Distribution of pCO2 and air-sea CO2 flux in Devi estuary, eastern India

Carbon dioxide (CO2) super-saturation in estuaries and the associated efflux to the atmosphere is an important part of the global carbon cycle. However, the net global estuarine emission of CO2 is based on limited spatial data. In this study, alkalinity, pH, salinity and temperature data were used to quantify the CO2 dynamics in the Devi estuary formed by the main distributary of the Mahanadi river on the east coast of India. The samples were collected during summer, monsoon and winter of 2018-19 in order to understand the seasonal and spatial variability of pCO(2) and CO2 efflux from the estuary. High pCO(2) (1565 +/- 782 mu atm) in comparison to the atmosphere (389 mu atm) was found in the estuary throughout the study period. The highest pCO(2) was observed during winter season (2071 +/- 828 mu atm) and the lowest one during monsoon (951 +/- 316 mu atm). A decreasing trend in pCO(2) from the upper estuary to lower estuary was observed. The mean dissolved oxygen saturation (DO %) was 137% throughout the study period indicating well-oxygenated waters. A decrease in DO% with increased pCO(2) accompanied by a decrease in pH suggests that organic matter degradation is a major controlling factor of pCO(2) in the estuary. Thus, heterotrophic environmental condition is prevalent in the estuary. Net annual CO2 efflux from the Devi river estuary was estimated to be 40.1 +/- 7.5 mol C m(-2) yr(-1), and the highest efflux was observed during summer season (126 +/- 69 mmol C m(-2) d(-1)) due to higher temperature and wind speed. The net efflux from the Devi river estuary is about 0.003-0.006% of the total global CO2 emission from estuaries. This study highlights the importance of small-scale estuaries in emitting large flux of CO2 to the atmosphere. The results contribute to the increasing dataset of pCO(2) and CO2 effluxes from estuaries worldwide in order to accurately estimate the role of estuaries in the global carbon budget.