Carbonate system in the subtropical Jiulong River Estuary and CO₂ flux estimation under modulation of tidal cycle

Estuaries are often a significant source of atmospheric CO₂. However, studies of carbonate systems have predominantly focused on large estuaries, while smaller estuaries have scarcely been documented. In this study, we collected surface and bottom seawater carbonate samples in the subtropical Jiulong River Estuary across different tidal levels from 2019 to 2021. The results showed that estuarine mixing of freshwater from the river with seawater was the dominant factor influencing the estuarine carbonate system. Moreover, estuarine mixing is concomitantly impacted by the net metabolism of biological production and decomposition, groundwater input, release of CO₂ from the estuary, and precipitation or dissolution of calcium carbonate. The estuarine partial pressure of CO₂ (pCO₂) varied from 530 µatm to 7 715 µatm, which represents a strong source of atmospheric CO₂. The mean annual air-sea CO₂ flux estimated from three different parameterized equations was approximately (25.63 ± 10.25) mol/(m²·a). Furthermore, the annual emission to the atmosphere was approximately (0.031 ± 0.012) Tg C, which accounts for a mere 0.007 7%–0.015% of global estuarine emissions. Dissolved inorganic carbon (DIC), total alkalinity (TA) and the pCO₂ exhibited high variability throughout the tidal cycle across all cruises. Specifically, the disparities observed between DIC and TA during low and high tides at identical stations during all cruises ranged from approximately 15% to 30%. The variance in the pCO₂ was even more pronounced, ranging from approximately 30% to 40%. Thus, tidal discrepancies may need to be taken into consideration to estimate the CO₂ flux from estuarine systems more accurately.