Characterisation of Plastics and their Relation to Benthic Organisms in Metsina Stream, Punakha Dzongkhag

Abstract

Plastics in freshwater ecosystem is an emerging environmental concern, yet spatial characterisation and ecological assessment remain limited in Bhutan. The study provides a baseline of the presence, types, and concentration of plastics classified as macro, meso, and microplastics across upstream, midstream, and downstream transects of the Metsina Stream. It also examined the relationship between plastic contamination and benthic organisms. Macro and mesoplastics were collected and weighed (items·kg⁻¹) from each plot while microplastics were extracted from the sediment samples using ZnCl2 density separation.  Macroinvertebrate diversity were assessed using the EPT index (57%) and HKH Biotic score (4.53). The Kruskal-Wallis test indicated significant difference between macro, meso and microplastics (p < .001) concentration across the stream transects with Mann-Whitney U tests confirming significant pairwise differences across stream transects. Inverse Distance Weighted interpolation revealed increasing plastic concentrations from upstream to downstream, suggesting anthropogenic influence. However, no significant correlation between plastic concentration and macroinvertebrate abundance (r = .029, p = .87) under current conditions. Principle Component Analysis explained 85.68% of variance among variables. These findings highlight the threats posed by plastics to freshwater ecosystem specifically benthic organisms and offers framework to integrate plastic contamination monitoring in Bhutan’s national freshwater assessment protocols for improved waste management strategies and conservation efforts.