Quantifying and characterizing the Mississippi River's contribution of microplastic debris to the Gulf of Mexico
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Plastics presence in the ocean is becoming increasingly ubiquitous which is widely recognized by the public, scientific communities, and government agencies. However, only in the past decade have plastics, specifically microplastics (MP) (<5 mm), in freshwater systems been quantified. MP sampling on surface waters usually consists of deploying drift nets behind or alongside a stationary or moving boat, restricting sampling to environments with low levels of suspended sediments and floating or submerged debris. This also limits quantification of MPs to particles >300 µm, as drift nets are limited in their pore size, allowing plastic debris (particulates and fibers) below this size to pass through the net and elude quantification. This project quantified and characterized MPs at 9 sites along the Mississippi River and its tributaries to assess their loading to the Gulf of Mexico by 1) creating a new method for sampling large rivers with high suspended sediment concentrations and large floating and submerged debris, 2) capturing and quantifying MP particulates and fibers <300 µm and 3) using micro-attenuated total reflectance Fourier transform infrared spectroscopy (µ-FTIR) to determine material type. A total of 7,600 suspect MPs were quantified from 24 samples and after µ-FTIR confirmation and blank correction, an average of ~11.6 ± 3.8 MP/L were found. Based on the average discharge of the Mississippi River into the Gulf of Mexico, ~4.6-5.4 quadrillion MPs and ~26.2-27.4 semi-synthetic MPs potentially enter the Gulf of Mexico annually. Once in the environment MPs and the harmful chemicals are consumed by organisms, potentially reaching humans by consumption of contaminated seafood. This project establishes a baseline to aid future fate and effects research as well as assist federal, state, and local policymakers in creating and assessing mitigation strategies to improve water quality.