Hydrological changes accelerate ecological degradation in a subtropical floodplain lake: A multi-proxy sedimentary...

Hydrological regulation has profoundly altered the natural hydrological regime of aquatic systems. Coupled with climate change and eutrophication, these alterations pose a significant threat to the stability of lake ecosystems. This study focuses on Qinglan Lake, a satellite lake in the southern Poyang Lake Basin. Based on ²¹⁰Pb_ex and ¹³⁷Cs dating results, we comprehensively analyzed multiple geochemical indicators, including sterols, n-alkanols, and δ¹³C_org, to reconstruct the ecological evolution of Qinglan Lake over the past century under multiple stressors. The results indicated a relatively natural state before 1958, characterized by lowsterol flux, organic carbon accumulation rate, and (n-C₂₆+n-C₂₈)/(n-C₂₂+n-C₂₄) n-alkanol ratio, while high δ¹³C_org and sitosterol/campesterol ratio, indicating limited primary productivity and macrophyte dominance. Between 1958 and 1990, the increases of (n-C₂₆+n-C₂₈)/(n-C₂₂+n-C₂₄) n-alkanol ratio and dinosterol abundance, and decrease in δ¹³C_org values, indicating the degradation of submerged macrophytes and restructured the algal community composition. This change might be triggered by the 1958 diversion of the Fuhe River, highlighting the role of riverine nutrients and hydrodynamics in facilitating phytoplankton growth. After 1990, under the combined effects of hydrological isolation, nutrient enrichment, and climate warming, sterol flux and cholesterol relative abundance increased consistently, indicating an intensified algal bloom, particularly of cyanobacteria. A comprehensive regional comparison reveals that restoring hydrological connectivity could be an effective strategy to curb ecological deterioration. These findings enhance our understanding of ecological degradation under multi-stresses in floodplain lakes and offer applicable insights for managing comparable systems worldwide.