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 pCO2 and CO2 efflux from the estuary. High pCO2 (1565 ± 782 μatm) in comparison to the atmosphere (389 μatm) was found in the estuary throughout the study period. The highest pCO2 was observed during winter season (2071 ± 828 μatm) and the lowest one during monsoon (951 ± 316 μatm). A decreasing trend in pCO2 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 pCO2 accompanied by a decrease in pH suggests that organic matter degradation is a major controlling factor of pCO2 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 pCO2 and CO2 effluxes from estuaries worldwide in order to accurately estimate the role of estuaries in the global carbon budget. •Seasonal and spatial distribution of pCO2 and CO2 efflux is studied.•Estimated CO2 efflux from Devi estuary is 40.1 ± 7.5 mol C m−2 yr−1.•Summer season has maximum efflux though winter season has maximum pCO2.•CO2 efflux is higher than some of major global estuaries.•Devi estuary has heterotrophic environmental condition.