AQUATIC MACROPHYTES IN LAKES: AN OVERLOOKED SOURCE FOR LONG CHAIN N-ALKYL LIPIDS IN LAKE SEDIMENTS

Aquatic macrophytes (aquatic plants and macro-algae growing in or near water) are critical components of lake ecosystems. Their contributions to total lipids in lake sediments, particularly long chain n-alkyl lipids (mainly n-alkanes and n-acids), have long been speculated. Increasing evidence from lakes at the Tibetan Plateau in China has indicated that large amounts of long chain (C₂₆–C₃₁) n-alkyl lipids in these lake sediments were derived from aquatic macrophytes, and some submerged plants and macro-algae contain similar or even higher concentrations of long chain n-alkyl lipids as those from terrestrial plants. Considerable amount of long chain n-alkyl lipids from submerged aquatic plants with C₄-like metabolism explained an underlying cause for the large discrepancies of carbon isotope values (δ¹³C) and uniformed hydrogen isotope (δD) variations among homologous lipids with different chain lengths.

δ¹³C and δD values from long chain n-alkyl lipids extracted from lake sediments have been widely used for the reconstruction of ancient vegetation, climate, altitude, etc. As such analyses have been built upon source-specific lipids, assuming that long chain n-alkyl lipids are primarily derived from terrestrial plants, the aquatic macrophytes’ input of lipids complicates the reconstruction of paleoenvironments, such as the C₃/C₄ ratio in ancient vegetation. The significant correlation between δD values of long-chain n-alkyl lipids from aquatic macrophytes and those of lake water, rather than those of precipitation, found in lakes at the Tibetan Plateau highlights both challenges and opportunities for paleohydological reconstructions employing the proxy of long chain n-alkyl lipids. The assessment of relative contributions of long-chain n-alkyl lipids from various sources in a particular lacustrine site becomes a critical prerequisite for the application of their isotope values as proxies. We found that examinations of inter-molecular isotope offsets among n-alkyl lipids with different carbon chain lengths from the lake sedimentary samples is a promising method to discriminate their terrestrial and aquatic sources. Factors influencing lipid biomass of aquatic macrophytes, their molecular isotope behaviors, and contributions beyond high plateau lakes require further studies.