It's a match: increasing understanding of dissolved organic matter processing of Tibetan catchments by combining optical spectroscopy and ultrahigh-resolution mass spectrometry

Dissolved organic matter (DOM) is an important carbon and nutrient source for biota in streams and lakes. The composition and bioavailability of DOM depends on the formation in the catchments as well as on the biogeochemical and photochemical processing during its fluvial transport. Our study aimed to understand how the chemical composition of DOM is shaped by its sources and how natural organic matter is modified by biogeochemical processes during the fluvial transport to the endorheic Lake Nam Co, Tibet. Three subcatchments of the Lake Nam Co watershed were selected, based on different biomes and various degrees of land degradation. Samples characterizing endmembers along the stream network were analysed for optical properties (UV/VIS, fluorescence matrices decomposed by parallel factor analysis). Solid phase extracted (SPE) DOM was further analysed on the molecular level using ultrahigh-resolution mass spectroscopy (FT ICR-MS).

FT ICR-MS analysis revealed that meltwater from glaciers was on the one hand relatively rich in polyphenols, potentially derived from atmospheric deposition. Optical properties on the other hand suggest a high biological lability together with mainly microbial DOM sources, probably from microbial primary production in the glaciers. DOM originating from peatlands and Kobresia pygmaea pastures showed a “terrestrial-like” optical DOM signature derived from plant litter and organic soil material and had a strong seasonal variability. Furthermore, grassland sites degraded by overgrazing released more organic compounds with higher molecular weight into the stream, likely due to hampered retention of water and organic matter in the disturbed topsoils. Molecular level data from FT ICR-MS analysis further revealed greater DOM processing in the terminal lake compared to the streams, which can be attributed to bio- and photooxidation in the lake water column. The analysis shows, that the use of complementary analytical techniques reveals matching indicators to unravel relevant biogeochemical processes in the catchment. This methodological approach allows novel, in-depth insights into the dynamics of DOM characteristics for the sensitive and threated environments of the Tibetan highlands.