Source identification of suspended and deposited organic matter in an alpine river with elemental, stable isotopic, and molecular proxies

Abstract

The source identification of organic matter (OM) is important for understanding its fate and effect in the aquatic environment. However, little relevant research has been reported from alpine rivers. In this study, elemental, stable isotopic, and molecular proxies were integrated to determine the relative contributions of various sources to suspended and deposited OM in the Yellow River source region, a typical alpine river system located in the Qinghai-Tibetan Plateau. The distribution of n-alkanes suggested that deposited OM was mainly derived from terrestrial sources, while the contribution of plankton was low, and the output result of the mixing SIAR model based on δ13C and N/C further showed that the proportional contributions of various sources were 70.7% for soil OM, 10.1% for terrestrial plants, 8.1% for livestock manure, and 11.1% for plankton. For suspended OM, the proportional contributions of various sources were 33.2% for soil organic matter, 25.2% for terrestrial plants, 20.4% for livestock manure, and 21.2% for plankton. The difference in source contribution between suspended and deposited OM might result from hydrodynamic separation and the alteration of environmental variables between the overlying water and riverbed. The contribution of plankton to riverine OM was lower in the Yellow River source region than in low-elevation rivers. This could be attributed to the specific climate condition, increasing turbidity, and low nutrient level in the study area. Moreover, the yields of the dissolved and particulate organic carbon in the Yellow River source region were estimated to be 0.53 t/km2/yr and 1.29 t/km2/yr, respectively, with the former lower than, and the latter comparable to other rivers around the world. This study suggests that the origin and load of riverine OM in the Yellow River source region are different from those of low-elevation rivers.