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

Dutta, Manab Kumar; Kumar, Sanjeev; Mukherjee, Rupa; Sanyal, Prasun; Mukhopadhyay, Sandip Kumar

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 inline-formulaCO2 outgassing dominated in the mixing regime. In the Sundarbans, the removal of DIC (via inline-formulaCO2 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 inline-formula∼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 inline-formulaδ13CPOC in the Hooghly suggested particulate organic matter contributions from different sources (freshwater runoff, terrestrial inline-formulaC3 plants and anthropogenic discharge), whereas the contribution from inline-formulaC3 plants was dominant at the Sundarbans. The significant departure of inline-formulaδ13CPOC from typical mangrove inline-formulaδ13C in the mangrove-dominated Sundarbans suggested significant particulate organic C (POC) modification due to degradation by respiration. The average inline-formulapCO2 in the Hooghly was higher by inline-formula∼1291inline-formulaµatm compared to the Sundarbans with surface runoff and organic matter degradation by respiration as dominant factors controlling inline-formulapinline-formulaCO2 in the Hooghly and Sundarbans, respectively. The entire Hooghly–Sundarbans system acted as a source of inline-formulaCO2 to the regional atmosphere with inline-formula∼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 inline-formulaCO2 emission from the anthropogenically influenced estuary than the mangrove-dominated ones.

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Dutta, Manab Kumar / Kumar, Sanjeev / Mukherjee, Rupa / et al: The post-monsoon carbon biogeochemistry of the Hooghly–Sundarbans estuarine system under different levels of anthropogenic impacts. 2019. Copernicus Publications.

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