Sedimentary organic carbon and nutrient distributions in an endorheic lake in semiarid area of the Mongolian Plateau

Abstract

Due to the lack of outlets, inflowing pollutants are often deposited in an endorheic lake, posing potential pressure on the environment. With climate change, extreme weather is expected to be more frequent and will contribute to the release of carbon and nutrients buried in the lakebeds. However, the distribution of sedimentary organic carbon and nutrients and the mechanisms that control the distribution are not fully understood, despite their significance to environmental development in endorheic lakes being widely recognized. In this study, the mechanisms controlling the sedimentary organic carbon and nutrient distributions in endorheic lakes were examined based on the analysis of an endorheic lake in the semiarid area of the Mongolian Plateau. The field survey results indicate that the concentrations of sedimentary organic carbon (TOC) and nutrients (NH₃-N and TP) on the lakebed have significant correlations and present spatial heterogeneities. To further study the distribution mechanisms, numerical models were established to calculate the age of the water discharged from the rivers around the lake, and satellite remote sensing data were applied to examine the external source of organic carbon and nutrients and the factors influencing their movements to the lake. Based on the distribution of the water age, the water flow and mass transport trends in Lake Hulun were determined, and the time scales of the environmental processes were compared with those of water circulation. Further analysis indicates that the water circulation in the lake favors the accumulation of sedimentary organic carbon and nutrients in the northwestern part of the lake, and the organic carbon produced in the lake is transported to this region within an ice-free period. Satellite remote sensing data indicate that the region on the northwest bank of the lake experiences a larger terrestrial slope and better vegetation coverage than that on the southeast bank, which corresponds to a higher concentration of sedimentary organic carbon and nutrients in the northwest of the lake. This suggests that the sediment quality is closely related to the environment around the endorheic lake, and the larger slope and better vegetation coverage are significant factors for the high concentration of sedimentary organic carbon and nutrients on the lakebed under the conditions of scarce precipitation and low temperature. This study provides a theoretical basis and direction for further protection and management of the ecological environment of endorheic lakes.

Keywords: Endorheic lake; Geological characteristics; Organic carbon; Sediments; Transport; Water age.