Climatic warming and overgrazing induced the high concentration of organic matter in Lake Hulun, a large shallow eutrophic steppe lake in northern China.

An abnormally high concentration of organic matter (OM) in Lake Hulun, a large shallow eutrophic lake located in the sparsely populated Hulun Buir Steppe, was observed in a field investigation. Little was known about the origin of the OM. To identify the source of the OM in Lake Hulun, the carbon/nitrogen (C/N) ratio, natural abundance of stable isotope and three dimensional excitation emission matrix (3DEEM) fluorescence spectroscopy techniques were employed. Furthermore, a cyanobacterial incubation and degradation experiment was conducted in the laboratory to quantify the contribution of algae to dissolved organic matter (DOM) in Lake Hulun. C/N, the stable carbon isotope (δ(13)C) values typical of C3 plant debris in particulate organic matter (POM) and the fluorescence indices of DOM indicate that most of the OM in Lake Hulun is of terrigenous origin. It was deduced that only about 10.2% and 7.3% of DOM were contributed by algae in September and January, respectively, according to the linear correlation between the concentrations of algae-derived DOM and the fluorescence intensities of tyrosine-like matter. According to the stockbreeding development and climate change in Hunlun Buir Steppe, we deduced that the destruction of the grassland ecosystem by overgrazing in specific locations and trends in climatic warming and drying were the main factors causing the increase of OM and nutrient concentrations in Lake Hulun. This result highlights the need to pay more attention to the inputs of terrigenous organic matter to the lakes in northern China.