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Microplastics in the Mississippi River System and at Oyster Reefs along the Mississippi Coast: An Update
Proceedings of the 2020 Mississippi Water Resources Conference
Year: 2020 Authors: Cizdziel J., Scircle A.
Microplastic (MP) concentrations along the northern Gulf of Mexico are among the highest levels reported globally. The most likely source of the plastic pollution is the Mississippi River (MR) which drains much of the central portion of the USA. Yet, surprisingly little is known about the concentrations, types, sizes, and loadings of MPs in the MR and its major tributaries. This lack of data is hindering our understanding of the magnitude and sources of the problem. Because the MR is an intricate system of waterways, tributaries, and commercial routes, an in-depth spatial study is needed to fully assess MP pollution in the system. Our research aims to systematically quantify the concentrations and loads of MPs in the MR system and at oyster reefs along the Mississippi Coast, and characterize their shapes, size distribution, and chemical composition. To that end, we developed and validated a one-pot method for the collection and preparation of water samples for microplastic analyses. The method prepares samples in the same vessel (Mason jars) that they are collected in right up until the MPs are transferred onto filters or spectroscopic windows for analyses. The method minimized contamination, degradation, and losses, while increasing recoveries and throughput when compared to conventional sieving. We applied it to surface grab samples collected from the Mississippi River and its major tributaries during and after historic flooding in 2019. Microplastics (>~30 µm) were detected by fluorescence microscopy and identified by Imaging Fourier Transform Infrared micro-spectroscopy (µFTIR-Imaging). Concentrations were lower during the flooding, likely due to dilution. Mean concentrations (counts/L) ranged from 14 in the Tennessee River during flooding to 83 in the Ohio River during low-flow (summer) conditions. Loads of MPs tended to increase down river and ranged from ~87 to ~129 trillion MPs/day near New Orleans. Most of the MPs (>60%) were in the lower size fraction (30-90 µm), consisted primarily of fragments (~85%), followed by fibers (~8%) and beads (~7%), with polyethylene, polyester, and polyacrylate as the primary MP type. Analyses of samples collected near Mississippi oyster reefs are underway and results will be presented at the meeting.