Interannual and seasonal variability and future forecasting of pCO2( water) using the ARIMA model and CO2 fluxes in a tropical estuary

Abstract

The seasonal and interannual variation in the partial pressure of carbon dioxide in water [pCO(2)(water)] and air-water CO2 exchange in the Mahanadi estuary situated on the east coast of India was studied between March 2013 and March 2021. The principal aim of the study was to analyze the spatiotemporal variability and future trend of pCO(2) and air-water CO2 fluxes along with the related carbonate chemistry parameters like water temperature, pH, salinity, nutrients, and total alkalinity, over 9 years. The seasonal CO2 flux over nine years was also calculated using five worldwide accepted equations. The seasonal map of pCO(2)( water) followed a general trend of being high in monsoon (2628 +/- 3484 mu atm) associated with high river inflow and low during pre-monsoon (445.6 +/- 270.0 mu atm). High pCO(2) in water compared to the atmosphere (average 407.6-409.4 mu atm) was observed in the estuary throughout the sampling period. The CO2 efflux computed using different gas transfer velocity formulas was also consistent with pCO(2) water acquiring the peak during monsoon in the Mahanadi estuary (6033 +/- 9478 mu mol m(-2) h(-1)) and trough during pre-monsoon (21.66 +/- 187.2 mu mol m(-2) h(-1)). The estuary acted as a net source of CO2 throughout the study period, with significant seasonality in the flux magnitudes. However, CO2 sequestration via photosynthesis by phytoplankton resulted in lower emission rates toward the atmosphere in summer. This study uses the autoregressive integrated moving average (ARIMA) model to forecast pCO(2)( water) for the future. Using measured and predicted values, our work demonstrated that pCO(2)( water) has an upward trend in the Mahanadi estuary. Our results demonstrate that long-term observations from estuaries should be prioritized to upscale the global carbon budget.