Coastal Acidification in China Seas KEY POINTS • Wintertime air-sea re-equilibration, summertime respiration and autumnal upset dominate subsurface carbonate chemistry in coastal seas. • High CO 2 solubility together with respiration leads to high DIC:TAlk ratios and low aragonite saturation state in the central Yellow Sea. • The northern East China Sea is subject to concurrent hypoxia and CO 2 acidification in summer, while the Yellow Sea is free from hypoxia.

Under climate change scenarios, the contribution of macroalgae to carbon sequestration has attracted wide attention. As primary producers, macroalgae can release substantial amounts of dissolved organic carbon (DOC) in seawater. However, little is known about the molecular composition and chemical properties of DOC derived from macroalgae and which of them are recalcitrant DOC (RDOC) that can be sequestered for a long time in the ocean. In the most intensive seaweed (kelp) farming area (Sanggou Bay) in China, we found that kelp mariculture not only significantly increased DOC concentration, but also introduced a variety of new DOC molecular species, many of which were sulfur-containing molecules. A long-term DOC degradation experiment revealed that those DOC with strong resistance to microbial degradation, i.e., RDOC, account for approximately 58% of the DOC extracted from kelp mariculture area. About 85% (3587 out of 4224 with different chemical features) of the RDOC molecular species were steadily present throughout the long-term degradation process. 15% (637 out of 4224 with different chemical features) of the RDOC molecular species were likely newly generated by microorganisms after metabolizing macroalgae-derived labile DOC. All these stable RDOC should be included in the blue carbon budgets of seaweed.

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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.