A DOM continuum from the roof of the world – Tibetan molecular dissolved organic matter characteristics track sources, land use effects, and processing along the fluvial-limnic pathway

Abstract. The Tibetan Plateau (TP) is the world largest and highest plateau, also comprising the biggest connected alpine pasture system of the world. Like other alpine systems, it is sensitive to impacts by climate change and increasing anthropogenic pressure. Carbon cycling at the TP is complex, including sources such as primary production in lakes, glaciers, and terrestrial plants, agricultural land use but also organic matter (OM) from aeolian deposition. Dissolved organic matter (DOM) connects these carbon reservoirs in the network, following the hydrological cycle from precipitation, glaciers, and headwaters to lakes. DOM is highly complex, its molecular composition holds information from its diverse sources and transformations during transport. However, due to its complexity, DOM cycling along the headwater-fluvial-limnic pathway and how terrestrial change can impact carbon cycling in the diverse water bodies is still not well understood. Here, we study DOM molecular transformations using ultrahigh-resolution mass spectrometry (FT-ICR-MS) along the TP alpine continuum from glacial, groundwater springs, and wetland biomes including pastures and alpine steppe, to the large saline endorheic Lake Nam Co. DOM molecular composition differed with respect to allochthonous sources between endmembers, as well as between stream samples, the brackish mixing zone, and the lake. Glacial meltwater DOM contained autochthonous signatures of low-oxidised, unsaturated molecular formulae together with terrestrial-like, dust-borne DOM sources. Glacial-fed streams were characterised by fresh autochthonous, probably algal DOM, and aromatic compounds likely originating from pastoral land sources. DOM from a groundwater spring had a highly degraded, strongly oxidised signature, probably related to the shallow upper aquifer, and degraded pastoral land sources. Wetland and stream DOM were characterised by less oxidised and less degraded inputs from vascular plants and soils. At the brackish zone of the lake shore, DOM contained a mixture of lake- and terrestrial DOM inherited from the streams. At Lake Nam Co, depletion of aromatic terrestrial molecular formulae suggested photooxidation at the surface, and relative enrichment of potentially recalcitrant DOM within the lake. Additionally, a relative enrichment of more aliphatic, nitrogen-containing DOM suggests autochthonous algal and microbial DOM sources in the lake. Our study revealed that DOM composition was largely influenced by local sources and transformations in glaciers, wetlands, and groundwater springs, also incorporating molecular signatures of pasture degradation. Streams with less glacial influence had plant- and soil borne aromatic-rich DOM sources, while the endorheic Lake Nam Co had a recalcitrant DOM composition comparable to millennial-scale stable marine DOM. This suggests that there is no typical high-alpine DOM signature, but that complex processes form DOM characteristics in the fluvial-limnic continuum. Small-scale catchment properties, land degradation and aquatic domains shape the differences. Alpine DOM compositions hence appear to be closely linked to landscape properties suggesting their susceptibility to changes in water quality and OM cycling in sensitive High Asian ecosystems.