Summer Carbonate Chemisty Of The Yellow Sea And Bohai Sea

Shelf marginal seas play an important role in the ocean carbon cycle.The Yellow Sea?YS?and Bohai Sea?BS?are two typical semi-enclosed marginal seas in China,which are influenced by two big estuaries,Changjiang Estuary and Yellow River Estuary.The carbonate chemistry in this area remains unclear as most achievements concentrated only on the surface layer with limited temporal and spatial coverage.Very few studies have considered the hydrographical conditions in this area,especially the driving force of different water masses.As the variations of carbonate characteristics may challenge marine organisms,we urgently need to have a deeper understanding of the carbonate chemistry characteristics in the YS and BS.To achieve this target,two cruises were performed in the YS and BS,from 17 August to 6 September in 2015 and from 28 June to 21 July in 2016,respectively.Samples of pH were taken during both cruises while total alkalinity?AT?,nutrients?NO3,P,Si?and chlorophyll?Ch1 a?were measured in 2016.Thus the data in 2016 were mainly used to show the distributions of carbonate system.According to the Temperature-Salinity diagram and the hydrographic characteristics in 2016,we first classified the waters into six types:Changjiang Diluted Water?CDW?;Yellow Sea Coastal Water?YSCoW?;Yellow Sea Warm Water?YSWW?;Yellow Sea Cold Water?YSCW?;Bohai Sea Central Water?BSCW?and Bohai Sea Coastal Water?BSCoW?.The distributions of pH and AT as well as other auxiliary variables were studied for both surface and bottom layers.In the surface,pH varied from 7.916 to 8.307 while in the bottom pH ranged from 7.849 to 8.074.AT varied from 2063 ?mol kg-1 to 2542?mol kg-1 in the surface and from 2205 ?mnol kg-1 to 2554 ?mol kg-1 in the bottom.pH was higher in coastal areas while AT was higher in the North Yellow Sea?NYS?and the BS.Three transects were selected to show the vertical characteristics of the carbonate system:transect CE?32°N,122.5?124°E?near the Changjiang Estuary;transect YS?36°N,121?124°E?in the central YS and transect BS?37.75?N,119.38°E?39.61°N,120.61°E?in the BS from the Yellow River Estuary to the northeastern BS.The vertical distributions showed that the carbonate characteristics were highly correlated to the properties of different water types.The CDW was indicated at a low-salinity of 23.67?31.97 and it dominated the areas around 32°N,while in the central YS,the YSCW occupied the bottom layer with a lowest temperature of 5.97 ? and the YSWW was limited to the upper layer?<20 m?.In the BS,the water was homogenous and the BSCoW only affected the area outside the Yellow River Estuary.In the surface,nutrients were higher in the CDW and BSCoW.But in the bottom,high nutrient concentrations were detected in the YSCW.Higher Ch1 a was detected in upper coastal waters,like the CDW and BSCoW.To study the main factors controlling the carbonate system,Pearson correlation analysis and multiple linear regressions?MLRs?were conducted on carbonate variables and environmental variables.The regression formulae between AT and salinity,pH and salinity were used to show the mixing behaviors between various water types.The results showed that temperature,salinity,nitrate,phosphate and silicate were the main variables defining the carbonate variables,although single variable varied a lot in different water types.Among all relationships,only the CDW showed a strong positive correlation between AT and salinity.The outflow from the BS served as a high alkalinity source of the surveying area.The biological activities had big influences on carbonate characteristics.In the surface,the biological activity increased the pH through primary production which consumed nutrients,while in the bottom,the respiration/remineralization processes of biogenic particles have caused a significant release of CO₂ which finally decreased the pH and increased total dissolved inorganic carbon?CT?,accompanying with the regeneration of nutrients in the bottom.Other processes like precipitation,typhoon,sediment and human activities can also affect the carbonate characteristics.To evaluate the temporal variations of pH,we compared the pH data between the two cruises.Generally,pH varied from 7.868?8.218 in the surface and from 7.787?8.051 in the bottom in Aug 2015.In the surface,the average pH values were 8.046±0.070 inJuly 2016 and 8.04±0.073 in August 2015 respectively,with a similar high pH zone detected in the plume of CD W.In the bottom,the pH values were 7.977±0.047 in July 2016 and 7.922±0.062 in August 2015.The pH in the bottom in August 2015 was lower than in July 2016 and the differences were mainly observed in the South YS and BS.Through a summary of reported observations on carbonate system,we discussed the variations of carbonate system and suggested the changes in hydrological and biological conditions between the years were the dominant factors causing these changes,like the change of current circulation and upwelling,and enhanced eutrophication.Ocean acidification?OA?will occur in the future as a result of interactions between many factors,including the excess CO₂ uptake,changes in ocean circulation,land-ocean exchanges,biological productivity and respiration,and freshwater contributions.The rate of carbonate system modification in the YS and BS will also be controlled by the change of riverine discharge?e.g.,Changjiang River,Yellow River?and the nutrients imported to the sea.Acidification will have potential effects on marine ecosystems,e.g.,it will decrease the ?Ar in the seawater and bring new challenges to the shell fish culturing in the YS and BS.