The Glacial – Terrestrial – Fluvial Pathway: A Multiparametrical Analysis of Spatiotemporal Dissolved Organic Matter Variation in Three Catchments of Lake Nam Co, Tibetan Plateau

Abstract

The Tibetan Plateau (TP) is a sensitive alpine environment of global importance, being Asia’s water tower, featuring vast ice masses and comprising the world’s largest alpine grasslands. Intensified land-use and pronounced global climate change put pressure on the environment of the TP. Here, we study the tempo-spatial variability of dissolved organic matter (DOM) to better understand fluxes of nutrients and energy from terrestrial to aquatic ecosystems on the TP. We approached this by a multiparametrical approach, based on inorganic water chemistry, dissolved organic carbon (DOC) concentration, dissolved organic matter (DOM) characteristics (chromophoric DOM, fluorescence DOM and δ13C of DOM), in stream samples of three catchments of the Nam Co watershed and the lake itself. Satellite based plant cover estimates were used to link the biogeochemical data to structure and degradation of vegetation zones in the catchments. Catchment streams showed site-specific DOM signatures inherited from glaciers, wetlands, groundwater, and pastures dominated by Kobresia pygmaea. Comparing stream and lake samples, we found DOM processing and unification by loss of chromophoric DOM signatures and a change towards an autochthonous source of lake DOM. DOM diversity was largest in the headwaters of the catchments and heavily modified in terminal aquatic systems. Seasonality was characterized by minor influence of freshet but a very strong impact of the Indian summer monsoon on DOM composition, with more microbial DOM sources. The DOM of Lake Nam Co differed chemically from stream water sample, indicating the lake to a be quasi-marine environment concerning the degree of chemical modification and sources of DOM. Our findings prove DOM to be a powerful marker to elucidate consequences of land use and climatic change on biogeochemical processes in High Asian alpine ecosystems

Keywords: Alpine pastures, dissolved organic carbon, PARAFAC, fluorescence, Tibetan Plateau, third pole environment