Characterization and enumeration of microplastic pollution in three fish species of the Upper Mississippi River

Abstract

Microplastics have become a widespread pollutant in terrestrial and aquatic ecosystems over the past 50 years. Microplastics can cause a variety of negative health effects in the organisms that consume them, from changes in feeding habits to increased exposure to toxic chemicals. The majority of recent research has focused on marine microplastics, so the extent that microplastics are impacting freshwater ecosystems is less resolved. In this project, we assessed microplastic pollution in three fish species collected in 2019 from Pools 4 and 8 of the Upper Mississippi River (UMR). Digestive tracts of emerald shiners (Notropis atherinoides) (n=89), yellow perch (Perca flavescens) (n=97), and shorthead redhorse (Moxostoma macrolepidotum) (n=95) were removed for microplastic analysis. Tissue and contents were digested, density separated and filtered for microplastic enumeration. Microplastics were counted and identified, and subsamples were verified via Raman Spectroscopy. In total, 891 microplastic particles were found among the 281 fish individuals and ranged from 0-22 particles per fish. The most prevalent type of microplastic found across species was fibers. Common colors included blue, black, red and clear. Within the size range of microplastics collected (250μm-5mm), microplastic particle prevalence decreased as size of particle increased across all species. Within each species, there were no significant differences in microplastic content when comparing fish from Pool 4 versus Pool 8 (p>0.05). In addition, habitat strata (e.g., backwater, main-channel, side-channel, etc.) did not have a significant effect on microplastic content in any species (p>0.05). Microplastic content of fish decreased as fish length (mm) increased (p<0.05). In addition, smaller fish tended to contain proportionately more microplastics than larger fish (microplastics per mm fish length) (p<0.05). Between the three species, emerald shiner contained significantly more microplastics per mm fish length than both yellow perch and shorthead redhorse (p<0.05). Raman verification was conducted on 115 randomly selected particles and revealed the most common microplastic polymers as styrene-isoprene, polyester (PES), and acrylonitrile butadiene styrene (ABS). This research confirms microplastic ingestion by UMR fish and highlights the need for further monitoring of microplastic pollution in the UMR system.