The post-monsoon carbon biogeochemistry of the Hooghly–Sundarbans estuarine system under different levels of anthropogenic impacts

Abstract

Abstract. The present study focused on understanding differences in the post-monsoon carbon (C) biogeochemistry of two adjacent estuaries undergoing different levels of anthropogenic stresses by investigating anthropogenically influenced Hooghly estuary and mangrove-dominated estuaries of the Sundarbans in the north-eastern India. The salinity of well-oxygenated estuaries of the Sundarbans (DO: 91 %–104 %) varied over a narrow range (12.74–16.69) relative to the Hooghly estuary (0.04–10.37). A mixing model suggested a combination of processes including freshwater intrusion, carbonate precipitation and carbonate dissolution to be a major factor controlling dissolved inorganic C (DIC) dynamics in the freshwater regime of the Hooghly, whereas phytoplankton productivity and CO2 outgassing dominated in the mixing regime. In the Sundarbans, the removal of DIC (via CO2 outgassing, phytoplankton uptake and export to the adjoining continental shelf region) dominated its addition through mineralization of mangrove-derived organic C. The concentration of dissolved organic C (DOC) in the Hooghly was ∼40 % higher than in the Sundarbans, which was largely due to the cumulative effect of anthropogenic inputs, DOC–POC interconversion and groundwater contribution rather than freshwater-mediated input. The measured δ13CPOC in the Hooghly suggested particulate organic matter contributions from different sources (freshwater runoff, terrestrial C3 plants and anthropogenic discharge), whereas the contribution from C3 plants was dominant at the Sundarbans. The significant departure of δ13CPOC from typical mangrove δ13C in the mangrove-dominated Sundarbans suggested significant particulate organic C (POC) modification due to degradation by respiration. The average pCO2 in the Hooghly was higher by ∼1291 µatm compared to the Sundarbans with surface runoff and organic matter degradation by respiration as dominant factors controlling pCO2 in the Hooghly and Sundarbans, respectively. The entire Hooghly–Sundarbans system acted as a source of CO2 to the regional atmosphere with ∼17 times higher emission from the Hooghly compared to the Sundarbans. Taken together, the cycling of C in estuaries with different levels of anthropogenic influences is evidently different, with significantly higher CO2 emission from the anthropogenically influenced estuary than the mangrove-dominated ones.

Low CO<sub>2</sub> evasion rate from the mangrove surrounding waters of Sundarban

“…The waters around mangroves are known to have a buffering capacity against coastal acidification (Sippo et al, 2016;Maher et al, 2018). In the present study, the data for the water around mangroves (and the pore water or groundwater data from other studies) consistently show a Talk:DIC ratio >1, [Dutta et al, 2019]), groundwater-and pore water-derived DIC (δ 13 CDIC = -18.0‰ in Sundarban [Dutta et al, 2019]; -14.5‰ to -10.0‰ [Maher et al, 2013]), and degradation and/or decomposition of marine phytoplankton (δ 13 CPOC = -21.8‰ [MEM, present study]; -22.0‰ to -20.0‰ [Rosentreter et al, 2018]). However, we found no significant correlation between δ 13 CDIC and δ 13 CPOC, which might be because the sources were well-mixed.…”

“…The Hooghly River is the main "artery" (Ray et al, 2018a) and only possible source of riverine freshwater for the Indian Sundarbans, and the adjacent sea is the BoB. For this reason, the near-zero salinity regime of the Hooghly River and the northern BoB have been universally used in previous works (Ray et al, 2015(Ray et al, , 2018aRay and Shahraki, 2016;Dutta et al, 2019) as freshwater and marine end-members, respectively, in their mixing models for the Indian part of the Sundarbans. As in these previous studies, we defined the observed salinity at the Diamond Harbour station (salinity approx.…”

“…Although the riverine freshwater supply is limited in our study area, some ends up in the river mouth. Moreover, the estimated Revelle factor of the pore water of Sundarban mangroves (computed from pore-water pH data of Mandal et al [2009] and pore-water DIC data of Dutta et al [2019]) was also much higher (17.6) than the Revelle factor in the creeks. Pore water is generated exclusively within the mangrove environment.…”

Reduction in Riverine Freshwater Supply Changes Inorganic and Organic Carbon Dynamics and Air‐Water CO2 Fluxes in a Tropical Mangrove Dominated Estuary

“…10.1029/2020JG006144 14 of 22 ( Akhand et al, 2016;Dutta, Kumar, Mukherjee, Sanyal, & Mukhopadhyay, 2019). The observed pCO (water) and air-water CO 2 efflux from the Dhamra were similar to those of the nearby Mahanadi Estuary (Dey et al, 2013;Ganguly et al, 2011).…”

Response to Reviewer 2

“…Authors' Response: Considering Reviewer 2's comments and rechecking previous studies (for example Ray et al 2018;Dutta et al 2019) in Sundarban (Indian part) on CO2/ carbon dynamics, we decided to eliminate all the wordings and sentences, which seems there is no riverine connection (or almost none), and replace the confusing phrases like "negligible riverine freshwater input" to "indirect and lesser riverine freshwater input". This is because the quantification of riverine freshwater input from Hooghly River to the Sundarban (Indian part) was not done previously using hydrological modelling and out of scope for the present study, It is well established fact, that Indian part of Sundarbans are not getting riverine freshwater directly from its upstream rivers (for example Matla, saptamukhi, Thakuran etc.…”