Quantification of allochthonous and autochthonous organic carbon in large and shallow Lake Wuliangsu based on distribution patterns and δ13C signatures of n-alkanes

Abstract

Identification and quantification of allochthonous and autochthonous organic carbon (OC) in large and shallow lakes are crucial for knowledge of OC burial and recycling behaviors, as well as sustainable management practices. However, quantitative methods to estimate the contributions from various OC sources are still limited. In this study, we analyzed distribution patterns and δ13C signatures of mid- and long-chain n-alkanes from aquatic plants, riverine sediment, and surface sediments from a typically large and shallow Lake Wuliangsu in the Hetao Irrigation District, China. The results indicate that n-alkanes among submerged macrophytes, emergent plants, and riverine sediment show unique distribution patterns and δ13C signatures, supporting the practicability of identification and quantification of OC sources by end-member mixing models of n-alkanes’ distribution patterns and 13C values. Our results also suggest that introducing δ13C values into the end-member mixing models could effectively reduce the uncertainty, as n-alkane distribution patterns might be modified by different species, habitat environments, and potential OC degradation. In the case of Lake Wuliangsu, the model results demonstrate that the riverine sourced OC from the main channel has decreased during southward transport, indicating that Lake Wuliangsu serves as an important trap and sink for OC transported from the upper reaches of the Yellow River. The model results also show a predominant contribution of the autochthonous OC to sediments in Lake Wuliangsu, with open-water areas dominated by submerged macrophyte-sourced OC and the other areas by emergent plant-sourced OC. Higher productivities of submerged macrophytes are mainly modulated by increases in water depth, water transparency, and nutrient concentrations, while higher productivities of emergent plants are mainly associated with increasing nutrient concentrations. To improve the ecological conditions and OC burial of Lake Wuliangsu, it is necessary to manage the quality of the inflowing water from the agricultural farmland, as well as the preservation of the water body from current paludification.