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

Abstract

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 (Ω 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 Ω arag was expected to be counteracted by a DIC removal of 10 μmol/kg relative to the baseline value along relevant conservative mixing line, when coastal Ω arag was close to a critical value of 1.5. Considering terrestrial carbonate inputs from Changjiang, however, the freshwaterdilution-induced coastal Ω 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.

Seasonal Mixing and Biological Controls of the Carbonate System in a River-Dominated Continental Shelf Subject to Eutrophication and Hypoxia in the Northern Gulf of Mexico

“…Though the riverine nitrogen-induced biological activity can trigger a larger variation than the influence of air-sea CO 2 gas exchange on Arag variations, we suggest that the former factor only controls the seasonal Arag variation and that the latter one controls the Arag value on a longer time scale (decadal). Low Arag values in low salinities (S < 18) in Figure 9E are consistent with results provided by Xiong et al (2019), who have simulated the effect of salinity changes on DIC and indicate that Arag decreases with a reduction in salinity at a given DIC. Besides, a few Arag values lower than the regression line (Figure 9E) may signal the benthic processes affecting the inorganic carbon system (Murrell et al, 2013;Fry et al, 2015;Hu et al, 2017).…”

“…Slope and intercept of this regression can reveal the effects of photosynthesis, respiration, and air-sea gas exchange on the Arag variation. Xiong et al (2019) have suggested that high Arag values were insensitive to salinity changes than low ones. They also have considered the combined effect of salinity and DIC changes on Arag variations.…”

Comparing Subsurface Seasonal Deoxygenation and Acidification in the Yellow Sea and Northern East China Sea Along the North-to-South Latitude Gradient

“…The summertime hypoxic area in the ECS was estimated at greater than 12,000 km 2 (or 432 km 3 volume, Chen et al, 2007), which was comparable to the largest coastal hypoxic zones observed in the world (Diaz, 2001;Fennel and Testa, 2019). To date, aragonite undersaturation has not been observed in the northern ECS, although the lowest summertime bottom-water arag value in the outer Changjiang Estuary has showed an alarming decline in recent years from 1.70 in July 2009 (Chou et al, 2013a) to 1.52 in July 2016 and to 1.39 in July 2017 (Xiong et al, 2019). Chou et al (2013a) predicted that the combination of intensifying eutrophication and increasing atmospheric CO 2 would push the bottom water of the Changjiang plume area toward undersaturated with respect to aragonite ( arag ∼0.8) by the end of this century.…”

“…Processes Driving Seasonal Variations of Bottom-Water DIC, pH T , and arag Dynamics of DIC, pH T , and arag in coastal zones are subject to multi-drivers, including coastal upwelling (Feely et al, 2008), riverine freshwater inputs (Salisbury et al, 2008;Rheuban et al, 2019;Xiong et al, 2019), vertical and lateral water mixing (Wang et al, 2013;Wanninkhof et al, 2015), and metabolic processes (Feely et al, 2010). In addition to these drivers, the seasonal temperature variability also affects the carbonate chemistry (Zhai et al, 2014b).…”

“…TAlk in the Yalu River is only 320-800 µmol kg −1 (Zhai et al, 2014b(Zhai et al, , 2015, approximately 1000 µmol kg −1 lower than the Changjiang TAlk value (1500-1900 µmol kg −1 , Xiong et al, 2019). As illustrated by Chen and Wang (1999) and Zhai et al (2014a), the ECS offshore waters originate from the Kuroshio tropical water.…”