Abstract
Globally, water bodies adjacent to mangroves are considered significant sources of atmospheric CO2. We directly measured the partial pressure of CO2 in water [pCO2(water)] and related biogeochemical parameters with high temporal resolution, covering both diel and tidal cycles, in the mangrove-surrounding waters around the northern Bay of Bengal during the post-monsoon season. Mean pCO2(water) was marginally oversaturated in two creeks (470 ± 162 µatm, mean ± SD) and undersaturated in the adjoining estuarine stations (387 ± 58 µatm) compared to atmospheric pCO2, and was considerably lower than the global average. We further estimated the pCO2(water) and buffering capacity of all possible sources of the mangrove-surrounding waters and concluded that their character as a CO2 sink or weak source is due to the predominance of marine water from the Bay of Bengal with low pCO2 and high buffering capacity. Marine water with high buffering capacity suppresses the effect of pCO2 increase within the mangrove system and lowers the CO2 evasion even in creek stations. The δ13C of dissolved inorganic carbon (DIC) in the mangrove-surrounding waters indicated that the DIC sources were a mixture of mangrove plants, pore-water, and groundwater, in addition to marine water. Finally, we showed that the CO2 evasion rate from the estuaries of the Sundarbans is much lower than the recently estimated world average. Our results demonstrate that mangrove areas having such low emissions should be considered when up-scaling the global mangrove carbon budget from regional observations.
Highlights
The carbon stocks within several coastal ecosystems, collectively referred to as ‘‘blue carbon’’, have drawn attention in the context of global climate change (Donato et al 2011; McLeod et al 2011; Pendleton et al 2012), and initiatives to characterize the functioning and long-term future of these blue-carbon ecosystems have begun (Macreadie et al 2019)
There were no significant differences in mean total alkalinity (TAlk) and dissolved inorganic carbon (DIC) between the creeks and estuary, but some creek samples had high values during ebb tide (TAlk, 2732 lmol kg–1; DIC, 2683 lmol kg–1) (Fig. 2a, b)
No significant difference was found in TAlk/DIC (p [ 0.05) (Fig. 2c). pCO2(water) and d13CDIC were significantly different between creeks and estuary (p \ 0.001); the mean pCO2(water) was oversaturated in the creeks (470 ± 162 latm, mean ± SD), and undersaturated in the estuary (387 ± 58 latm) with respect to pCO2(air) (408 latm) (Fig. 3 and Table 1), whereas, the mean pH was slightly lower in the creeks (7.91 ± 0.16) than in the estuary (7.96 ± 0.06); the difference was not significant (p [ 0.05, Table 1). d13CDIC varied over a wide range, from –1.5% to –7.6% with a mean of –3.4% ± 1.9% and –1.9% ± 0.2% in the creeks and estuary, respectively (Fig. 2d)
Summary
The carbon stocks within several coastal ecosystems (e.g. mangroves, tidal marshes and seagrass beds), collectively referred to as ‘‘blue carbon’’, have drawn attention in the context of global climate change (Donato et al 2011; McLeod et al 2011; Pendleton et al 2012), and initiatives to characterize the functioning and long-term future of these blue-carbon ecosystems have begun (Macreadie et al 2019). These ecosystems are known to be carbon sinks; mangroves deserve special mention owing to their large soil organic carbon pool and for being a center for active deposition of both autochthonous and allochthonous organic matter (Breithaupt et al 2012; Sanders et al 2016a, 2016b). Several studies have attributed the high CO2 emissions from mangrove-surrounding waters to the mixing of surface water with pore-water through tidal pumping, which leads to enrichment of the partial pressure of CO2 in water [pCO2(water)] and dissolved inorganic carbon (DIC), as well as to metabolic activity in sediments (Bouillon et al 2007b; Gleeson et al 2013; Li et al 2009; Maher et al 2013; Robinson et al 2007; Santos et al 2012; Sippo et al 2016)
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