Low CO₂ evasion rate from the mangrove surrounding waters of Sundarban

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.