Spectroscopic fingerprints to track the fate of aquatic organic matter along an alpine headstream on the Tibetan Plateau.

The fragile aquatic ecosystem on the Tibetan Plateau is severely threatened by human activities and climate change. Dissolved organic matter (DOM) is a vital indicator of surface water quality; however, its comprehensive molecular analysis is challenged due to its low concentration (total organic carbon less than 0.5 mg/L) in alpine areas. This study proposes the fluorescence excitation-emission matrix (FEEM) to fingerprint DOM in a typical headstream in the Namco basin, one of the largest lake regions in Tibet. We found that the FEEM can sensitively detect low-concentration pollution traces and the variation of DOM along the flow from the ice sheet, through the wetland, eventually to the estuary of the lake. The fluorescence intensity indices for biodegradable carbon (fT/C) and humification (HIXem) responded drastically along the flow. Fluorescence regional integrals (FRIs) clearly reflected the overall increase of protein-like substances and decrease of humus-like substances along the flow, whereas this tendency was reversed when passing through the wetland. The FRIs-derived secondary parameters (HPP, HMP, WLP and SSP) further sensed likely variations in hydrophobicity, humification degree, excited-state fluorophore energy and Stokes shift. Parallel factor analysis (PARAFAC) and two-dimensional correlation spectroscopy (2DCOS) of the FEEM signals witnessed the trade-off among tyrosine-like organics (C1 peak), tryptophan-like byproducts (C2 peak) and humus-like remains (C3 peak) along the flow. The C1 component can be traced back to the vicinity of the ice sheet exit, presumably due to human and animal activities. The wetland can absorb or convert part of the C1 component into C2 or C3 products, demonstrating the function of regulating water quality and buffering environmental impacts. The spectroscopic indicators evaluated in this study may provide tools for diagnosing early traces of water pollution and ecological instability in alpine areas.