Non-conservative behaviors of dissolved organic matter derived from the analysis of concave and convex estuarine mixing lines in a large-river estuary

Dissolved organic matter (DOM) in large-river estuaries has multiple sources and complex compositions. The non-conservative behaviors and source-sink budgets of DOM during estuarine mixing have always been a highly studied and challenging topic in marine biogeochemical studies. In this study, systematic samplings were conducted in the Changjiang (Yangtze) River estuary in March and July 2019 and in July and August 2020 to determine the concentrations of four important DOM components, i.e., dissolved organic carbon, chromophoric dissolved organic matter, monosaccharides, and polysaccharides. Linear fitting lines have generally been used to describe the variation of DOM concentrations with increasing salinity from land to sea, with concave and convex trends (although probably slight) being largely ignored. We found that these concave and convex trends reflected the source and sink dynamics of DOM, respectively, and could be well described by quadratic and cubic polynomial functions. The mathematical principles underlying these curves were also investigated. Compared to traditional laboratory or on-site incubation experiments, the analysis of estuarine mixing curves was based on data observed in situ, resulting in a more accurate and reliable description of the source-sink dynamics of various DOM components transported in large-river estuaries. Moreover, estuarine mixing curves accounted for both static and dynamic non-conservative processes. Overall, this study provided a novel and efficient method to study the source-sink dynamics and non-conservative behaviors of DOM in large-river estuaries worldwide.