Partial pressure of carbon dioxide (pCO2) and air-water CO2 exchange in the tropical semidiurnal estuarine system

Abstract

Time-series observations of the Vellar estuary between May 2013 and December 2019 showed clear variability with respect to space and time in the distribution of nutrients, partial pressure of carbon dioxide (pCO₂) and air-water CO₂ exchange. Lower and higher salinities revealed significant seasonality in estuarine pCO₂, as well as variations in the seasonal pattern due to the freshwater discharges during monsoon rainfall. The pCO₂ attained the highest levels (8457 µatm) during monsoon which coincided with the lowest pH (7.498) and the undersaturation of pCO₂ (322 µatm) was observed with maximum pH (8.182) during pre-monsoon. The Principal Component Analysis (PCA) identified four components that accounted for 77.28 % of the total variance and explained the significant influence of nutrients, chlorophyll and temperature on pCO₂ distribution. Similarly, the multiple linear regression analysis showed significant influence of environmental variables on pCO₂ variability with a R² of 0.957, SEE±230.816, p < 0.001. The surveyed area of the Vellar estuary had an overall pCO₂ of 1068 µatm and was supersaturated with regard to the atmospheric pCO₂ throughout the year, with an average CO₂ flux of 4.13±5.59 mmol C m^-2 d^-1 to the atmosphere. During the study period, the Vellar estuary actively supplied 650.2 mol C m^-2 Y^-1 to the atmosphere. Hence, the metabolic balance of the estuarine ecosystem is aided by land derived organic carbon accompanied with freshwater flows from the Vellar river, constituting the estuary as a substantial source of atmospheric CO₂.