Quasi-synchronous Cumulative Oxygen Utilization And Dissolved Inorganic Carbon In The South Yellow Sea Cold Water Mass

Ocean acidification is a complex phenomenon of decreasing the pH value of seawater,which is the result of many factors,including increased dissolution of atmospheric CO2(exogenous acidification)and the addition of dissolved inorganic carbon from the re-mineralization of primary producers and dissolved organic matter(endogenous acidification).Ocean acidification leads to changes in seawater carbonate system,affecting CO2 partial pressure(pCO2),dissolved inorganic carbon(DIC).pH,total alkalinity(TAlk)and aragonite saturation(?arag).and then causing a series of chemical changes in the ocean system,thus influencing the growth,reproduction,metabolism,survival and other processes of marine organisms to varying degrees.Among them,marine calcifying organisms are the first to suffer from environmental stress.The Yellow Sea is a highly productive marginal sea in the northwest Pacific Ocean,where seasonal acidification is a serious problem and develops rapidly.The Yellow Sea is susceptible to various ecological environmental impacts caused by acidification.The purpose of this paper is to elucidate the spatiotemporal dynamic distribution and the main regulatory processes of the Yellow Sea carbonate system in order to understand and evaluate the environmental status of the Yellow Sea and the development trend of acidification.Based on the carbonate system data from our four field surveys in the Yellow Sea in 2019,combined with the hydrological conditions of the Yellow Sea and the dissolved oxygen data,we clarified the spatial distribution pattern and seasonal variation characteristics of hydrology with carbonate system parameters in the Yellow Sea.The results show that the survey-average values in the South Yellow Sea in April of dissolved oxygen,pHT and ?arag were 287±19 ?mol·kg-1?8.08±0.06?2.12±0.25,respectively.In the Yellow Sea Cold Water Mass,the dissolved oxygen in the bottom water decreased from 204±24 ?mol·kg-1 in August to 184± 16 ?mol·kg-1 in October and continued to descend to 154±17 ?mol·kg-1 in November.Correspondingly,the bottom pHT decreased from 7.89±0.04 in August to 7.86±0.03 in October,and continued to drop to 7.84±0.04 in November.The underlying ?arag of 1.42±0.12 in August reduced to 1.37±0.08 in October,and continued to decline to 1.32±0.09 in November.The lowest annual value of dissolved oxygen in the whole sampling area was?69 ?mol·kg-1(dissolved oxygen saturation?32%),close to the critical value of hypoxia,and the corresponding pHT was 7.77 in the southern Yellow Sea in summer.The ?arag reached the lowest value of 1.15 at the bottom of the central South Yellow Sea station in late autumn.Combined with the analysis of historical data in 2018,the regulation mechanism of DIC and acidification indexes in the South Yellow Sea was clarified by comparing the measured values of carbonate system parameters and their equilibrium values with the atmosphere.The results show that the hydro-chemical parameters and acidification indexes in the bottom of the South Yellow Sea were mainly controlled by physical processes in spring,while the primary production played a leading role in the removal of DIC and the rise of ?arag in surface seawater.Community respiration consumed dissolved oxygen in summer and autumn,which played a leading role in the accumulation of DIC and the decline of ?arag in the bottom of the Yellow Sea Cold Water Mass.The surface algal blooms in spring may provide sufficient biogenic debris for the oxygen consumption and acidification in the bottom of the Yellow Sea Cold Water Mass in summer and autumn.The settled biogenic debris was quickly ngested by zooplankton and decomposed by microorganisms,causing rapid oxygen consumption and acidification of the lower seawater,and even ?arag rapidly falling below the critical value of calcium carbonate corrosive seawater.As the seasons changed,the degradable biogenic debris gradually decreased,and the remineralization rate of organic matters gradually decreased.The end-member mixing model was used to carry out an approximate simulation of the hydrological dynamics of the South Yellow Sea.The hydrological characteristics of the South Yellow Sea in spring were analyzed.The results show that the spring hydrological conditions of the South Yellow Sea were controlled by the Subei Coastal Water,the Lubei Coastal Current and the Yellow Sea Warm Current The Yellow Sea Warm Current dominated most area of the South Yellow Sea,while the coastal water was limited to the shoal of northern Jiangsu and the south coast of Shandong Peninsula.The Yellow Sea Warm Current at varying temperatures carried different concentrations of dissolved inorganic carbon into the South Yellow Sea,which determined the initial ?arag in the South Yellow Sea in spring,while the influence of the internal circulation in the South Yellow Sea Cold Water Mass in summer and autumn can be ignored.The sensitivity of ?arag to DIC accumulation in the bottom of the cold water mass was weaker than that in the general shelf sea,and the annual initial water temperature is the key factor to control this sensitivity difference.