Abstract: Lake Xiliang, a representative macrophyte-dominated lake in the middle and lower reaches of the Yangtze River, offers critical insights into regional environmental management through the historical record of heavy metal pollution preserved in its sedimentary deposits. By integrating 210Pb and 137Cs dating of a sediment core with comprehensive geochemical analyses (Al, K, Ti, Ca, Mg, Zn, Cr, Cu, As, Cd, and Pb), organic matter content, enrichment factors (EF), and potential ecological risk indices (Er, RI), this study reconstructs the temporal evolution of heavy metal contamination and associated ecological risks in Lake Xiliang from 1858 to 2021. The findings indicate that the history of heavy metal pollution and ecological risk can be divided into three distinct phases. Stage I (1858—1963) represents a period dominated by natural processes, characterized by negligible heavy metal enrichment (EF≈1) and low ecological risk (RI<75). During Stage II (1963~1988), agricultural activities led to increased accumulation of As, Cd, and Pb (EF>1.5); however, the buffering capacity of the macrophyte-dominated ecosystem, evidenced through carbonate precipitation (as reflected by elevated Ca/(Mg+Al) ratios) and dilution effects, effectively mitigated observable ecological risks. In Stage III (1988~2021), industrial discharges significantly intensified contamination, particularly for Cd, Pb, and Zn, with Cd showing the most pronounced enrichment (EF>8). The RI increased to 111, indicating moderate ecological risk, with Cd contributing up to 84%. Positive Matrix Factorization (PMF) analysis revealed that the contribution of natural sources decreased from 54% to 3% since 1858, while contributions from agricultural sources (dominated by As, Zn, and Pb) and industrial sources (dominated by Cd and Zn) increased to 44% and 54%, respectively. The ecological risk index for Cd exceeded the high-risk threshold (>80) in 1993, likely due to the combined effects of industrial pollutant inputs and reduced ecological buffering capacity resulting from vegetation degradation, which promoted remobilization of heavy metal species. This study elucidates a dual mechanism of "pollution buffering–risk transformation" in macrophyte-dominated lakes during heavy metal accumulation and release phases, a process distinct from that in algal-dominated lakes, thereby providing a scientific basis for differentiated management strategies and ecological restoration in diverse lake ecosystems of the middle and lower Yangtze River.