Role of inter-annual variability in the magnitude of river discharge on CO2 fluxes in the Godavari tidal river zone, India

Abstract

Variations in the magnitude of river discharge may alter pH, pCO2 of surface water, and flux of CO2 to the atmosphere in the Tidal River Zone (TRZ). To test this hypothesis, a nearly decade-long time-series data of salinity, pH, and pCO2 data were collected and analyzed in the TRZ of Godavari River. The higher freshwater discharge occurred in the TRZ during the wet period (June to December) compared to the dry period (January to May). An increase in annual mean discharge was noticed between 2008 and 2019 due to surplus rainfall and the expansion of the hydropower plant. The surface water pH and pCO2 in the TRZ were controlled by the river discharge as evidenced by the linear positive and negative relationships with the rate of river discharge respectively. A strong relationship was observed between apparent oxygen utilization (AOU) and excess CO2 indicating a strong influence of heterotrophic activity on pCO2 levels in the TRZ. The respiration quotient (RQ) was above the aerobic respiration value of 1.35 indicating that terrestrial sources, such as sewage, and groundwater discharge, may have contributed to pH and pCO2 levels in the TRZ. The CO2 fluxes during the peak discharge period contribute to 48–88% of annual mean fluxes followed by moderate (12–52%) and dry periods (<4%). The annual mean CO2 emission amounted to 0.04 ± 0.05 TgC for the area of 36.9 × 106 m2 and increased at the rate of 0.005 TgC y−1 between 2008 and 2019 due to an increase in river discharge. The CO2 flux from the Godavari TRZ (0.04 TgC y−1) contributes to ∼0.2% of global tropical estuaries (18 TgC y−1). The CO2 flux contributed to ∼1.5% of the total carbon (both particulate and dissolved) exported from the Godavari TRZ to the Bay of Bengal and it is lower than the global mean contribution of 16%.