Mangrove, seagrass, and coral habitats often lie adjacent to each other in the tropics and subtropics. Lateral carbon fluxes and their consecutive effects on CO2 dynamics and air-water fluxes along the ecosystem continuum are often overlooked. We measured the partial pressure of CO2 in water and associated biogeochemical parameters with a high temporal resolution and estimated air-water CO2 fluxes along the ecosystem continuum. Their lateral fluxes were estimated by using a biogeochemical mass-balance model. The results showed that the waters surrounding mangrove, seagrass, and coral habitats acted as a strong, moderate, and weak source of atmospheric CO2, respectively. The mangrove zone acted as a net source for TAlk, DIC, and DOC, but as a net sink for POC. The contribution of riverine and mangrove-derived OM was substantially high in mangrove sediment, indicating that net transport of POC towards the coastal sea was suppressed by the sediment trapping function of mangroves. The seagrass zone acted as a net source of all carbon forms and TAlk, whereas the coral zone acted as a net sink of TAlk, DIC, and DOC. The lateral transport of carbon from mangroves and rivers offset atmospheric CO2 uptake in the seagrass zone. DOC degradation might increase DIC, and other biogeochemical processes facilitate the functioning of the coral zone as a DOC sink. However, as a result of DIC uptake by autotrophs, mainly in the coral zone, the whole ecosystem continuum was a net sink of DIC and atmospheric CO2 evasion was lowered. We conclude that lateral transport of riverine and mangrove-derived DIC, TAlk, and DOC affect CO2 dynamics and air-water fluxes in seagrass and coral ecosystems. Thus, studies of lateral carbon fluxes at local and regional scales can improve global carbon budget estimates.

Mangroves are active carbon sequesters playing a crucial role in coastal ecosystems. In the present study, aboveground biomass (AGB) was estimated in a 5-year-old Avicennia marina plantation (approximate area $≈190ha$) of Indian Sundarbans using high-resolution satellite data in order to assess its carbon sequestration potential. The reflectance values of each band of LISS IV satellite data and the vegetation indices, viz., normalized difference vegetation index (NDVI), optimized soil adjusted vegetation index (OSAVI), and transformed difference vegetation index (TDVI), derived from the satellite data, were correlated with the AGB. OSAVI showed the strongest positive linear relationship with the AGB and hence carbon content of the stand. OSAVI was found to predict the AGB to a great extent ($r2=0.72$) as it is known to nullify the background soil reflectance effect added to vegetation reflectance. The total AGB of the entire plantation was estimated to be 236 metric tons having a carbon stock of 54.9 metric tons, sequestered within a time span of 5 years. Integration of this technique for monitoring and management of young mangrove plantations will give time and cost effective results.

The Sundarban is the world’s largest contiguous mangrove forest and stores around 26.62 Tg of blue carbon. The present study reviewed the factors causing a decline in its blue carbon content and poses a challenge in enhancing the carbon stock of this region. This review emphasized that recurrent tropical cyclones, soil erosion, freshwater scarcity, reduced sediment load into the delta, nutrient deficiency, salt-stress-induced changes in species composition, mangrove clearing, and anthropogenic pollution are the fundamental drivers which can potentially reduce the total blue carbon stock of this region. The southern end of the Ganges–Brahmaputra–Meghna Delta that shelters this forest has stopped its natural progradation due to inadequate sediment flow from the upper reaches. Growing population pressure from the north of the Sundarban Biosphere Reserve and severe erosion in the southern end accentuated by regional sea-level rise has left minimal options to enhance the blue carbon stock by extending the forest premises. This study collated the scholarly observations of the past decades from this region, indicating a carbon sequestration potential deterioration. By collecting the existing knowledge base, this review indicated the aspects that require immediate attention to stop this ecosystem’s draining of the valuable carbon sequestered and, at the same time, enhance the carbon stock, if possible. This review provided some key recommendations that can help sustain the blue carbon stock of the Indian Sundarban. This review stressed that characterizing the spatial variability of blue carbon with more sampling points, catering to the damaged trees after tropical cyclones, estuarine rejuvenation in the upper reaches, maintaining species diversity through afforestation programs, arresting coastal erosion through increasing sediment flow, and combating marine pollution have become urgent needs of the hour. The observations synthesized in this study can be helpful for academics, policy managers, and decision makers willing to uphold the sustainability of the blue carbon stock of this crucial ecosystem.