A High Accumulation of Dissolved Organic Matter in the Water Resulting from Terrestrial Input into a Large, Shallow Steppe Lake

Dissolved organic matter (DOM) is generally the dominant carbon pool in freshwater lakes and plays a vital role in the regional or even global carbon cycle. In recent years, steppe lakes have been subject to two stressors: eutrophication and abnormally high concentrations of organic matter. However, little is known about the sources and composition of the organic matter. In this study, carbon/nitrogen (C/N) ratios and three-dimensional excitation–emission matrix (3DEEM) fluorescence spectroscopy were adopted to identify the sources and composition of DOM in Hulun Lake, a large shallow steppe lake. The physicochemical and water quality parameters of Hulun Lake and three inflow rivers, Crulen River, Orshen River and Hailar River, were also investigated. The results showed that visible fulvic-like fluorescence (peak C) and tryptophan-like substances (peak T) were the main components of the DOM. The C/N atomic ratios and spectral characteristics of the DOM suggested that approximately 82% of the DOM in Lake Hulun is derived from terrigenous material. The results underscore that it is necessary to devote more research attention to terrestrial inputs into steppe lakes.