Microplastic Chemostasis and Homogeneity During a Historic Flood on the Mississippi River

Microplastics are ubiquitous environmental pollutants, yet little is known about their transport and distribution in large rivers. This study quantified and characterized microplastics under varying flow conditions and throughout the water column of the Mississippi River (United States). Temporal samples were collected from the river’s surface at St. Louis, Missouri, every ∼4 days during a historic flood in 2019 (n = 21) and biweekly under lower flow conditions in 2019–2020 (n = 7). The microplastic concentrations (6.0 ± 3.0 counts/L) and compositions (predominantly fibers that were frequently clear, blue, black, and red) in our temporal samples did not fluctuate as a function of discharge or other indicators of new water inputs (e.g., specific conductivity and turbidity). When we assessed samples collected throughout the Mississippi River’s water column (n = 10) at East Alton, Illinois, we found that the microplastic amounts (7.0 ± 3.5 counts/L) and assemblages (mostly clear, blue, black, and red fibers) were similar to the time series samples. We observed no relationship between water velocity and microplastic abundances across the channel. Instead, microplastic quantities near a developed bank with heavy barge traffic were higher than those found near the opposite, forested bank. Localized variations in land use may thus explain the minor differences in microplastic concentrations across the river. The microplastic amounts did not change as a function of water column depth. Nevertheless, we saw some evidence of varying microplastic compositions between surface and deeper samples, which is potentially the consequence of dissimilarities in polymer buoyancy. Microplastic chemostasis and homogeneity at our Mississippi River sites during a historic flood event contrast with prior observations of changing microplastic amounts and assemblages during discharge perturbations. Our results may therefore indicate variable transport processes with river scale that could be applicable to additional segments of the Mississippi River or other large river systems.