Characterization of CDOM in saline and freshwater lakes across China using spectroscopic analysis

Colored dissolved organic matter (CDOM) is a major component of DOM in waters, and plays a vital role in carbon cycling in inland waters. In this study, the light absorption and three-dimensional excitation-emission matrix spectra (EEMs) of CDOM of 936 water samples collected in 2014-2017 from 234 lakes in five regions across China were examined to determine relationships between lake water sources (fresh versus saline) and their fluorescence/absorption characteristics. Results indicated significant differences regarding DOC concentration and a_CDOM(254) between freshwater (6.68 mg C L^-1, 19.55 m^-1) and saline lakes (27.4 mg C L^-1, 41.17 m^-1). While humic-like (F₅) and fulvic-like (F₃) compounds contributed to CDOM fluorescence in all lake waters significantly, their contribution to total fluorescence intensity (F_T) differed between saline and freshwater lakes. Significant negative relationships were also observed between lake altitude with either F₅ (R² = 0.63, N = 306) or F_T (R² = 0.64, N = 306), suggesting that the abundance of humic-like materials in CDOM tends to decrease with increased in lakes altitude. In high-altitude lakes, strong solar irradiance and UV exposure may have induced photo-oxidation reactions resulting in decreased abundance of humic-like substances and the formation of low molecular weight compounds. These findings have important implications regarding our understanding of C dynamics in lacustrine systems and the contribution of these ecosystems to the global C cycle.