The Mechanism Of Oxygen Deficit In Specific Areas Of The East China Sea By Using Dissolved Oxygen Properties

The dissolved oxygen in seawater is important for marine organisms,and plays an important role in stabilizing the marine ecological environment and the normal circulation of various geochemical parameters.When a large amount of dissolved oxygen in seawater is consumed and maintained for a long time,the concentration of dissolved oxygen eventually falls below 63?mol/L,it will lead to the occurrence of hypoxia.The hypoxia waters influence the health of the marine environment,and constantly threatens the economic activities such as human fishery production.Studies have shown that frequent occurrence of hypoxia events in many sea areas has become a universal phenomenon.Therefore,investigating hypoxia,understanding hypoxia and treating hypoxia have become a hot topic in the research of marine science.In this paper,Changjiang Estuary and its adjacent sea and the Aojiang Estuary and its adjacent sea are selected as the study areas respectively.These two study areas belong to the same large scope(East China Sea),but they are different.Oxygen isotope(¹⁸O)is used as a technical breakthrough.From the perspective of dissolved oxygen isotope analysis in seawater,the distribution characteristics of dissolved oxygen properties(concentration+isotope)in specific areas of the East China Sea(the Changjiang Estuary and its adjacent sea,and Aojiang Estuary and its adjacent sea in Lianjiang County)are explored,and the succession characteristics and main control factors of contribution rates of oxygen consumption in the water column respiration(WCR)and sediment oxygen respiration(SOR)to total apparent oxygen utilization(AOU)in the water column are interpreted,The major work contents and conclusions are as follows:1?Comparing the dissolved oxygen concentrations of the three voyages in July 2018,July 2020 and August 2020,the maximum values of average[O₂]in surface and near bottom waters appeared in July 2018(227?mol/L,148?mol/L),while in August 2020,the average[O₂]in surface and near bottom waters was the lowest (217?mol/L,88?mol/L).During the three voyages,the consumption of dissolved oxygen in the water column was accompanied by significant oxygen isotope fractionation,that is,the change of[O₂]showed the opposite relationship with ?¹⁸O.This negative correlation is significant and universal.2?Analyze the oxygen properties(concentration+?¹⁸O) by using respiratory models (AS model and IR model);Taking the results of AS model as an example,SOR% contributed 54%of AOU and WCR%contributed the remaining 46% in the water below the mixed layer of Changjiang Estuary and its adjacent sea.The preliminary analysis shows that the influence of Changjiang diluted waters and the respiratory fractionation factor ranging from 12‰ to 25‰ on the model analysis results of ?¹⁸O in the study area is very small(less than 10%).3?In July,2018,the overall trend of surface AOU and WCR%in near bottom waters showed an obvious negative correlation,that is,the smaller the surface AOU,the greater the photosynthetic oxygen production in surface water(implying more organic matter produced on site),and correspondingly,the greater contribution rate of WCR%to AOU will appear in near bottom waters.At the same time,it is found that the change of SOR%is related to the[O₂]of near bottom waters:the higher the[O₂]of near bottom waters,the higher SOR%will appear,but the SOR% is very low(<30%)under the condition of low dissolved oxygen.In addition,the model results all show that SOR%of near bottom waters has a weak negative correlation with the density difference between surface layer and near bottom waters;As far as the data in this study are concerned,surface sediment grain size,percentage of organic carbon and salinity have little influence on SOR.4?A number of investigations on Aojiang Estuary and its adjacent sea in Lianjiang county from May 2019 to January 2020.The results showed that:Similar to the Changjiang Estuary,the consumption of dissolved oxygen in seawater is coupled with the increase of?¹⁸O,and there is an obvious negative correlation between [O₂]in the near bottom waters and?¹⁸O.In January,2020,the surface seawater of Aojiang Estuary and its adjacent sea area was collected for respiration culture experiment,and the experimental results showed that?_WCR=18.7‰.The?¹⁸O of near bottom waters in Aojiang Estuary and its adjacent sea area is calculated by using the model.The results show that there is no hypoxia in Aojiang Estuary,and the main oxygen-consuming machine contributes SOR.Based on the survey results near the Changjiang Estuary and Aojiang Estuary,we propose that the contribution of WCR and SOR is a dynamic process below the mixed layer which is not invariable.When the[O₂]was higher,SOR was the main factor contributing to AOU.However,WCR was the main contributing factor(70%or more)during the occurrence of hypoxia.This study suggested that the dynamic change relationship between WCR and SOR and its influencing factors should be considered in the prediction and treatment of hypoxia in the future.