Seasonal shifts in diurnal variations of <i>p</i>CO<sub>2</sub> and O<sub>2</sub> in the lower Ganges River

Abstract

High-frequency, paired measurements of the partial pressure of CO2 (pCO(2)) and dissolved O-2 were combined with dissolved organic matter (DOM) characterization to investigate diurnal (7:00-19:00) and seasonal variations in CO2 dynamics in the lower Ganges River. Diurnal variations in pCO(2) shifted seasonally, concurring with changes in DOM optical properties, biodegradable DOM, and O-2 departures from atmospheric equilibrium (Delta O-2). Increased pCO(2) and CO2 flux (FCO2) in August-September corresponded to monsoonal floods carrying more terrestrial materials. Lower dry-season pCO(2) and FCO2 concurred with higher Delta O-2, and exhibited strong relationships with DO and pH, indicating a dominant influence of phytoplankton production. Despite large variations in pCO(2), the year-long positive FCO2 implied that the lower Ganges acted as a CO2 source, reaching 122 mmol m(-2) d(-1). Increasing hydrologic variability and pollution in the Ganges Basin can amplify pCO(2) variations linked to seasonal shifts in terrestrial inputs and phytoplankton production.