Identifying sources and variations of organic matter in an urban river in Beijing, China using stable isotope analysis

Urban river ecosystem has been greatly altered by human activities, especially the input and transformation of allochthonous organic matter (OM) in urban river. Understanding of spatial and temporal variation of OM sources is critical to urban river management. Compared to previous studies, this study mainly focused on the spatial-temporal variations of OM sources in North Canal, an urban river in Beijing. Carbon stable isotopic (delta C-13), nitrogen stable isotopic (delta N-15), and isotopic multivariate mixture models were used to assess OM sources and their relative contribution. The result showed that autochthonous OM (macrophytes 68%, phytoplankton 16%) was dominant for suspended particulates, while allochthonous OM (C4 plants: 61%-76%; terrestrial soil: 36%-16%) was predominant in benthic fine particulates sediment. The composition of OM sources in suspended particulates and sediment showed a similar spatial pattern. Aquatic macrophytes were the main OM sources in upstream reaches, while terrestrial soil and sewage were the primary OM sources in midstream and downstream. Moreover, aquatic macrophytes and phytoplankton OM were the primary OM sources of main stream, whereas terrestrial soil and sewage were the main OM sources of tributaries. In addition, OM sources composition showed seasonal variations, and the terrestrial erosion caused by rainfall was the crucial factor for various OM sources in different seasons. In wet season, terrestrial soil erosion (24%-56%) and C4 plants (2%-28%) were primary sources of sedimentary OM, while macrophytes (32%-58%) and sewage (10%-42%) were the dominant OM sources in dry season. This study illuminated the effect of the human activities and rainfall on the sources of organic matter in North Canal. The control of sewage effluents and terrestrial soil erosion may be the best way to manage OM in North Canal.