N-Alkyl lipid concentrations and distributions in aquatic plants and their individual δD variations

Abstract

Aquatic plants are major input sources of autochthonous organic matter in lake sediments, but investigations on fatty acid (FA) and n-alkane distributions in aquatic plants are currently limited, greatly hindering the applications of their isotope geochemistry in lacustrine environments. Here, the reported n-alkyl lipid distributions of aquatic plants in globally studied lakes, together with newly obtained aquatic plant n-alkyl lipid data in Chinese lakes (Yunnan and Inner Mongolia), are used to understand their distribution characteristics. The results show that aquatic plants have predominantly mid-chain lipids (C23–C25n-alkanes and C22–C24 FAs), differing from that of terrestrial plants (dominant by long-chain lipids), but the long-chain n-alkanes (e.g., C27 and C29) and long-chain FAs (e.g., C26 and C28) also show high abundances in most samples. Submerged plants have high concentrations of long-chain n-alkanes (avg. 47 µg g−1) and long-chain FAs (avg. 170 µg g−1), close to those in terrestrial plants, indicating that submerged plants may make large contributions of long-chain n-alkyl lipids to lake sediments, while the contributions of long-chain n-alkyl lipids derived from algae to lake sediments may be small because of their low concentrations (avg. 2 µg g−1 for n-alkanes and 9 µg g−1 for FAs). We find that lipid molecular proxies (including ACL14–32 and ATR14–18) can be reliably used to distinguish the FAs sourced from algae and other plants, and Paq’ values can be utilized to distinguish the n-alkane sources between submerged plants and terrestrial plants. Aquatic plants do not have significant δD differences among different chain-length n-alkanes and FAs for each sample, suggesting that the offset between δD values of different chain-length n-alkyl lipids in lake sediments can help determine sedimentary lipid input sources and infer paleohydrological changes.