Improving The Understanding Of Seasonal Acidification In Different Coastal Systems And Its Relationship To Hypoxia Based On An Investigation Along The Latitude Gradient

The absorption of atmospheric anthropogenic carbon dioxide(CO2)has acidified the ocean,threatening marine calcifying organisms and even the whole marine ecosystem.In productive coastal zones,the respiration of algae and/or remineralization processes of other biogenic particles consume dissolved oxygen(DO)and release a great deal of CO2 in subsurface waters,leading to more rapid seawater acidification in coastal seas,compared with the open ocean.The coastal acidification can develop within a few months and is bound to enhance negative effects related to elevated atmospheric CO2-induced ocean acidification.It is shown that various degrees of respiration-induced seasonal deoxygenation and acidification occur along China's coasts in recent years.However,the transregional pattern of coastal acidification is still unclear because previous studies have been primarily confined to individual coastal seas or the same circulation system.Studies on individual coastal sea also are not sufficient to elucidate the coupling of coastal acidification and hypoxia.Therefore,a comparative study of different coastal systems helps improve the understanding of regional.differentiation of seasonal acidification and hypoxia and identify areas vulnerable to ocean acidification,which will assist future prediction of marine environmental changes and formulation of management policies.Aiming at the global environmental problem of hypoxia-acidification in productive coastal zones,we examined carbonate system parameters in the Yellow Sea(seasonally seriously acidified but free from hypoxia)and northern East China Sea(ECS)(frequent summertime hypoxia but inconspicuous acidification)based on a full-seasonal-covering investigation along the latitude gradient.Spatial-temporal pattern of carbonate chemistry and its controlling mechanisms in the two contrasting coastal seas were revealed.Together with hydrological data and DO measurements,the relationships between seasonal acidification and hypoxia were investigated by comparing yearly initial conditions and summertime respiration processes in the two coastal seas.Further,effects of riverine carbonate inputs on coastal acidification in China's and American estuaries were studied as well as the fate of the terrestrial carbonate system.Finally,latitude gradients of coast acidification along China's and American east coasts were compared to identify areas vulnerable to ocean acidification.In this study,seven field surveys were conducted on the Yellow Sea and the northern ECS during 2017-2018,spanning a wet summer(July-August 2018)and a dry winter(December 2017-January 2018),as well as those transitional seasons of spring(March-May 2018)and autumn(October-November 2017 and October-November 2018).In winter and spring,DO,pH at total hydrogen ion concentration scale(pHT)and aragonite saturation state(flarag)in the Yellow Sea were averaged at 291±27 ?mol O2kg-1,8.06±0.05 and 1.92±0.18,respectively;DO,pHT and ?arag in the northern ECS were averaged at 258±33?mol O2 kg-1,8.03±0.06 and 2.20±0.45,respectively.In the Yellow Sea Cold Water Mass(YSCWM),summertime bottom-water DO,pHT and ?arag were 248±25?mol O2 kg-1,7.97±0.05 and 1.59±0.18,while autumnal bottom-water DO,pHT and ?arag were 195±21 ?mol O2 kg-1,7.85±0.04 and 1.28±0.09,respectively.In the northern ECS,summertime bottom-water DO,pHT and?arag were 126±29?mol O2 kg-1,7.85±0.07 and 1.91±0.32,while autumnal bottom-water DO,pHT and ?arag were 203±27?mol O2 kg-1,7.98±0.05 and 2.60±0.29,respectively.Lowest pHT(7.71)and ?arag values(1.07)along with undersaturated DO(>150 ?mol O2 kg-1)occurred in the YSCWM area in autumn,while hypoxic DO values of 49-63 ?mol O2 kg-1 and extremely low pHt values of 7.68-7.74 as well as critically low ?arag values of 1.21-1.39 were observed in the northern ECS in summer.By comparing field-measured carbonate system parameters in the two coastal seas with their air-equilibrated levels,processes driving seasonal variations of dissolved inorganic carbon(DIC),ratio of dissolved inorganic carbon and total alkalinity(DIC:TAlk ratio),pHT and ?arag in the bottom-water were investigated.The air-sea re--equilibration played a fundamental role in wintertime carbonate system parameters,which naturally preconditioned the water column to form yearly initial conditions.In summer,the community respiration beneath thermoclines dominated the increases in bottom-water DIC and DIC:TAlk ratio and the declines in bottom-water pHT and ?arag.Moreover,the autumnal collapse of thermoclines driven by cooling-induced water-column overturning or episodic wind-driven mixing events led to bottom-water ventilation,causing these carbonate system parameters to approach their air-equilibrated levels.Subsurface seasonal deoxygenation and acidification in both the Yellow Sea and northern ECS were induced by the community respiration beneath thermoclines during warm seasons.The northern East China Sea is subject to concurrent hypoxia and CO2 acidification