Sources And Transformation Of Dissolved Organic Matter As Indicated By Amino Acids In The Pearl River Estuary,China And Northern South China Sea

In the estuary,production and transformation of dissolved organic matter(DOM)regulate the carbon export and transformation from land to ocean.It's challenging to trace the sources and transformation of DOM because of the complex environments in the estuary.The application of amino acids in marine research mainly includes abundance and composition of amino acids,AA carbon yield(%),principal component analysis(PCA),and degradation index.As a kind of biomarkers,amino acids has been used to trace the biogeochemical processes,which happen to organic matter in the ocean.Here we collected samples from different regions of Pearl River Estuary(PRE)and Northern South China Sea(NSCS)in July 2015.The biogeochemical processes that occur in different regions were better defined by the physical and biogeochemical parameters.The sources and lability of DOM in different regions of PRE and NSCS at a molecular level were obtained by the analysis of particulate hydrolyzed amino acids(PHAA),total dissolved amino acids(TDAA)and their enantiomers.The relationship between particulate organic matter(POM)and DOM in the PRE area were explored in our investigation.Further,the mechanism of generating and accumulating bioavailable DOM were evaluated in PRE and NSCS areas.Three conclusions are listed as the followings:(1)The fresh water input from the land dominated the discharge of upper reach of PRE,and the middle reach of PRE was a conservative mixing area by the two end members(fresh water and sea water).Glycine(Gly)and gama-aminobutyruc(GABA)are composition of TDAA,which were richer in DOM in the upper and middle reach of PRE than those of lower reach of PRE.The D/L ratios of four major TDAA(aspartic acid,glutamic acid,serine,alanine)were also relative higher in the upper and middle reaches of PRE,indicating that DOM in the upper and middle reaches of PRE were highly altered by microorganisms and the diagenetic status were relatively higher.There was a significant stratification in the water column because of the salt wage.Due to the abundant nutrients input from the land and appropriate light conditions,the higher primary production activities produced more labile POM and DOM,which were inferred from relative abundance of glutamic(Glu)and phenylalanine(Phe),as well as the relative higher TDAA carbon yield(%).Based on the two-end member model,it was found that 7%dissolved inorganic carbon(DIC)in the surface water of lower-PRE were transformed to dissolved organic carbon(DOC)during the primary production.(2)Degradation index(DI),which based on PCA analysis,were applied to estimate the diagenetic status of POM and DOM,indicating that the digenetic status of DOM and POM from upper-and mid-PRE are more degraded than those in the lower-PRE.In all regions of PRE,POM was found to be more bioavailable and fresher than DOM.The production of POM and DOM was coupled and POM might be one of the sources of DOM in the PRE area.(3)Driven by monsoons,the Pearl River plume was rushed to the NSCS from the PRE.Combining the physical and biochemical parameters,bioavailable DOM seems to be accumulated in the Pearl River plume area.It was found that the bioavailable DOM accumulation was regulated by the primary production,the release of bio-DOM caused by zooplankton grazing and the limitation of phosphate concentrations.According to the PCA analysis of TDAA composition,DOM in PRE was enriched in Phe,and DOM in NSCS was enriched in Gly and GABA.Theses results imply that the DOM in PRE was more bioavailable than those in NSCS and that DOM in NSCS was more degraded,but DOC concentrations did not change significantly during the transportation process.Overall,DOM was not only to be transported from the PRE to the NSCS,but its lability had also been come to be more degraded.In this study,PHAA,TDAA,and their enantiomers were measured in the PRE area simultaneously by the high solution sampling to study the sources,transformation of DOM with the PCA analysis in the PRE area and the Pearl River plume area.The comparison of POM and DOM lability were discussed preliminarily by statistical analysis.