Biogenic controls on the air—water carbon dioxide exchange in the Sundarban mangrove environment, northeast coast of Bay of Bengal, India

Abstract

The Sundarban mangrove forest (4,264 km 2 ) constitutes about 3% of the total area of the world mangrove. We measured diurnal and seasonal variations of air-water CO 2 exchange in relation to the occurrence of phytoplankton during January-December 2001. Diurnal variations of airflows showed that the minimum and maximum CO 2 flux of Ϫ16.2 mol m Ϫ2 h Ϫ1 and 49.9 mol m Ϫ2 h Ϫ1, respectively, occurred during the higher sea breeze. The average ratio of dissolved inorganic nitrogen (DIN ϭ 13.85 Ϯ 7.19 mol L Ϫ1 ) to dissolved inorganic phosphorus (DIP ϭ 1.23 Ϯ 0.57 mol L Ϫ1 ) was 11 Ϯ 4 and the surface water was undersaturated with respect to dissolved oxygen. The mean value of 0.1 Ϯ 0.08 for the ratio of phytoplankton production (P) to community respiration (R) indicated that the ecosystem was heterotrophic. The saturation of dissolved carbon dioxide with respect to the atmosphere varied seasonally between 59% and 156%, with minimum levels in postmonsoon and maximum levels in premonsoon/early monsoon (June/July). Out of the 36 genera of diatoms, 1 blue green alga, and 3 dinoflagellates that occurred throughout the year, only 6 reached bloom proportions in postmonsoon, when mangrove water was a sink of atmospheric CO 2 . Although 59.3% of the emitted CO 2 was removed from the atmosphere by biological processes, on an annual basis, the Sundarban mangrove forest supplies 13.8 kg C ha Ϫ1 yr Ϫ1 of CO 2 from water surface to the atmosphere. Even though it is important to compare all in and out fluxes, there is no direct link between CO 2 emission and the later CO 2 removal by biological processes.

Reviews and syntheses: Anthropogenic perturbations to carbon fluxes in Asian river systems: Concepts, emerging trends, and research challenges

“…As summarized in Table 3, many studies of aquatic CO2 dynamics in India have been conducted in estuaries and coastal areas (e.g., Mukhopadhyay et al, 2002;Biswas et al, 2004;Gupta et al, 2009;Sarma et al, 2012;Samanta et al, 2015), except for the secondary data of pCO2 that can be estimated based on CO2 system calculations (Pierrot et al, 2006) and water quality data collected in various headwaters (Sarin et al, 1989;Bickle et al, 2003;Chakrapani and Veizer, 2005) and 440 lower reaches (Manaka et al, 2015) of the Ganges-Brahmaputra. The values of pCO2 estimated for some headwaters, lower reaches, and tributaries of the Ganges basin (mean: 891; range: 16-2,778 µatm) were relatively low compared to other Asian rivers (Table 3).…”

“…In a study of human impacts on C dynamics in the Cochin estuary, Southern India, Gupta et al (2009) ascribed monsoonal pCO2 increases up to 6,000 atm to the enhanced decomposition of 445 the OM released from anthropogenic sources upstream. A particular attention has been paid to the emission of CO2 and CH4 from the Indian part of the deltaic region of the Ganges-Brahmaputra system that includes estuaries with contrasting biogeochemical features: the anthropogenically impacted Hooghly estuary and the mangrove-dominated estuaries of Sundarbans, the world's largest mangrove ecosystem (Mukhopadhyay et al, 2002;Biswas et al, 2004;Biswas et al, 2007;Dutta et al, 2015Dutta et al, , 2017Samanta et al, 2015). The Hooghly estuary was found as net heterotrophic, with the fugacity of 450 CO2 (fCO2) and CH4 levels varying from ~ 400-700 µatm and 10.3-59.25 nM, respectively (Mukhopadhyay et al, 2002;Biswas et al, 2007).…”

“…On an annual scale, the Hooghly estuary acts as a source of both CO2 (2.78 to 84.4 mmol m 2 d 1 ; Mukhopadhyay et al, 2002) and CH4 (36.7-6193.5 nmol m 2 hr 1 ; Biswas et al, 2007) to the atmosphere. The estuaries of Sundarbans were reported as being net heterotrophic, with fCO2 and CH4 concentrations varying between 160-797 µatm (Biswas et al, 2004) and 54.20 (± 5.06)-90.91 (± 21.20) nM, respectively, and hence as functioning as a source for both CO2 455 (314.6 mmol m 2 d 1 ) and CH4 (4.66-17.59 μmol m 2 d 1 ) to the atmosphere (Dutta et al, 2015). Based on the mass balance between source and sink inventories, Dutta et al (2015) estimated that annually ~0.5 Gg of CH4 was exported from the estuaries of Sundarbans to the northern Bay of Bengal.)…”

Nutrient budgets (C, N and P) and trophic dynamicsof a Brazilian tropical estuary: Barra das Jangadas

“…Due to the intense anthropogenic disturbance, estuaries are often considered to be net heterotrophic ecosystems and act as a source of CO 2 (Biswas et al 2004, Mukhopadhyay et al 2006. The increased nutrient load leads to eutro-442 CARLOS E.D.…”

Methane and sulfate dynamics in sediments from mangrove-dominated tropical coastal lagoons, Yucatán, Mexico

“…Despite brackish to marine salinities, methane fluxes comparable to those measured in freshwater wetlands have been reported for coastal mangrove-dominated lagoon systems in several places around the world, including Florida (Barber P.-C. Chuang et al: Methane and sulfate dynamics et al., 1988), Puerto Rico (Sotomayor et al, 1994), India (Biswas et al, 2004(Biswas et al, , 2007Ramesh, 2000, 2001;Ramesh et al, 1997Ramesh et al, , 2007Verma et al, 1999), Tanzania (Kristensen et al, 2008), Thailand (Lekphet et al, 2005), China (Alongi et al, 2005), the Andaman Islands (Linto et al, 2014) and Australia (Call et al, 2015). The anaerobic and organic-rich sediments found in these systems provide a suitable environment for methanogenesis, yet the extensive supply of sulfate from seawater should favor sulfate reducers over methanogens in the shallow sections of the sediments (Kristensen et al, 2008;Lee et al, 2008).…”