Characteristics of Chromophoric Dissolved Organic Matter (CDOM) in Natural Rivers of Western Sichuan Plateau

Abstract

The concentration of dissolved organic carbon (DOC) in some rivers in the western Sichuan Plateau is extremely high. Due to the unique optical properties of colored dissolved organic matter (CDOM), a thorough understanding of the photodegradation characteristics of CDOM helps in revealing regional DOC dynamics and carbon cycle at the water-land interface in alpine natural waters. Surface water samples were collected from three rivers (the upper reaches of the Minjiang, Zagunao, and Fubian rivers) in the alpine-gorge region, and the Baihe and Heihe rivers in the plateau planation surface distributed among the watersheds in the western Sichuan plateau, southwest China. Ultraviolet-visible spectra(UV-vis) absorbance and excitation emission matrix (EEM) fluorescence spectroscopy with two-dimensional correlation spectrum (2D-COS) and principal component analysis (PCA) were used to characterize the environmental behavior responses to light irradiation. The first-order decay constants and photodegradation rates of the Baihe and Heihe rivers are 0.167 d-1,0.173 d-1 and 64.85%,63.43%, respectively. Due to the low concentration (0.71 m-1) of the Fubian River CDOM, photodegradation behavior is limited. The photodegradation behavior of CDOM in the Zaogunao and Minjiang rivers is relatively complicated. Exposure to sunlight led to decreases in the chromophores and changes in origin of CDOM from the four rivers, except for the Minjiang River. The aromaticity, hydrophobicity, and humification degree of CDOM in the rivers from the plateau planation surface were decreased with increasing exposure time. PARAFAC produced a four-component model:C2[280(<250)/400 nm], C3 (255/440 nm), and C4[270(360)/492 nm] represented terrestrial UVA humic-like fluorophores, and C1 (275/310nm) belonged to a tyrosine-like substance, as humic-like FDOM was found to be more readily photodegraded. 2D-COS indicated that UVA humic-like FDOM showed a higher susceptibility in the rivers from plateau planation surface. The preferential removal of UVA humic-like FDOM (especially at 500 nm emission wavelength), and delayed response of tyrosine-like fluorescence, were revealed from the five rivers upon irradiation. Two factors were identified in the principal component analysis of the Baihe River, explaining 87.28% of these parameters, which comprehensively reflected the effects of the photodegradation process on CDOM characteristics and fluorescent components.