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Mississippi River Microplastics on the Menu

As the Mississippi River tributaries drift across 31 states, they pick up an assortment of passengers. Fertilizers, pesticides, animal waste and sewage all travel down the river toward Louisiana. One pollutant, often ignored, but equally detrimental to water quality, is plastic. A plastic bottle tossed in Minnesota can travel the entire length of the river with Louisiana as the last stop before the Gulf on the landfill-like tour. And unlike other trash, plastic does not decompose; it only breaks into smaller pieces.

“We are pulsing massive amounts of plastic into our waterways and no one has looked at the implications. It’s a chronic environmental stressor,” said Mark Benfield, professor at Louisiana State University. His lab is one of the first to look at plastics in the river and what impacts they are having.

Benfield spent much of his career studying marine zooplankton, but has long been interested in plastics, especially microplastics (defined as less than 5 millimeters, smaller than a Mardi Gras bead). It took a class field trip with students to really expose the pervasiveness of the problem. In 2015, Benfield and his students began collecting plankton samples. They pulled up one sample and found plastic in it. Then another. And another. All day long, samples with plastic were brought on board.

Since then, Benfield and others in his lab have been using Louisiana Sea Grant support to collect and examine samples. Aboard a small johnboat dodging shipping vessels, Benfield and post-doctoral research scientist, Matt Kupchik pull samples both north and south of Baton Rouge and New Orleans. “There is a lot of plastic in the river. And there is a heck of a lot going into our estuaries and the Gulf,” observed Benfield.

Most of the plastic is really small – on the order of 10 – 100 micrometers, similar in size to many single-celled algae. Too small to pick up. “The sizes are literally infinite. The plastic breaks down into smaller and smaller pieces. The only limitation for collecting them is our current technology,” said Kupchik.

Using a combination of nets, pumps, filters, acid digestions and cross-polarized microscopes, they have confirmed both the presence and abundance of microplastics in the Mississippi River. These ubiquitous, microscopic plastics are problematic, because in addition to being a physical pollutant, plastics leach toxic chemicals like bisphenols, phthalates, styrene and other plasticizers into our waters and our bodies.

The worst offenders are fibers. These small threads are shed every time you wash fleece, polyester and other synthetic fabrics. Given their flexibility, size and appearance to organic materials, they are also the hardest to collect. “To their credit, a lot of plastic is trapped by wastewater treatment plants, but the fibers still find a way through,” said Benfield.

Thus far Benfield’s lab has sampled in the summer, fall and winter. Water sampling trips as short as 10 minutes yield nets full of plastic. The real test will be this spring as large quantities of plastic-laden water chug down the Mississippi River and into our waterways.