What is the influence of centennial evolution from macrophytes to algae on organic matter burial in the upper reaches of Yangtze River from the biomarker carbon isotope fingerprint

Abstract

Identification and quantification of sediment organic matter (OM) in the plateau eutrophic lakes, which are strongly influenced by human activities, is noticeable in the study of regional carbon (C) burial and global change. The abundance of n-alkanes and compound-specific carbon isotope (δ¹³C_n-alk) in sediment cores have been widely utilized as proxies. Here, the n-alkane diagnostic pattern, δ¹³C_n-alk end-member model, ecological data, and the historical OM input, associated with the ecosystem evolutionary processes were investigated. The results show the long-term ecological evolution under climate warming and anthropogenic interferences promoted the organic C (OC) and nitrogen (ON) burial in the sediments. The n-alkane distribution pattern, diagnostic proxies, and δ¹³C_n-alk signatures supported the degradation of the ecosystem from macrophyte-dominated (before the 1970s) to mixed (1980s–1990s) and then to algae-dominated (after 2000s) ecosystem. The conversion between aquatic macrophytes and prevailing algae and intensive anthropogenic activities increased OC and ON burial by 3.43 and 3.56 folds, respectively. The differential input of nutrients exacerbated the imbalance of N and phosphorus and stimulated the expansion of algae. Although exogenous OM inputs from terrestrial vascular plants increased rapidly with anthropogenic interference, the LMW-derived n-alkanes and lower C/N ratio and TAR also provide evidence for the predominance of typical endogenous OM input. Meanwhile, an explanation of the photosynthesis pathway from δ¹³C_n-alk of long-chain n-alkanes indicated that C₃ plants in the lake ecosystem were dominant with more than 70 % of the total over the past century. These results indicated that OM input and burial are closely related to the evolution of the lake ecosystem especially in the context of climate changes and extensive anthropogenic activities. It provides perspectives for biomarker and compound-specific stable isotope interpretation of the OM from input to sink in sedimentary records of eutrophic lakes.