Multi-annual variability of pCO2(aq) and air-water CO2 flux in the mangrove-dominated Dhamra Estuary draining into the Bay of Bengal (India)

Abstract

Small estuaries often remain neglected while characterizing air-water CO₂ flux dynamics. This study reports the seasonal, spatial, and multi-annual variability of carbon biogeochemistry, emphasizing air-water CO₂ flux from a small tropical mangrove-dominated estuary (Dhamra Estuary) of the Bay of Bengal, based on the 9-year-long sampling survey (2013 to 2021). The sampling covered twelve pre-fixed locations of this estuary. A suite of biogeochemical parameters was kept within the purview of this study to deliniate the interrelationship between CO₂ fluxes and potential factors that can regulate/govern pCO₂(aq) dynamics. Air water CO₂ exchange rates were calculated using five globally accepted empirical gas transfer velocity equations and varied in a range of - 832.5 to 7904 μmol m^-2 h^-1. The estuary was a sink for CO₂ in monsoon season, having the highest average flux rates of - 380.9 ± 125.5 μmol m^-2 h^-1, whereas a source in pre-monsoon (38.29 ± 913.1 μmol m^-2 h^-1) and post-monsoon (91.81 ± 1009.8 μmol m^-2 h^-1). The significant factors governing pCO₂ were pH, salinity, total alkalinity and dissolved inorganic carbon (DIC). This long-term seasonal study emphasizes the need to include small regional estuaries for more accurate estimates of global CO₂ flux to upscale the global carbon budget and its controlling mechanism.

Keywords: Air-water CO2 flux; Coastal region; Estuary; Inter-annual variation; Partial pressure of CO2.