Assessing Microplastic Debris in the Mississippi River Watershed Across Space, Time, and Varying Flow Conditions

Plastic debris is ubiquitous in marine settings. While the public, government agencies, and the scientific community have largely focused on macroplastic debris (plastic >5 mm in diameter), the overwhelming majority of plastics in aquatic systems are microplastics (plastic <5 mm in diameter). Microplastics are emerging contaminants that, due to their small size, were understudied until the last decade, but may cause significant harm to aquatic life. Marine plastic debris is principally derived from terrestrial inputs, but these sources have not been systematically quantified. The Mississippi River is likely one of the largest sources of plastic debris, not only to the Gulf of Mexico, but also the global ocean. Thus, our research assessed microplastics in the Mississippi River watershed across space, time, and varying flow conditions. We characterized microplastic debris (e.g., size, shape), quantified microplastic concentrations, and estimated microplastic loads from the Mississippi River to the Gulf of Mexico. We found that the vast majority of microplastics from the Mississippi River watershed were fibers (98%). Microplastic concentrations averaged at ~80 counts/L, but could be as high as several hundred counts/L. Our results suggest that loads of >30×10 15 microplastics are discharged to the ocean every year. Moreover, initial results indicate that microplastics and suspended solids are positively correlated and increase with river depth. Ongoing work will 1) assess how particle size distributions in the water column relate to microplastic type and quantity, 2) establish a rating curve method to quantify temporal trends in microplastic loads using discharge data, and 3) employ mesocosm experiments to understand the degradation of macroplastics to microplastic under conditions similar to those in the Mississippi River. Our research can be widely applied to river systems to inform mitigation strategies closer to pollution sources that eventually impact marine environments.