Abstract:To systematically reveal the sediment pollution characteristics, endogenous release risks, and primary pollution sources in Qilu Lake, a heavily polluted plateau lake in Yunnan, and to provide a scientific basis for its precise and effective management, a comprehensive sediment investigation was conducted across the entire lake from 2024 to 2025. This study involved collecting samples from 20 sites to determine the concentrations of total nitrogen (TN), total phosphorus (TP), organic matter (OM), and various heavy metals, with their spatial distribution patterns analyzed using GIS. Concurrently, laboratory static incubation experiments were performed on sediments from three typical areas—the western estuary, the lake"s center, and the northeastern region—to quantify the release fluxes and transformation patterns of nitrogen and phosphorus. Finally, methods such as Nemerow"s comprehensive pollution index and Principal Component Analysis (PCA) were employed to assess the pollution degree and identify the main sources. The results indicated that the sediments of Qilu Lake act as a massive "pollution reservoir" and are extremely polluted, with surface sediment concentrations of TN, TP, and OM ranging from 2120–11300 mg/kg, 270–2280 mg/kg, and 41.4–260 g/kg, respectively. The Nemerow"s index assessment revealed that 70% of the sampling sites reached moderate to severe pollution levels (PN > 2.0), with TN being the primary determining factor for the pollution. The pollutant distribution exhibited significant spatial heterogeneity, forming "pollution hotspots" in the southwestern estuary area, which receives terrestrial inputs, and in the central lake area, which serves as a deposition center for endogenous biomass. PCA clearly distinguished two major categories of pollution sources: (1) a composite source of agricultural non-point and industrial pollution, characterized by TP and various heavy metals (especially Cd, Pb, Cu, Zn); and (2) an organic pollution source, characterized by TN and OM, originating from domestic sewage and endogenous biological contributions. The static release experiments confirmed that the sediment is a strong internal source of nitrogen and phosphorus. The central and northeastern parts of the lake were identified as the main "high-efficiency release zones," with a maximum TP release flux of up to 2.80 mg/(m2·d) and an ammonia nitrogen release flux as high as 146.1 mg/(m2·d). In contrast, the western estuary area exhibited a unique net absorption of total nitrogen, suggesting its potential as a "denitrification functional zone." The core innovation of this study lies in revealing, for the first time, the spatial heterogeneity and functional differentiation between the "stock" (pollutant storage) and "risk" (release potential) of internal pollution in Qilu Lake. The study found that the western estuary area, despite having the highest pollutant stock, exhibited unique net total nitrogen absorption in its release mechanisms, identifying it as a potential "denitrification functional zone." Conversely, the central and northeastern areas, although some parts do not have the highest pollutant stock, function as the "high-efficiency release zones" for nitrogen and phosphorus. This finding not only deepens the understanding of the complexity of biogeochemical processes in heavily polluted shallow lakes but, more importantly, This provides the critical scientific evidence for evolving endogenous pollution remediation in lakes, moving away from conventional, simplistic, holistic approaches to a precision, zone-based management and control strategy informed by "source-sink" patterns and functional differentiation.