Occurrence of microplastics in water and sediment of a highly urbanized lake ecosystem in Addis Ababa, Ethiopia

• Microplastics (MPs) were assessed along the land contamination gradient using water and sediment samples. • Measured MPs concentrations were different between water and sediment samples. • MPs found mainly PE followed by PET, small (< 100 µm), fragment, and transparent. • Water and sediment can reflect quantitative and qualitative MPs contamination of the lake. Microplastic (MP) pollution poses a growing global environmental threat; however, its occurrence and characteristics in Ethiopian freshwater ecosystems remain largely unexplored. This critical knowledge gap has hampered the development of targeted mitigation strategies and environmental policies in this region. To address this, the present study investigated the composition, distribution, and potential ecological risks of MPs in both surface water and sediment from Lake Aba Samuel, Addis Ababa, Ethiopia. Water and sediment samples were collected from 12 sites and processed by density separation and oxidative digestion. Isolated MPs were analyzed visually, spectroscopically, and microscopically (SEM and FTIR). MPs were detected in all samples, with mean concentrations of 29 ± 0.6 particles/L in water and 42 ± 0.5 particles/kg in sediment, showing spatial variation influenced by pollution sources, hydrodynamics, and sedimentation. The MPs were predominantly fragments, transparent in color, and smaller than 100 µm. Polymer analysis identified polyethylene, polypropylene, and polyethylene terephthalate as the most common in water, whereas sediment samples were rich in polyethylene terephthalate, polyethylene, and polyvinyl chloride. Risk assessments using the polymer hazard index and potential ecological risk index indicated a high ecological risk associated with sediment MPs, particularly owing to the presence of PVC. These findings offer the first detailed account of MP pollution in an Ethiopian freshwater body, underscoring the urgent need for improved waste management, ecological risk assessments, and future research in underrepresented regions.