in summer,while the seasonally acidified Yellow Sea is free from hypoxia.At the beginning of the warm-season stratification formation,yearly initial DO values in cold Yellow Sea waters were higher than those in the warm northern ECS waters The yearly initial pHT values in the two coastal seas were similar,whereas yearly initial DIC:TAlk ratio and ?arag values were quite different.The Yellow Sea had higher yearly initial DIC:TAlk ratio and lower yearly initial ?arag values than those in the northern ECS.In comparison with the relatively low-latitude northern ECS,relatively high-latitude Yellow Sea exhibited higher O2 and CO2 solubility in winter.The different yearly initial conditions preconditioned by wintertime re-equilibration were key reasons why the Yellow Sea suffers from serious seasonal acidification but is still free from hypoxia.Coastal seasonal acidification is also affected by riverine water inputs.To investigate the potential environmental impacts of riverine carbonate flux in different system,we compared buffering effects of terrestrial carbonate inputs on coastal acidification in Changjiang River,Yalu River and Mississippi river plume areas based on historical data,incorporating an ideal rainwater dilution case.Without riverine carbonate inputs,the effect of a unit of salinity decrease(due to rainwater dilution)on?arag was expected to be counteracted by a DIC removal of 10?mol kg-1 relative to the baseline value along relevant conservative mixing line,when coastal Qarag was close to a critical value of 1.5.Considering terrestrial carbonate inputs from Changjiang,however,the freshwater-dilution-induced coastal Qarag suppression decreased by 12%(8.75-8.86 versus 10).The buffering effect of Changjiang River plume against the freshwater-dilution-induced Qarag suppression was between the Mississippi river plume(9.43)and Yalu River plume(6.73)cases.In addition,the riverine Ca2+ inputs further increased the buffering effect of coastal carbonate system against the freshwater-dilution-induced Qarag suppression by 5-12%.Then,we used DIC:TAlk ratio to describe the transformation process from the terrestrial carbonate system into seawater carbonate system,attempting to better understand the mixing behavior and fate of terrestrial inorganic carbon on the shelf.Based on historical data along Changjiang Estuary and the ECS in 2005-2017,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.In the biological productivity fronts,the biological drawdown of DIC induces a substantial decline in DIC:TAlk ratio from>1 at the river end to as low as?0.7.Our findings support Alfred C Redfield's argument on "the influence of organisms on the composition of seawater" in the 1960s or earlier.Globally,the marine alteration of terrestrial carbonate system by biological production exists in many large river plume areas,such as the Mississippi plume,the Amazon plume,and the Pearl River plume,China.In order to explore the latitude gradient of coastal acidification,historical dataset in east coasts of China(July-August 2018)and America(June-July 2018)were collected to compare their summertime seawater carbonate chemistry and acidification parameters along the north-to-south latitude gradient.Along the two east coasts,summertime bottom-water DIC:TAlk ratios decreased from north to south,while bottom-water ?arag values increased from north to south.Seawater ?arag had a strong negative correlation with DIC:TAlk ratio.The correlation between ?arag and DIC:TAlk ratio exhibited little differences among those diverse waters involved in this study.More CO2 dissolved in seawater as indicated by higher DIC:TAlk ratio,more CO32-ions were titrated,resulting in lower ?arag values.North-to-south declines in sensitivity to ocean acidification stresses were revealed along east coasts of China and America.The lowest bottom-water ?arag values in northern shelf waters along China's and U.S.eastern coasts,i.e.,the North Yellow Sea and Gulf of Maine(with very high DIC:TAlk ratios of?0.96)were close to the critical value for the ideal aragonite dissolution(?arag?1).In summary,subsurface seasonal deoxygenation and acidification in both the Yellow Sea and northern ECS were induced by the community respiration beneath thermoclines during warm seasons.The northern ECS was subject to concurrent hypoxia and CO2 acidification in summer.However,seasonal acidification in the central Yellow Sea free from hypoxia was more serious than that in the northern ECS.In comparison with the relatively low-latitude northern ECS,relatively high-latitude Yellow Sea exhibited higher O2 and CO2 solubility in winter.The different yearly initial conditions preconditioned by wintertime re-equilibration were key reasons why the Yellow Sea suffers from serious seasonal acidification but is still free from hypoxia,which improved the understanding of the coupling of respiration-induced coastal acidification and hypoxia.Seawater ?arag had a strong negative correlation with DIC:TAlk ratio among diverse waters involved in this study,suggesting DIC:TAlk ratio might have a potential for application in theoretical study of ocean acidification.