The seasonal variations in sources of dissolved inorganic carbon (DIC) with reference to river discharge were evaluated using concentrations and isotopic composition of DIC ( \(\delta\) 13CDIC) in the Godavari estuary. Near freshwater was observed throughout the entire estuary during the wet period, whereas mixing of freshwater and marine water occurred during the dry period. Concentrations of DIC displayed an inverse relationship with salinity, with lower \(\delta\) 13CDIC values in the upper than the lower estuary during the dry period, suggesting that DIC variability in the estuary can be explained through the estuarine mixing. In contrast, no significant trends in the concentration and \(\delta\) 13CDIC were observed during the wet period in the estuary due to dominant freshwater flow. The deviation of the DIC (ΔDIC) and \(\delta\) 13CDIC (Δ \(\delta\) 13CDIC) from the mixing line between freshwater and seawater end members indicates that the DIC is modified within the estuary. The plot of ΔDIC and Δ \(\delta\) 13CDIC indicates that biological production contributed to DIC changes mostly during the dry period. Dominant contribution through organic matter decomposition was observed in the lower estuary from February to April and August. DIC contribution through the dissolution of carbonate mineral particles was noticed in the lower estuary of March and the entire estuary during October. The lower estuary from January to June seems to be influenced by carbonate production. Submarine groundwater discharge contributed to < 5% of the total DIC in the estuary during the dry period. The annual DIC flux to the Bay of Bengal amounted to 1.8 ± 0.4 Tg y−1 (1 Tg = 1012 g) with maximum contribution during the wet period (~ 86% of annual flux). The annual water flux from the Godavari River is ~ 0.15% of the global river water, whereas the annual DIC flux from the Godavari is ~ 0.5% of the global river DIC flux. This study suggests that DIC was significantly modified within the Godavari estuary and supplies disproportionately to the coastal ocean.
