Bioconversion of heavy metals in river sediments: Seasonal dynamics, driving mechanisms, and BSAF-based assessment in the Liujiang River basin

The bioconversion of heavy metals (HMs) in sediments has received limited research attention, despite its critical importance for ecological risk assessment. The Liujiang River Basin, a typical karst catchment in Southwest China, provides an ideal context for investigating HM bioconversion due to the generally high bioavailability of HMs in its sediments. This study employed the Biotic Sediment Accumulation Factor (BSAF) to analyze spatio-temporal variations in the bioconversion of seven priority metals between sediments and wild fish. Results demonstrated that BSAF values for most HMs were significantly higher in the wet season than in the dry season. Zinc demonstrated the highest bioaccumulation potential, with its BSAF frequently exceeding 1 and reaching up to 1.99, unlike the other studied HMs. A clear correspondence was observed between BSAF values and oxidizable fractions, suggesting the crucial role of oxidizable forms in driving HM bioconversion. Multivariate correlation analysis further revealed that combined proportions of oxidizable and carbonate-bound fractions shaped three distinct bioconversion patterns: inhibited bioconversion under dual low fractions (resulting in low BSAFs), promoted bioconversion with a single high fraction, and significantly promoted bioconversion under dual high fractions (producing high BSAFs). Notably, seasonal variations in BSAF values between Pseudohemiculter dispar and Cyprinus carpio indicated that pelagic feeding amplifies the impact of exogenous HMs, while benthic feeding intensifies the influence of sediment-destabilized HMs. Given the dominant roles of carbonate-bound and oxidizable fractions, we recommend implementing strict wastewater emission controls coupled with enhanced dissolved oxygen levels in water bodies to effectively mitigate HM bioconversion in sediments.