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The reliability of Δ7/Δ5 sterol ratios as an indicator of trophic status in Lake Fuxian was tested with in situ nutrient enrichment incubation experiment and a comparison of Δ7/Δ5 values in the strata of a sediment core with the known chronology of nutrient enrichment in the lake. The results indicated that the Δ7/Δ5 sterol ratios (including sum of Δ7/sum of Δ5 sterol ratios and C27-C29 Δ7/C27-C29 Δ5 sterol ratios) in the incubation water increased consistently with total phosphorus (TP) and total nitrogen (TN) concentration across the full experimental range from 0.012 to 0.130 mg/L and from 0.068 to 1.351 mg/L, respectively. The covariation between Δ7/Δ5 sterol ratios and TP and TN concentration could be explained by enhanced productivity of Δ7 sterol-producing green algae (e.g., Scenedesmus sp.) under the prevailing nutrient conditions. A sediment core from Lake Fuxian showed a two-stage increase in C27-C29 Δ7/C27-C29 Δ5 sterol ratios over time, including a small rise from 1951 to 1986, and a much greater increase since 1986. These results coincide with the two-stage eutrophication previously documented in Lake Fuxian using geochemical evidence and water quality monitoring data. This study therefore demonstrates Δ7/Δ5 sterol ratio as a credible descriptor of trophic status in Lake Fuxian. [ABSTRACT FROM AUTHOR]

Lake Fuxian is a large, deep, and oligotrophic freshwater lake. In recent sediment of this lake, a series of branched and cyclic alkanes, including branched alkanes with one or two quaternary carbon atoms (C compounds), alkylcycloheptane, and alkylcyclohexane, were detected in high abundance. For each homolog of long-chain branched/cyclic alkanes, molecules with only an odd or even carbon number were present in the sediment. The abundance variation of each compound homolog corresponded to those of other compound homolog in the section, indicating the same origin of these compounds. The concentrations of long-chain branched/cyclic alkanes in the sediment profile exhibited variations similar to those of phytoplankton biomarkers, such as 3-methyl-alkanes, short-chain odd carbon n-alkanes, hopene, polyunsaturated fatty acids, phytol, sterols, and long-chain alkyl diols, but slightly different from those of long-chain odd carbon n-alkanes, which are ubiquitous biomarkers for higher plants. Therefore, the C compounds and long-chain cyclic alkanes in the lake sediment were most likely produced by algae and bacteria in the water, primarily, by the heterotrophic bacteria in the moderately oxygenated waters, which flourishing might be the results of specific water environments in Lake Fuxian. [ABSTRACT FROM AUTHOR]