Decoding riverine control of air-sea CO2 flux in the Pearl River Estuary: the dominant role of terrestrial organic matter quantity and stoichiometry.

Estuarine air-sea CO2 flux is governed by terrestrial inputs with opposing effects, yet their quantitative controls remain unclear, complicating predictions under anthropogenic change. To address this, we used an observationally constrained physical-biogeochemical model to conduct a process-based attribution of air-sea CO2 flux in the eutrophic Pearl River Estuary during summer, when peak river inputs drive the most active carbon cycling of the year. Results show that the system acts as a net source because CO2-enhancing processes (high-pCO2 river input and terrestrial organic matter (OMterr) remineralization) outweigh nutrient-fueled CO2 consumption. The estuary-wide air-sea CO2 flux is an order of magnitude more sensitive to changes in OMterr load than to equivalent variations in nutrients. OMterr stoichiometry regulates the compensation effect, i.e., the extent to which CO2 uptake fueled by OMterr-remineralization-derived nutrients offsets OMterr-released CO2; this effect increases from 16% to 29% as the C:N ratio decreases from 12 to 6. The net flux response depends strongly on whether the stoichiometric shift is driven by changes in organic carbon or organic nitrogen loading: a two-unit decrease in OMterr C:N ratio reduces outgassing by ∼14% when the shift is driven by organic carbon change, but by only ∼2-4% when driven by organic nitrogen change. This suggests that the decline in OMterr C:N ratios observed in many rivers worldwide has likely attenuated estuarine CO2 emissions, with the magnitude depending on the driver of C:N decline. These findings underscore the importance of OMterr quantity and stoichiometry, alongside riverine carbonate chemistry, in shaping estuarine CO2 emissions.