Seasonal and Spatial Variation in Microplastic Morphology in a Protected Tropical Freshwater Ecosystem: Evidence from Okomu National Park, Nigeria

Microplastic (MP) pollution in tropical freshwater ecosystems remains poorly characterized, particularly within protected landscapes. This study assessed the spatial and seasonal variation in MP morphology in surface waters of Okomu National Park, Nigeria. Water samples were collected from five sampling stations across two dry and two wet seasons, yielding 170 particles classified into six morphological categories: filament, fibre, foam, bead, fragment and plastic film. Filaments dominated the assemblage, accounting for 71.0% of particles during the dry season and 49.1% during the wet season. In contrast, fragments increased from 8.1% to 17.6%, while plastic films increased from 0% to 14.8% and occurred at all stations during the wet season. Seasonal morphology composition differed significantly (χ² = 20.45, df = 5, p = 0.001; Cramér’s V = 0.347), with filaments and plastic films representing the principal contributors to the observed shift. Morphological richness and Shannon diversity generally increased during the wet season, although paired station-level differences were not statistically significant. Spatially, morphological composition showed substantial overlap among stations, with no significant station-level association (χ² = 18.09, df = 20, p = 0.581; Cramér’s V = 0.163). These findings demonstrate seasonal restructuring of MP morphology despite limited spatial differentiation and provide a baseline for continued 29/09monitoring of plastic pollution in protected tropical freshwater ecosystems.