Population density and agricultural land cover influence microplastic concentrations in river sediments

Rivers transport microplastics (MPs) from terrestrial sources to marine environments, and are often the environment where MPs enter aquatic food webs. However, the spatial distribution of MPs in river sediments remains inconsistently reported, and the environmental factors driving MP accumulation are not well understood. This study investigates MP spatial distribution (n = 48) in sediment along multiple longitudinal profiles in streams spanning nine watersheds in the Mid-Atlantic United States and compares findings reported for sediment concentrations (n = 525) from 18 countries located across five continents. We hypothesized that MPs would accumulate at higher concentrations downstream with larger drainage areas, particularly in urbanized and populated areas. Median MP concentrations in the Mid-Atlantic region were 516 ± 765 particles kg^-1, and 2508 ± 7422 particles kg^-1 globally. No clear longitudinal trend was observed from headwaters to downstream reaches in the regional dataset. Interestingly, accounting for collinearity, vegetation significantly influenced MP levels in freshwater sediments, with certain types potentially influencing MP concentrations. A regional regression model identified vegetation cover as significantly correlated with MP concentrations, while global-scale data showed positive associations between MP counts, population density, and agricultural land cover. These findings highlight the need to consider land cover when evaluating spatial MP distribution and suggest that vegetation management could be explored as a potential strategy to reduce MPs in aquatic environments.