Drivers of Microplastic Contamination in Tropical Freshwater: Evidence from Three Ethiopian Rift Valley Lakes

Abstract

Freshwater lakes in the Main Ethiopian Rift Valley are increasingly exposed to agricultural, urban, and industrial pressures, yet the drivers governing microplastic contamination across tropical lake systems remain poorly resolved. We quantified microplastics and water-quality parameters in Lakes Ziway, Hawassa, and Koka across 24 sites during the wet (July 2021) and dry (February 2022) seasons using coordinated manta trawling and depth-integrated bulk-water filtration, followed by wet peroxide oxidation, NaI density separation, and ATR-FTIR polymer identification. Microplastics were dominated by secondary particles, with fibers (45%) and fragments (32%) comprising the principal morphologies and Polyamide (48.2%), Polypropylene (30.1%), and Polyethylene (21.7%) as the dominant polymers. Mean microplastic concentrations increased significantly from 311 ± 34 particles m⁻³ in the dry season to 418 ± 34 particles m⁻³ during the wet season (p < 0.001), while Lakes Koka (485 ± 42 particles m⁻³) and Hawassa (452 ± 42 particles m⁻³) exhibited substantially greater contamination than Lake Ziway (288 ± 42 particles m⁻³). Water chemistry showed pronounced seasonal variation, with elevated turbidity, sulfate, and nutrient concentrations during the wet season. Integration of ESA WorldCover land-use data with multivariate modeling revealed that the primary land-use gradient (PC1; 73% of variance), representing an urbanization–naturalness continuum, was significantly associated with microplastic density (β = −35.44, p = 0.014), explaining approximately 27% of the observed spatial variation, while substantial site-level effects indicated strong local environmental controls. Comparison of sampling methods further demonstrated that manta trawling underestimated microplastic abundance by approximately 288 particles m⁻³ relative to bulk-water filtration, indicating that surface-net surveys provide conservative estimates of contamination. Collectively, these findings demonstrate that microplastic pollution in tropical Rift Valley lakes is governed by interacting catchment-scale land use and seasonal hydrological transport, with local lake characteristics further modulating contamination patterns. This study provides one of the first integrated watershed-to-lake assessments of freshwater microplastics in East Africa and establishes a framework for understanding microplastic dynamics in rapidly urbanizing tropical catchments.