Microplastics in East African Nearshore Sediments

Global plastic waste accounts for 60–80% of marine litter, largely driven by inadequate land-based waste management and growing coastal populations. Mismanaged macro-plastics entering the oceans undergo ultraviolet (UV) degradation, thermal oxidation, and mechanical fragmentation, breaking down into persistent microplastics (< 5 mm). Due to biofouling and polymer density alterations, nearshore surface sediments serve as the primary global sink for these widespread environmental contaminants. In December 2019, surface sediment grab samples were collected from 14 coastal sites across Kenya, Tanzania mainland (Dar es Salaam coast), and Zanzibar. Microplastics were extracted using NaCl density separation and vacuum filtration. Physical characterization involved quantification, sizing, and colour categorization under stereomicroscope. Chemical polymer identification was performed micro-FTIR spectroscopy (600–4000 cm-1). Log-transformed data were evaluated using ANOVA and PCA. Across East African nearshore seabed sediments, abundances ranged from 33 to 397 particles kg⁻¹ DW, yielding an overall mean concentration of 110 ± 12 MPs kg-1 DW. Zanzibar recorded significantly higher concentrations (p = 0.032) than Kenya and Tanzania mainland. Extracted sizes were dominated by the 4–50 µm category (63%). Fibers/filaments dominated at 97% over fragments (3%), while black was the dominant colour (71%), alongside red, blue, white, and green. Micro-FTIR analysis identified synthetic polymers including polyester (18%), polyethylene (6%), and polyvinyl chloride (6%), alongside prevalent natural cellulose variants. Nearshore seabed sediments across East Africa were ubiquitously contaminated with microplastics, predominantly black fibers < 50 µm. Abundance strongly correlates with local population density and riverine discharge rather than direct sediment grain-size sorting. Upgrade of coastal municipal wastewater filtration and solid waste infrastructure to trap fibers is paramount, including the establishment of long-term, multi-seasonal monitoring programs to track plastic polymer deposition trends and ecological risks to benthic marine fauna.