Characteristics of dissolved organic nitrogen in overlying water of typical lakes of Yunnan Plateau, China

Abstract

In this study, Ultraviolet–visible spectroscopy (UV/Vis) and excitation-emission matrix fluorescence combined with parallel factor analysis (the EEM-PARAFAC model) were used for the characterization of the components and structure of the dissolved organic nitrogen (DON) in the overlying water of three typical lakes from Yunnan Plateau, China. Four distinct components of Erhai Lake, three components of Chenghai Lake, and three components of Lugu Lake were identified by the PARAFAC model, based on fluorescence excitation-emission spectra. These included two terrestrial and marine humic-like components (E1 and E3) for Erhai Lake, one terrestrial and marine humic-like component (C1) for Chenghai Lake, and one autochthonous tryptophan-like component (L1) for Lugu Lake. The autochthonous tryptophan-like component S1 (E4, C2, and L3) and autochthonous tyrosine-like component S2 (E2, C3, and L2) were also identified. In addition, the S1 and S2 components were highly similar to one another and had almost identical extraction and emission wavelengths. Both S1 and S2 components were highly prevalent in plateau lakes but rarely appeared in plain lakes. Principal component analysis (PCA) found a great correlation between the S1 and S2 components in all studied lakes, indicating that they had a common source or the same variation trend. To be noted, we found that with the decrease of eutrophication level, relative abundance of S1 and S2 presented significance increase, the relative abundance of S1 and S2 components can be used as an indicator of lake trophic status in the Yunnan Plateau. Special attention should be given to these components with regard to plateau lake protection. PCA score analysis for the studied lakes found obvious quantitative and qualitative differences in the fluorescence components of different regions of Chenghai Lake. No significant discrepancy was found between Erhai Lake regions.