Seasonal shifts in diurnal variations of p CO ₂ and O ₂ in the lower Ganges River

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