Provenance And Dynamics Of Particulate Organic Matter In A Well-mixed Water Column From The Continental Shelf Of East China Sea

Previous work in the East China Sea(ECS)indicated that particulate organic matter(POM)in the deep chlorophyll maximum layer was largely derived from phytoplankton production in summer 2013,when the shelf water column was stratified.However,less is known about the provenance and dynamics of POM,especially carbon and nitrogen isotopic(?13C and 815N)variability and their controlling mechanisms,in autumn/winter when the shelf water column is generally well-mixed in the ECS.Since the pool of POM in the shallow continental margin of the ECS is a mixture of terrestrial and phytoplankton-derived(marine)organic matter and this mixture is expected to vary seasonally,distinguishing these sources is essential to understand the role of marginal seas in the global carbon cycle.The present study therefore aims to characterize the POM from the continental shelf of the southern ECS.Twenty-seven total suspended matter(TSM)collected at-30 m water depth from the inner shelf of the ECS during autumn 2013 were analyzed for particulate organic carbon and nitrogen(POC and PN)concentrations and their stable isotopic compositions(?12C and ?15N),to understaId the distribution,sources,and the fate of POM.Hydrographic properties,including temperature,turbidity,salinity and chlorophyll a,indicated that the shelf water was well-mixed and weakly influenced by Taiwan Warm Current Water during the sampling season(autumn 2013).Low ratios of POC to chlorophyll a(POC:Chl a)(almost less than 200 g g-1)and molar C/N(2.4-6.5,lower than that of Redfield ratio)suggested that the POM in the ECS was predominantly sourced by phytoplankton.Isotopic results however showed wider?13C and ?15N compositions from-24.3 to-21.3‰and from 2.3 to 7.4‰,respectively.This implies that ?13C was probably attributed to a mixture of terrestrial-and marine-sourced organic matter,or it may be resulted from the degradation of selective components of OM or the active bacterial contribution to the POM,as evidenced by low C/N ratios.The 815N composition of POM seems to be controlled by the different inorganic nitrogen sources and/or metabolic process.Estimation using a simple two endmember carbon isotope mixing model revealed that on average-40%of organic carbon is from terrigenous sources,while the remaining comes from marine sources during autumn.