fluxes were salinity, porewater, groundwater, and freshwater discharge. The degree of primary productivity was the main regulating factor for air-water CH 4 fluxes.
This review indicates that a substantial number of studies characterizing the airwater CO 2 and CH 4 fluxes exist in the estuaries of the Sundarbans. However, all these studies concentrated only on the estuaries that flow through the Indian counterpart of the Sundarbans. An adequate number of studies in the Bangladesh Sundarbans would enable us to draw a scenario about the entire Sundarbans estuarine network. Each major estuary flowing through the Sundarbans has unique freshwater-seawater admixing dynamics and a varying influence of adjacent mangroves. The present observations indicate that such subtle differences in the salinity regime and mangrove influence might lead to varying partial pressures of CO 2 and CH 4 [pCO 2(water) and pCH 4(water) ] and air-water CO 2 and CH 4 fluxes. Thus, sampling all the major waterways in the vast Sundarbans is necessary. Among the Indian estuaries, the Hooghly (flowing through the western boundary and carrying the bulk load of freshwater) and the Matla (flowing through the central part with minimal freshwater from the upper reaches) have received the most scientific attention. Other estuaries such as Raimangal, Bidya, Saptamukhi, and Thakuran should receive similar attention in the future.
The studies conducted so far have had varying sampling strategies. Some studies concentrated on fewer sampling points but stressed high temporal resolution, whereas others tried to cover more locations with a lower temporal resolution. In a dynamic estuarine environment such as the Sundarbans, tide-induced variability in carbon-biogeochemistryrelated parameters poses a challenge to characterizing the mean and range of a particular parameter. Thus, more studies focusing simultaneously on the spatial extent and temporal resolution can alleviate the uncertainties in the data range. Besides the estuaries of the Bangladesh Sundarbans, which have yet to receive proper scientific attention in this regard, the estuarine waters flowing through the core area of the Indian Sundarbans also need rigorous sampling in the future, or else a substantial part of this unique eco-region remains undersampled. Whatever data on freshwater discharge are available at present mainly focus on the Hooghly estuary. Future researchers should model the estuarine flow through the various other estuaries of the Sundarbans that do not have a proper connection with the upper reaches. This would help in delineating the lateral fluxes of various forms of carbon from this system to the adjacent ocean and vice versa. Most of the studies accomplished so far have relied on discrete sampling techniques, which constrained the number of observations, as they require manual presence. Automated sensors on fixed platforms and the deployment of buoys should be on future research agendas. Otherwise, characterizing the temporal changes in the behavior of these estuaries with the changing climate and anthropogenic influences upstream would be highly challenging. , total alkalinity (TAlk), partial pressure of CO2 (pCO2), and concentration of CH4 in the Indian Sundarbans' adjoining estuaries according to different studies. Hooghly River is the only source of riverine freshwater in the Indian Sundarbans. The major organic matter degradation/ mineralization pathway is denitrification. The mangrove surrounding waters of these estuaries acted as a source and sink for CO2 and CH4 depending on the seasonality and spatiality; the source and sink capacity varied over a wide range. These estuaries also transfer a substantial amount of carbon to the adjacent Bay of Bengal, mainly in POC, DOC, and DIC forms.
This review indicates that a substantial number of studies characterizing the air-water CO2 and CH4 fluxes exist in the estuaries of the Sundarbans. However, all these studies concentrated only on the estuaries that flow through the Indian counterpart of the Sundarbans. An adequate number of studies in the Bangladesh Sundarbans would enable us to draw a scenario about the entire Sundarbans estuarine network. Each major estuary flowing through the Sundarbans has unique freshwater-seawater admixing dynamics and a varying influence of adjacent mangroves. The present observations indicate that such subtle differences in the salinity regime and mangrove influence might lead to varying partial pressures of CO2 and CH4 [pCO2(water) and pCH4(water)] and air-water CO2 and CH4 fluxes. Thus, sampling all the major waterways in the vast Sundarbans is necessary. Among the Indian estuaries, the Hooghly (flowing through the western boundary and carrying the bulk load of freshwater) and the Matla (flowing through the central part with minimal freshwater from the upper reaches) have received the most scientific attention. Other estuaries such as Raimangal, Bidya, Saptamukhi, and Thakuran should receive similar attention in the future.
The studies conducted so far have had varying sampling strategies. Some studies concentrated on fewer sampling points but stressed high temporal resolution, whereas others tried to cover more locations with a lower temporal resolution. In a dynamic estuarine environment such as the Sundarbans, tide-induced variability in carbon-biogeochemistry-related parameters poses a challenge to characterizing the mean and range of a particular parameter. Thus, more studies focusing simultaneously on the spatial extent and temporal resolution can alleviate the uncertainties in the data range. Besides the estuaries of the Bangladesh Sundarbans, which have yet to receive proper scientific attention , and dissolved inorganic carbon (DIC)], total alkalinity (TAlk), partial pressure of CO 2 (pCO 2 ), and concentration of CH 4 in the Indian Sundarbans' adjoining estuaries according to different studies. Hooghly River is the only source of riverine freshwater in the Indian Sundarbans. The major organic matter degradation/ mineralization pathway is denitrification. The mangrove surrounding waters of these estuaries acted as a source and sink for CO 2 and CH 4 depending on the seasonality and spatiality; the source and sink capacity varied over a wide range. These estuaries also transfer a substantial amount of carbon to the adjacent Bay of Bengal, mainly in POC, DOC, and DIC forms.

Supplementary Materials:
The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/life13040863/s1, Table S1. The list of studies on air-water CO 2 and CH 4 fluxes, along with other related carbon-biogeochemistry parameters carried out in the Indian Sundarbans estuaries. Table S2. List of observations on TAlk and DIC from several studies on the Indian Sundarbans estuaries. The results are displayed either as mean ± standard deviation from the mean or as the range (minimum to maximum). NA denotes not available. Table S3. List of observations on DOC and POC from several studies on the Indian Sundarbans estuaries. The results are displayed either as mean ± standard deviation from the mean or as the range (minimum to maximum). Single magnitudes, in some instances, represent the mean (without any reported standard deviation). Table S4. List of observations on pCO 2(water) and air-water CO 2 fluxes from several studies on the Indian Sundarbans estuaries. The results are displayed either as mean ± standard deviation from the mean or as the range (minimum to maximum). Single magnitudes, in some instances, represent the mean (without any reported standard deviation). Table S5. List of observations on CH 4 concentration in water and air-water CH 4 fluxes from several studies on the Indian Sundarbans estuaries. The results are displayed either as mean ± standard deviation from the mean or as the range (minimum to maximum). Single magnitudes, in some instances, represent the mean (without any reported standard deviation).