Partitioning dynamics and microbial responses drive the fate of polycyclic aromatic hydrocarbons in a plateau lake

Polycyclic aromatic hydrocarbons (PAHs) are persistent organic contaminants of concern in freshwater drinking-water sources, yet their environmental fate in hydrologically dynamic plateau lakes remains poorly constrained. Here, we investigated the multi-compartment distribution, apparent particle-water partitioning, microbial functional potential, and associated ecological and human-health risks of PAHs in Erhai Lake, a large subtropical plateau lake in southwestern China. PAHs were quantified simultaneously in the dissolved, suspended particulate, and surface-sediment phases across three seasons. Dissolved low-molecular-weight PAHs dominated the water column, exhibiting pronounced seasonal variability with summer peaks (12.9-49.8 ng L^-1) driven by intensified tourism and hydrological inputs. In contrast, high-molecular-weight PAHs preferentially accumulated in sediments, forming depositional hotspots in the southern basin. Apparent organic-carbon-normalized partition coefficients (K_OC,app) revealed that suspended particulates act as dynamic regulators of PAH transport and microbial accessibility. Metagenomic analysis revealed that the distribution of PAH-degradation-related functional genes co-varied with chemical partitioning patterns, suggesting potential microbial mediation of PAH fate. Ecological and human health risk assessments, including probabilistic Monte Carlo simulations, indicated that PAH-related risks remained within a low-risk regime across all seasons. Collectively, these findings provide a mechanistic framework for understanding how partitioning and microbial metabolism jointly regulate the fate of PAHs in plateau lakes, offering critical insights for the management of sensitive freshwater resources.