Export Flux, Biogeochemical Effects, and the Fate of a Terrestrial Carbonate System: From Changjiang (Yangtze River) Estuary to the East China Sea

Seasonal variations in the transports of total alkalinity (TAlk) and dissolved inorganic carbon (DIC) from the Lower Changjiang (Yangtze) River/Estuary to the East China Sea were investigated based on a series of field surveys in 2015-2017, including monthly samplings at Datong Station and seasonal mapping cruises in the Changjiang Estuary and the adjacent northwestern East China Sea. In comparison with historical data sets, the Changjiang TAlk flux varied around a nearly stable average over the past 55 years. This is much different from some American rivers, where TAlk export fluxes increased for a century long. To assess effects of riverine carbonate inputs on coastal carbonate chemistry, we compared several cases showing freshwater-dilution-induced decline in coastal aragonite saturation state (omega(arag)), including rainwater dilution and riverine water dilution. Without riverine carbonate inputs, the effect of a unit of salinity decrease (due to rainwater dilution) on omega(arag) was expected to be counteracted by a DIC removal of 10 mu mol/kg relative to the baseline value along relevant conservative mixing line, when coastal omega(arag) was close to a critical value of 1.5. Considering terrestrial carbonate inputs from Changjiang, however, the freshwater-dilution-induced coastal omega(arag) suppression decreased by 12%. Our data also showed that more than 10% of wet-season DIC flux discharged from the Changjiang Estuary was sequestered by biological activities in nearshore areas, while the TAlk flux was rarely affected. This biological alteration effectively transformed the terrestrial carbonate system from a feature of DIC:TAlk >1.0 to the usual seawater feature of DIC:TAlk <0.9. Plain Language Summary Changjiang (Yangtze River) serves as the second largest carbonate contributor to the ocean among the world large rivers. We examined riverine/estuarine transport fluxes of total alkalinity (TAlk) and dissolved inorganic carbon (DIC) in the continuum from the Lower Changjiang to its estuary and to the adjacent northwestern East China Sea. In comparison with historical data, the Changjiang TAlk flux varied around a nearly stable average over the past 55 years, which was much different from the American case of century-long TAlk increase in some rivers. We also assessed effects of riverine carbonate inputs on the coastal carbonate chemistry. Results suggest that terrestrial carbonate inputs decreased the freshwater-dilution-induced carbonate mineral suppression in coastal zones. Based on field data, we estimated that more than 10% of wet-season DIC flux discharged from the Changjiang Estuary was sequestered by biological activities in nearshore areas, while the TAlk flux was rarely affected. We explained how biological drawdown of riverine DIC transformed the terrestrial feature of DIC:TAlk ratio higher than 1.0 to the usual seawater feature of DIC:TAlk ratio less than 0.9, supporting Alfred C. Redfield's argument on "the influence of organisms on the composition of seawater" in the 1960s or earlier.