Characterization of chromophoric dissolved organic matter in lakes on the Tibet Plateau, China, using spectroscopic analysis

Abstract. Spatiotemporal variations in the characteristics of fluorescent dissolved organic matter (FDOM) components from 63 lakes across the Tibet Plateau, China, are examined using excitation-emission matrix spectra (EEM) and fluorescence regional integration (FRI) from 2014 to 2017. Freshwater (N = 135) and brackish water (N = 109) samples from 63 lakes were grouped according to salinity or electrical conductivity. In order to compare results between the lakes, cumulative volumes beneath the EEM values (φ_i, i = I, II, III, IV, V) were normalized to a DOC concentration of 1 mg/L. EEM-FRI identified tyrosine-like (φ_I), tryptophan-like (φ_II), fulvic-like (φ_III), microbial protein-like (φ_IV), and humic-like (φ_V) fluorescence regions, as well as their proportions (P_i). Chromophoric dissolved organic matter (CDOM) absorption parameters, fluorescence indices, average fluorescence intensities of the five fluorescent components and total fluorescence intensities (φ_T) differed under spatial variation among brackish and freshwater lakes (ANOVA, p < 0.05). Principal component analysis (PCA) was used to assess and group five normalized FDOM components for all of the water samples. These results show that microbial protein-like (φ_IV), fulvic-like (φ_III) and humic-like (φ_V) have positive correlations (R² > 0.79, t-test, p < 0.01), indicating that these FDOM components may originate from similar sources. A correlation also exists between normalized φ_i (i = I, II, III, IV, V) and DOC concentrations with a salinity > 19 ‰ (averaged EC, 23 764 μs cm ⁻¹) (t-test, p < 0.01), of which R² f regression analysis showed a decreasing tendency with EC. Similar correlations between a(254) and DOC concentrations (t-test, p < 0.01) are also evident for sunshine hours > 2900 h. Redundancy analysis (RDA) indicates that a(254) and a(350) have a correlation with CDOM in brackish lakes. a(254), HIX and a(350) were also correlated with water quality. Strong evapoconcentration, intense ultraviolet irradiance and landscape features of the Tibet Plateau may be responsible for the FDOM characteristics identified in this study.