Stable Isotope Of Dissolved Inorganic Carbon In Bohai And Yellow Seas In Summer And Autumn,and Its Implication On The Origin Of Oxygen-consuming Organic Matters In Central Bohai Sea

Dissolved inorganic carbon and its stable isotope(δ13CDIC)can provide information about the source of carbon in the marine ecological cycle,and can be used to explore anthropogenic and natural effects on the ecosystem.As an important marginal sea of China’s continental shelf—Yellow Sea and Bohai Sea,the phenomena of seasonal hypoxia and acidification of them have gradually become the research focus of scientists.In recent years,δ13CDIC has become an effective tool to help scientists to understand the formation mechanism of ocean hypoxia and acidification,and thus has important significance for the study of marine biogeochemical cycle and marine economic development.Water samples were collected from Bohai and Yellow Seas in August and October of 2020 for DO,DIC,TAlk,δ13CDIC and δ18OH2O.Water samples were collected from central Bohai Sea in July of 2021 for DO,DIC,δ13CDIC,and from Laizhou Bay in June 2021.Meanwhile,the corresponding environmental parameters and historical data were obtained.Seasonal hypoxia was observed in the Huanghe estuary in summer of 2020,and was reappeared in 2021.High DIC values and negative δ13CDIC values were observed in the region corresponding to seasonal hypoxia area.The cold water mass in the middle subsurface of the Yellow Sea have low temperature and high salinity.High DIC values and negative δ13CDIC values were observed in the cold water mass area.The area of cold water mass decreased in autumn,but acidification phenomenon was still existed.Based on the relationship between δ13CDIC and AOU and DIC,it is inferred that the dominant factor of DIC growth in north Yellow Sea cold water mass is bio-dominated organic mineralization.On the basis of estimating the dominant role of biological process in the seasonal lowoxygen region,a reasonable three-end mixing model was established by selecting appropriate endmembers and taking salinity and seawater oxygen isotope as conservative parameters.The△(DIC×δ13CDIC)and △DIC were calculated and fitted according to the isotope conservation formula.The δ13COC obtained by fitting data points in the oxygen consumption area(-19.47±1.85‰)is approximate to the end member value δ13COCmarine(i.e.,organic matter in situ produced by marine phytoplankton).Therefore,the organic matter consumed by biological respiration in the Huanghe estuary of Bohai Sea in 2020 is dominated by the Marine organic matter produced by the in-situ photosynthesis of Marine phytoplankton.According to theδ13CDIC AOU and DIC-AOU fitting lines and the simplified isotopic conservation formula,it can be inferred that δ13COC in summer of 2021(-21.21～-18.66‰)was same as in 2020.There was no difference between 2021 and 2020 in the composition of oxygen consumption organic matter in Bohai Sea,but the annual initial values were different.In this study,δ13CDIC was fully used to understand and analyse how seasonal hypoxia and acidification respond to environment variation and biological process in the Bohai Sea and the Yellow Sea.For the first time in the Bohai Sea,a reasonable three-terminal mixed model was constructed by proper data processing,and used for further studies.Based on the principle of isotope conservation and its reasonable derivation and simplification,this paper analyzed the sources of oxygen consuming organic matter in the seasonal low oxygen area of Bohai Sea,that is,Marine organic matter plays a dominant role in the oxygen consuming organic matter.This study provides a potential reference for subsequent research.