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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-388
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-388
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 02 Oct 2019

Submitted as: research article | 02 Oct 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Low CO2 evasion rate from the mangrove surrounding waters of Sundarban

Anirban Akhand1, Abhra Chanda2, Kenta Watanabe1, Sourav Das2, Tatsuki Tokoro1, Kunal Chakraborty3, Sugata Hazra2, and Tomohiro Kuwae1 Anirban Akhand et al.
  • 1Coastal and Estuarine Environment Research Group, Port and Airport Research Institute, 3-1-1 Nagase, Yokosuka 239-0826, Japan
  • 2School of Oceanographic Studies, Jadavpur University, 188, Raja S. C. Mullick Road, Kolkata 700 032, West Bengal, India
  • 3Indian National Centre for Ocean Information Services, Ocean Valley, Pragathi Nagar (BO), Nizampet (SO), Hyderabad 500 090, India

Abstract. Globally, water bodies adjacent to mangroves are considered sources of atmospheric CO2. We directly measured the partial pressure of CO2 in water, pCO2(water), and other related biogeochemical parameters with very high (1-min) temporal resolution at Dhanchi Island in India’s Sundarbans during the post-monsoon season. We used elemental, stable isotopic, and optical signatures to investigate the sources of dissolved inorganic carbon (DIC) and organic matter (OM) in these waters. Diel mean pCO2(water) was marginally oversaturated in creeks (efflux, 69 ± 180 µmol m−2 h−1) and undersaturated along the island boundary and in the main river (influx, −17 ± 53 and −31 ± 73 µmol m−2 h−1, respectively) compared to the atmospheric CO2 concentration. The possibility in earlier studies of over- or underestimating the CO2 flux because of an inability to capture tidal minima and maxima was minimized in the present study, which confirmed that the waters surrounding mangroves in this region can act as a sink or a very weak source of atmospheric CO2. δ13C values for DIC suggest a mixed DIC source, and a three-end-member stable isotope mixing model and optical signatures of OM suggest negligible riverine contribution of freshwater to OM. We conclude that the CO2 sink or weak source character was due to a reduced input of riverine freshwater [which usually has high pCO2(water)] and the predominance of pCO2-lean water from the coastal sea, which eventually increases the buffering capacity of the water as evidenced by the Revelle factor. Up-scaling the CO2 flux data for all seasons and the entire estuary, we propose that the CO2 evasion rate observed in this study is much lower than the recently estimated world average. Mangrove areas having such low emissions should be given due emphasis when up-scaling the global mangrove carbon budget from regional observations.

Anirban Akhand et al.
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Anirban Akhand et al.
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Short summary
Globally, mangrove surrounding waters are considered net sources of atmospheric CO2. We estimated the air–water CO2 flux with high temporal resolution in mangrove surrounding waters in the Indian Sundarbans and found that the water can act as a sink or weak source of CO2. We suggest that the CO2 efflux from the mangroves surrounding waters of Sundarbans are much lower than the recently estimated world average, and that such low-efflux areas should be included in global mangrove carbon budgeting.
Globally, mangrove surrounding waters are considered net sources of atmospheric CO2. We...
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