Optical properties as indicators of in-lake dissolved organic matter components, sources and transformation

Abstract Dissolved organic matter (DOM) plays an essential role in the biogeochemical cycle of carbon (C) in inland lakes. Combining ultraviolet–visible (UV–vis) absorption and fluorescence spectroscopy–parallel factor analysis (PARAFAC), we examined spatial-temporal variations in chromophoric DOM (CDOM) composition and sources in Lake Erhai, the seventh largest lake in China. The results showed that dissolved organic carbon (DOC) concentrations were obviously higher in the wet season than in the dry season. The optical indices of lake DOM exhibited regular seasonal variability. Larger SR values presented in the wet season indicated strong photobleaching activity. Humic-like substances (C1 and C3) dominated the lake DOM in the dry season, while protein-like substances (C2) were abundant in the wet season. The proportion of autochthonous DOM exceeds 60% in the northern region of Lake Erhai owing to algal proliferation. Evident correlations between DOC and Chl a, BIX, and YFI supported that most lake DOM originate from the photosynthesis of phytoplankton in the eutrophic transitional lake. Autochthonous DOM was associated with DOC, Chl a, C2%, YFI, and BIX, whereas terrestrial DOM was closely linked to a254,a350, C3/C1% and HIX, based on principal component analysis (PCA) results. Higher HIX values for the inflowing river in comparison to lake water demonstrated the allochthonous characteristics of river DOM. High variability of the DOM sources, compositions, and dynamics were observed across the lake catchment as evidenced from the CDOM optical properties. These findings further underscore the significant role of in-lake autochthonous DOM in the global C cycle for inland lakes toward eutrophication.