Spatiotemporal Patterns And Influencing Factors Of Dissolved Oxygen And Nutrients In The South Yellow Sea And The East China Sea

The South Yellow Sea(SYS)and East China Sea(ECS)are typical continental shelf marginal seas in the Northwest Pacific Ocean.Influenced by multiple pressures such as human activities,ecological and environmental problems such as bottom hypoxia and acidification problems occur frequently in the typical areas of the SYS and ECS,potentially threatening the sustainable development of the coastal ecosystem.At the same time,the physical and biogeochemical processes in theareas are complex,which significantly affect the spatial-temporal distribution patterns and transport of dissolved oxygen(DO)and nutrients.In this paper,based on the hydrological,chemical and biological data obtained from the SYS-CE(Changjiang Estuary)in spring and autumn 2017 and the ECS in summer and autumn 2019,the DO distributions and low oxygen,acidification characteristics and its influencing factors were studied,the DO spatial pattern in the ECS during autumn and its relationships with the hydrological and ecological processes were analyzed,the spatial-temporal distribution of nutrients in the ECS during spring and autumn and transport and its physical and biogeochemical processes were deeply discussed.The main conclusions are as follows:(1)The contents and distributions of DO corresponded to the flow field/water mass patterns in the SYS-CE,and regulated the formation of bottom hypoxia and acidification of the sea area.The bottom TWC(Taiwan Warm Current)-a dominated area off the CE was characterized by low oxygen in summer,the bottom oxygen-deficient waters off the CE could be uplifted to the upper layers and expanded northeastward.The upwelled low-oxygen waters off the CE and the uplift of the middle DO-maximum layer in the YSCWM(Yellow Sea Cold Water Mass)area were basically consistent with the uplifting trend of the thermocline in summer,which not only reflected the impacts of upwelling on the vertical distributions of DO,but also indicated the existence of upwelling around the YSCWM boundary and in the nearshore area off the CE.The offshore diluted water in the northeastern region off the CE also contributed to the formation of the local low-oxygen zone.A low-oxygen area existed in the lower layer of the YSCWM in autumn.Besides,acidification occurred off the CE during summer and also in the YSCWM-dominated area during autumn.Specifically,the acidified area off the CE was generally consistent with the hypoxic zone in summer,and the acidified area located within the YSCWM-dominated region in autumn was basically in accordance with the low-oxygen zone.(2)In autumn,there exists a bottom low-oxygen area of Zhejiang offshore,which is retained by the southward subsidence of bottom low-oxygen from the CE in summer,and its maintenance is closely related to hydrodynamic conditions and organic matter subsidence and decomposition.After autumn the low-salt Changjiang Dilutes Water(CDW)and Zhejiang-Fujian Coastal Current(ZFCC)extended southward,and interacted with the low-temperature and high-salt Kuroshio Subsurface Water(KSW)that invaded northward,thermocline and halocline are formed near the bottom layer,and the vertical oxygen exchange is limited between upper oxygen-enriched water and the bottom.The DO consumed near the bottom could not be replenished in time,thus maintaining the hypoxic zone retained in the summer.However,compared with the summer,the vertical mixing of water bodies is gradually strengthened in autumn,the hydrocline is obviously subsided compared with the summer,resulting in the thinning and narrowing of the thickness of the hypoxic water bodies.At the same time,the spatial distributions of the bottom low-oxygen correspond to the surface high chlorophyll a(Chl a)concentration,indicating that the deposition and decomposition of organic debris is the biogeochemical basis for maintaining the bottom low-oxygen in autumn.(3)The distributions and changes of nutrients in the ECS during spring and autumn are mainly regulated by water mass patterns.Under the influence of the CDW and the ZFCC,the upper high nutrient areas are formed in the southern Zhoushan Islands and offshore Zhejiang,which are diffused and decreased with the continental coastal water to the Kuroshio regions.The surface low nutrient characteristics in the southeastern ECS are controlled by the KSW and the Taiwan Strait Water,while the bottom high PO₄-P water of this area is mainly affected by the expansion of Kuroshio Subsurface Water(KSSW)(the main source of bottom water of TWC).The KSSW is transported across the continental shelf to the ECS and an important channel for material exchange is formed between the Kuroshio and the ECS areas,which regulated the abnormally high N/P ratio in the ECS.The high PO₄-P in the 30m water layer near Zhoushan Islands in autumn are caused by the upwelling along the Zhejiang-Fujian coast.The northeastern bottom high nutrient areas in spring were formed by the southward transport of the Yellow Sea Coastal Current in winter and retained in the northern ECS.