Time-series observations were conducted in the Dowleiswaram dam reservoir that was constructed on the largest monsoonal river in India to understand the source of inorganic carbon, and fluxes to the atmosphere. The reservoir stores water during dry period of six months and water increases during the period when Indian subcontinent receives significant rainfall. Significant modification of organic matter was noticed during storage period indicated by decrease in pH from 7.5 to 6.4 and oxygen saturation from ~95% to 65%. The relationship of dissolved inorganic carbon (DIC) with oxygen saturation, dissolved organic carbon (DOC) and isotopic ratios of DIC suggests that heterotrophic activities are the major source of inorganic carbon to the reservoir. In addition to this, ground water exchange also contributes significantly to the inorganic carbon pool in the reservoir. Nutrients released due to decomposition of organic matter in the reservoir supports both autotrophic and heterotrophic activities. The pCO2 levels in the reservoir varied between 3944 and 16,042 μatm and higher pCO2 levels were noticed during peak discharge period. The annual mean CO2 fluxes from the reservoir amounted to 112 ± 126 mmolC m-2·d-1 and ~6 times higher fluxes were noticed during discharge period compared to dry period and such high fluxes during discharge period were contributed by both high pCO2 levels and winds. It was further noticed that dam reservoir is a strong source of pCO2 to the estuary wherein <1000 μatm of pCO2 during dry period and >15,000 μatm during discharge period were observed. Our study also indicates that Dowleiswaram dam reservoir is a strong source of CO2 to atmosphere, even though it is much smaller than Brazilian (tropical) reservoir but higher than European reservoirs.
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