Production and Transformation of Dissolved and Particulate Organic Matter as Indicated by Amino Acids in the Pearl River Estuary, China

Production and transformation of dissolved and particulate organic matter (DOM, POM) in estuaries regulate the carbon export from land to ocean, yet tracing their source and diagenetic status is challenging in these dynamic environments. Here we study the production, transport, and diagenetic status of DOM in the Pearl River Estuary (PRE), China, based on total dissolved amino acids, their enantiomers (D/L ratios), and other ancillary biogeochemical parameters with a complete coverage of upper, middle, and lower estuary. Inferred from amino acid composition and D/L ratios, DOM in the upper PRE was highly altered by bacteria, while the carbon yields of total dissolved amino acids were relatively high showing a conflicting pattern likely contributed by multiple sources including planktonic production, soil leachates, and sewage discharge. Conservative mixing between freshwater and seawater predominantly controlled DOM dynamics in the middle PRE. In contrast, the lower PRE was characterized by high planktonic production, leading to the accumulation of labile DOC in the surface water. The compositional pattern of amino acids differed significantly between dissolved and particulate phases, yet the diagenetic indices of POM and DOM were both relatively lower in the upper and middle PRE compared to the lower PRE, suggesting that primary production is a major driving factor in the lower PRE. Overall, our study emphasizes the highly dynamic and spatially heterogeneous nature of the PRE, and the results from molecular level characterization provide new insights into the complex sources, diagenesis, and transformation processes of DOM in different regions of the estuary, as well as its connection with POM. Amino acids and their enantiomers were applied as tracers for diagenetic status of organic materials in the Pearl River EstuaryDiagenetic status of DOM was regulated by the extent of bacterial alteration, mixing, and primary production in the estuaryThe covaried diagenetic indices of DOM and POM implied that the two organic pools were somewhat coupled in the Pearl River Estuary