Assessment of Microplastic Bioaccumulation and Trophic Transfer in Snow Trout (Schizothorax richardsonii) in Dangchhu and Beychhu River, Bhutan

Microplastic pollution in high-altitude freshwater ecosystems remains insufficiently documented, particularly in Bhutanese river systems. This study assessed microplastic bioaccumulation and potential trophic transfer in Snow trout (Schizothorax richardsonii), algae and surface water from the Dangchhu and Beychhu rivers. Stratified systematic random sampling was used to collect 30 Snow trout samples, 30 algae samples and 30 surface water samples along a 15 km river transect. Microplastics were detected in all sample types. Fish gastrointestinal tracts showed the highest mean concentration (3.62 particles/g), followed by algae (0.96 particles/g) and surface water (0.03 particles/L). The dominant microplastic characteristics were line-shaped particles, mainly blue and black, with sizes ranging from 0.01 to 5 mm in the reported component-level assessment. The bioaccumulation factor between Snow trout and algae was 3.8, while the bioconcentration factor between Snow trout and surface water was 133.2, indicating higher accumulation in fish relative to both algae and water. Correlation analysis showed weak or negative associations among fish, algae and water microplastic concentrations, suggesting that exposure may be influenced by multiple environmental and biological factors rather than a single pathway. Cluster analysis further indicated an association between microplastic concentration and water quality, with downstream areas showing higher pollution signals. These findings indicate that Snow trout in the Dangchhu and Beychhu rivers are exposed to microplastic contamination and support the need for improved freshwater monitoring and waste management.