Morphological Predictors of Microplastic Accumulation in the Ona River System, Southwestern Nigeria

The study meticulously evaluated the spatiotemporal dynamics of microplastic (MP) pollution across 12 distinct sites in the Ona River Basin, southwestern Nigeria, with the objective of assessing the influence of catchment characteristics, including elevation, slope, channel width, and depth, on the abundance of MPs. Samples were systematically collected during four distinct seasonal phases: the onset of the dry season, the peak of dry season, the onset of the rainy season, and the peak of rainy season. The concentrations of MPs ranged from 16.93 to 77.67 items per liter, with the highest concentrations recorded during the peak of the dry season, particularly at the Orogun Upstream, Alaro, and Onigbodogi sites. Fibers emerged as the most frequently identified category of MPs across all rivers and seasonal contexts, whereas fragments, films, and beads were observed to be more prevalent in urbanized locales such as Gege, Oke Ado, and Ogunpa Downstream. Regression analysis indicated a statistically significant negative correlation between elevation and MP concentration (p < 0.001), while channel width (p = 0.002) and depth (p = 0.036) revealed positive relationships. Slope did not exhibit a significant effect on the distribution of MPs. These findings imply that lower elevation zones and rivers characterized by broader and deeper channels are more susceptible to the accumulation of MPs. Seasonal variations further elucidate the influence of hydrology, with reduced flow and dilution during dry periods promoting retention. This study highlights the critical role of landform and seasonal determinants in shaping the patterns of MP pollution and reinforces the necessity for targeted mitigation strategies in urban and low-elevation watersheds.