Experimental and geochemical evidence for Δ7/Δ5 sterol ratios as indicators of trophic status in Lake Fuxian, a great lake in Yunnan Province, SW China

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.