Excitation-emission matrix fluorescence spectra of chromophoric dissolved organic matter reflected the composition and origination of dissolved organic carbon in Lijiang River, Southwest China

Aquatic photosynthesis transforms inorganic carbon to organic carbon (OC), which contributes to autochthonous organic carbon (AOC) in sediment and particulate organic carbon in surface karst aquatic systems. Aquatic plant and microbes are participated in autochthonous dissolved organic carbon (ADOC) formation in surface karst aquatic systems, but the composition and formation of ADOC remains little known, which leaves problem on calculation of organic carbon flux. In this study, the Lijiang River was chosen as typical surface karst river to identify the DOC composition and its origin, and to explore the environmental influencing factors. Samples were collected seasonally from the upper to lower reaches of the river to analyze hydrochemical parameters and the excitation-emission matrix (EEM) spectrum of chromophoric dissolved organic matter (CDOM). Three CDOM components were calculated by parallel factor analysis (PARAFAC) from EEM spectra, which were allochthonous soil-sourced DOC (SDOC), autochthonous aquatic plant-sourced DOC (APDOC), and microbial-sourced DOC (MDOC). Based on the DOC component concentrations, SDOC is induced by large amounts of precipitation causing soil erosion in summer. APDOC formation is encouraged by moderate water temperatures in spring and fall restricted by high water turbidity in summer. The significant positive linear correlations between APDOC and dissolved inorganic carbon (DIC) and MDOC indicate DIC fertilization effect in karst aquatic systems directly promotes APDOC formation and indirectly promotes MDOC. Our study develops a relatively simple way to calculate composition of karst aquatic DOC, which demonstrates the participation of aquatic plants and microbes in APDOC production and reiterate that autochthonous DOC should be considered when calculating the carbon sink in surface karst aquatic systems.