Microplastics in freshwater environments across the high altitude region of Gilgit-Baltistan, Pakistan

Microplastics are increasingly recognized as pervasive contaminants in remote mountain freshwater ecosystems, yet evidence from high-altitude regions remains limited. This study investigated microplastics occurrence and characteristics in surface water, sediments, and fish from the Braldu-Shigar-Skardu freshwater system in Gilgit-Baltistan, northern Pakistan. Samples were collected from remote headwaters, tributaries, main river channels, and the Satpara Lake-Dam system. Fish originated from wild populations, farming facilities, and local markets to provide a broader assessment of microplastic exposure. Matrix-specific extraction protocols were applied, followed by stereomicroscope sorting and polymer identification by µFT-IR spectroscopy. Microplastics were detected in nearly all sites and matrices, with concentrations ranging from 1 to 17 MPs L^-1 in water, 55-537 MPs kg^-1 in sediments, and 1-11 MPs individual^-1 in fish gastrointestinal tracts. Notably, highest frequency of microplastics occurrence was measured in farmed individuals (78%-83%), suggesting that aquaculture-related practices may represent an additional source of microplastics. Wild fish showed the lowest occurrence (25%), whereas those obtained from local markets revealed intermediate values (62%), indicating variable exposure across different pathways. Surface waters were dominated by small fragments and polypropylene, indicative of recent inputs and origin from fragmentation. Sediments showed a broader size distribution, and greater polymer diversity, consistent with their role as long-term sinks. The highest polymer diversity was highlighted in fish, indicating multiple exposure pathways. Spatial patterns did not show a geographical downstream increase, but rather a patchy distribution shaped by local anthropogenic input, hydrological connectivity, and depositional dynamics. Glacier-fed rivers primarily act as transport corridors, whereas low-energy environments such as Satpara Lake function as sinks promoting microplastics accumulation. Overall, the study provides the first integrated multi-compartment baseline for microplastics in Gilgit-Baltistan and supports future monitoring and management actions for these high altitude critical freshwater resources.