Non-point source pollution poses a significant threat to global water security, and risk assessment and key source area (CSA) identification are critical for its management. While source–sink landscape models are widely used for non-point source pollution evaluation, their application in karst regions is challenged by ecological fragility, shallow soil layers, and severe soil erosion, limiting their effectiveness in accurately identifying pollution risks and CSAs. This study focuses on the Caohai Lake basin in southwestern China; it integrates the landscape-weighted load index (LWLI) and the universal soil loss equation (USLE) to assess non-point source pollution risks in the basin with the aim of precisely delineating critical source areas (CSAs). Total phosphorus (TP) and total nitrogen (TN) served as key predictors of water quality, and their responses to the LWLI and USLE were analyzed in the karst environment. The results revealed the following: (1) source landscapes cover 65% of the basin area, with cropland (40%) being the primary contributor to nitrogen pollution; (2) the LWLI and USLE explain 50–67% of the TP and TN variations during the wet season, with a sharp increase in water quality risk when the LWLI exceeds 0.75; and (3) high-risk and very high-risk areas account for 36.3% and 15.3% of the basin, respectively, and are concentrated in the northwest and south, where intensive agriculture and severe soil erosion dominate. These findings provide a scientific basis for non-point source pollution control in the Caohai Lake basin.

Combined use of in situ passivation and macrophytes is a valuable technology that exerts remarkable effects on aquatic systems. However, the effectiveness and ecological functions of this combined technology for macrophytes-dominated eutrophic (MDE) lakes with organophosphorus-controlled internal phosphorus (P) loading were poorly understood. In this study, aquatic simulation experiments were performed to study the combination of La-modified materials (LMM; La-modified bentonite (LMB), and La/Al co-modified attapulgite (LAA)) with macrophytes (Myriophyllum verticillatum L. (MVL), Hydrilla verticillata (Linn. f.) royle (HVR), and Ceratophyllum demersum L. (CDL)) for the control of P mobility in the water column, and to investigate the passivator effects on the physiological characteristics of macrophytes. The mineralization of organophosphates (BD−Po, HCl−Po, and Res−Po) is an important factor for maintaining high internal P loadings and overlying water P concentrations in the experiments. Compared with individual treatment groups, the reduction of internal P release flux and porewater SRP concentrations was more obvious in the combined treatments. Moreover, the redox-sensitive P forms transformation is more pronounced in the surface sediments. In the LAA+M group, internal P release flux was reduced by 55% and 55% compared with individual passivators and macrophytes retreatment groups, respectively. In contrast, the LMB+M group decreased by 16% and 46%, respectively. Simultaneously, LMM had less effect on macrophytes traits compared with individual macrophytes group and enhanced the absorption of phosphate by macrophytes. The phosphate content of macrophytes in the LAA+M and LMB+M groups increased by 24% and 11%, respectively, in comparison with the individual macrophytes group. Results concluded that the combination of passivator and macrophytes enhanced the effect of ecological restoration and exerts a synergistic effect on internal P pollution with macrophytes.