Long-chain n-alkenes in recent sediment of Lake Lugu (SW China) and their ecological implications

Long-chain n-alkenes showing a predominance of n-C_25:1 and n-C_27:1 were detected in recent sediment of Lake Lugu, an oligotrophic alpine lake in Yunnan Province, SW China. The abundances of n-C_25:1 and n-C_27:1 alkenes varied considerably in the sediment core, most obviously in showing higher values during the period of 1821–1982, followed by a significant decrease between 1982 and 2012. Such variations were similar to those of long-chain 1,15-alkyl diols, biomarkers of eustigmatophytes, and a significant correlation was observed between n-C_25:1 (or n-C_27:1) alkene and C₃₂ 1,15-alkyl diol in the sediment core. This correlation and the fact that long-chain n-alkenes in some eustigmatophytes (e.g., Nannochloropsis sp.) were dominated by C₂₅ and C₂₇ compounds indicates that eustigmatophytes were the most likely contributor of the long-chain n-alkenes (especially the n-C_25:1 and n-C_27:1 alkenes) in Lake Lugu sediment. Chlorophytes, on the other hand, cannot be excluded as a possible contributor of the long-chain n-alkenes because these algae are common in Lake Lugu and they are known to biosynthesize n-C_27:1 alkene. Productivities for some species of eustigmatophytes and chlorophytes considerably decreased when the lake water was heavily influenced by catchment soil erosion and anthropogenic sewage inputs.