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

Abstract

Estuaries act as an important land-ocean transition zone where river nutrients and organic matter experience complex biogeochemical processes commonly leading to a super-saturation of CO2. The CO2 super-saturation makes them significant emitters of this trace gas to the atmosphere via air-sea exchange. Globally, total CO2 emission from estuaries is estimated to be in the range of 0.27 to 0.50 Pg C yr-1. This counteracts for about 21% of the total uptake of CO2 by the continental shelf. Thus, the huge estuarine emission or efflux rates of CO2 causes them to play an immense role in the global carbon budget. The rate of CO2 flux depends on several factors such as temperature, wind speed, depth of water etc and thereby, shows wide variability in accordance to the environmental setup of an estuary. However, CO2 flux from several small-scale estuaries are till date not evaluated. Also, the method of determination of CO2 flux in estuaries (especially shallow depth estuaries) is still under debate, which calls attention to the ambiguousness of the global carbon budget. This study aims to present data on the spatial and seasonal variability of CO2 and CO2 flux and discuss the factors controlling this variation in the Devi river estuary formed by a major distributary of the Mahanadi River draining into the Bay of Bengal. The surface water pCO2 in the Devi estuary is 1565 ± 782 μatm with pCO2 maxima and minima in the winter and monsoon season respectively. This high pCO2 is attributed to intense organic matter decomposition, which suggests a net heterotrophic environment prevailing in the estuary. Such a high pCO2 makes this estuary a significant source of CO2 to the atmosphere with an annual flux of 40.1 ± 7.5 mol C m-2 yr-1. Strong seasonal variation of CO2 flux with higher flux rates in summer than winter is encountered in the estuary due to changes in temperature and wind speed. The net annual flux from the estuary contributes about 0.003 to 0.006% of the global estuarine CO2 emission. Therefore, this study shows the importance of small-scale estuaries in the global carbon budget and highlights the role of various local-scale atmospheric factors in controlling the pCO2 and its flux to the atmosphere.