Microplastic abundance and composition along an anthropogenic pressure gradient in an Andean river (Colombia)

Abstract This study evaluated the presence, distribution, and composition of microplastics in the Combeima River sub-basin (Tropical Andes, Colombia), under the hypothesis that their abundance increases with greater anthropogenic pressure. Sampling was conducted along an altitudinal gradient to characterize microplastics in riverbanks, water, sediment, and freshwater fish, and to assess the influence of hydrological regimes and flow variations on their accumulation across environmental compartments. Microplastics were extracted using alkaline digestion and density separation, visually classified under a stereomicroscope, and chemically identified through Fourier transform infrared spectroscopy. The highest concentrations were found in downstream areas associated with urban and agricultural land use. Fibers, particularly polymeric blends and polypropylene, were the most abundant type, especially in fish samples. A wide variety of polymers were identified, including polyethylene terephthalate, high-density polyethylene, and others related to synthetic textiles and industrial materials. Water and fish showed higher microplastic abundance compared to soil and sediment, where larger particles tended to accumulate. This study is the first report of microplastics in freshwater fish from the Tolima region and provides essential information on spatial variation in composition and the influence of land use and hydrological dynamics on distribution, valuable for future monitoring and management strategies in tropical Andean ecosystems.