Removal of dissolved inorganic carbon in the Yellow River Estuary

The Yellow River of China runs mainly through an arid and semiarid midlatitude region that has experienced substantial anthropogenic and climatic change. This area includes the carbonate-rich Loess Plateau and carries water of exceptionally high carbonate content. To investigate the processes by which dissolved inorganic carbon (DIC) is biogeochemically modified as the river approaches the sea, a multipronged field investigation was conducted in the Yellow River estuary, 2005-2009. The project included four research cruises (spring and fall), a year of monthly sampling at a lower-river hydrological station (Lijin), and in situ bottle incubations. Our study revealed that 4-11% of the Yellow River DIC was removed from the water column in the estuarine mixing zone and thus was not transported to the sea. DIC removal was greater in the spring and occurred at a higher salinity range than in the fall. As a unique feature of the Yellow River estuary, calcium carbonate (CaCO3) precipitation was nearly as important as net biological production in the DIC removal. Longer freshwater-seawater mixing distances (and times) and higher DIC concentrations in the freshwater end member also promoted net biological production and CaCO3 precipitation, thus encouraging DIC removal.