| Both p H and carbonate saturation states(Ω) are important parameters of the seawater carbonate system, indicating the status and the ecological threats of ocean acidification(OA).Riverine transportation of dissolved inorganic carbon plays an important role in the global carbon cycle. Also, river discharge serves as a factor enhancing the vulnerability of coastal sea to OA. However, so far our understanding of the relationship between p H/Ω and riverine export of inorganic carbon is relatively poor. In this study, we attempt to study the export flux of inorganic carbon from Changjiang(Yangtze River) and its impacts on p H and Ω in the important estuarine region so as to understand how the large river dominated ocean margins response to OA in the future.In March and July 2015, two field surveys were conducted in the Changjiang Estuary and the adjacent East China Sea. Total alkalinity(TAlk) and dissolveld inorganic carbon(DIC) samples were collected onboard. To examine the reliability of estuarine water sample storage techniques, several water samples were collected onboard using 250 m L Teflon–coated glass bottles(together with ground–glass stoppers) and 60 m L borosilicate glass vials, respectively. Results showed that DIC and TAlk samples stored in 250 m L Teflon–coated glass bottles were quite stable, with a difference no more than 10 μmol kg–1 in a time fram of ~70 days, suggesting that the Teflon–coated glass bottle is suitable for the relatively long–term storage of freshwater carbonate samples. However, a DIC loss rate of 0.9—2.6 μmol kg–1 d–1 was observed in those freshwater samples that were stored in 60 m L borosilicate glass vials.Basing on the inorganic carbon results collected in the Changjiang extuary in 2015, riverine export fluxes of DIC and TAlk from the Changjiang River were estimated at 1.53 × 1012 mol yr–1 and 1.52 × 1012 mol yr–1, respectively. However, seaward DIC and TAlk export fluxes in 2015 from the Changjiang River were 1.63 × 1012 mol C yr–1 and 1.59 × 1012 mol yr–1, respectively, which show insignificant difference from the riverine DIC/TAlk export flux. Based on historical data obtained from 2008 to 2012(Zhai W-D, unpublished), the annual rinverine export flux of DIC ranges from 1.13 × 1012 mol C yr–1to 1.73 × 1012 mol C yr–1(with an average DIC flux of 1.43 × 1012 mol C yr–1) in the past decade, while the annual rinverine export flux of TAlk ranges from 1.12 × 1012 mol C yr–1 to 1.71 × 1012 mol C yr–1(with an average TAlk flux of 1.42 × 1012 mol C yr–1) during the same period. Comparing with the riverine inorganic carbon export flux from 1963 to 1999(before the the three gorges dam completed), the Changjiang River show no signigicant change in export flux of inorganic carbon during the past decade(after the fullfilling of the three gorges dam). The inoranic carbon export flux of the Changjiang River show a fluctuation around 1.6 × 1012 mol C yr–1 during the past 50 years, wihich is different from the Mississippi and other American rivers that show an increasing trend in TAlk export flux during the past 50 years.In March 2015, the Changjiang dilution water(CDW, with salinity lower than 31) occupied west of 123 °E. In the CDW, p H and carbonate saturation of seawater show tender increase from 122.06 °E to 123 °E. Away from the CDW, i.e. east to 123 °E, those parametes showed quite narrow change. In the area with salinity of higher than 10, p HT ranges from 8.0—8.16, and Ωcal ranges from 2.0 to 4.36, while Ωarag ranges from 1.1 to 2.79. Due to dilution effects of the CDW, the aragonite saturation state is no more than 1.5 in the low salinity(<21) area. Additionally, stations A1–1 and A1–3 represent a water mass of higher DIC, which is associated with the Yellow sea coastal current. As a result, the region showed comparitively lower p HT(8.02—8.05) and lower carbonate saturation state(Ωcal from 2.73 to 3.0 and Ωarag from 1.71 to 1.89) than other areas with the same salinity range.In July, However, p H and carbonate saturation were controlled by the combined effects of riverine dilution and biological process. The whole water mass has a stronge thermocline and weak halocline in the region with salinity lower than 31, average river discharge during survey was ~49500 m3 s–1, the CDW mainly occupied the west part of 123.5 °E. Besides, algae bloom lead dissolved oxygen, p HT, Ωcal, Ωarag of the surface water to relatively high level(DO% 85%—212%, p HT 7.87—8.55, Ωcal 2.04—9.78, Ωarag 1.2—6.15) in the area with salinity higher than 10. However, the remineralization of organic mater consumed O2 and released large amount of CO2 in the bottom water, driving the bottom water mass with low DO%, p HT, Ωcal and Ωarag(DO% 47%—82%, p HT 7.84—8.08, Ωcal 2.61—4.12, Ωarag 1.67—2.60).Model analyses showed that the effects of 1 unit salintiy decline on Ωarag can be expected to be counteracted by a △DIC decrease of 8.97 μmol kg–1, when seawater aragonite saturation was close to 1.5; 1 unit salintiy decline on p HT can be expected to be counteracted by a △DIC decrease of 3.41 μmol kg–1, when seawater p HT was close to 7.9. However, 1 unit salintiy decline on p HT can be expected to be counteracted by a △DIC decrease of 1.07 μmol kg–1, when seawater p HT was close to 8.1. Comparing modeled results of Ωarag with those in the Yalu River estuary, we get that Ωarag in the Yangtze River estuary is more sensitive to the DIC addition or the Changjiang dilution water. |
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