Assessing Plastic Pollution on Shorelines and in Coastal Sediments

Plastics have become pollutants around the world, entering the marine environment predominantly from many land-based sources. A small proportion of plastics in the marine environment also come from sea-based activities such as fishing, boating, and shipping. Once in the environment, the low density of most plastic polymers allows transport around the globe by winds, rivers, currents, waves, floodwaters, and tides. As a result, the ultimate fate of present and past plastic pollution is poorly constrained; however, much material in the sea is thought to accumulate in marine sediment over time. Plastics in the environment have a wide range of morphologies (e.g., films, foams, and fibers), colors, and chemical compositions. As plastic products break down, due to mechanical, chemical, thermal, or UV degradation pathways, they become small particles known as microplastics (MPs; 1 µm - 5 mm) or nanoplastics (< 1 µm). These small particles retain their polymeric makeup but may change in color or morphology due to degradation processes. Ingestion of macroplastics (> 5 mm) and MPs has been shown to negatively impact over 100 marine species. MPs are hypothesized to be transported similarly to small sediment particles, allowing researchers to draw upon previous sediment transport work to inform models of MP transport and fate. The overarching goal of this dissertation is to increase our understanding of the anthropogenic and environmental forcings controlling the historic and modern-day distributions of plastic pollution in the marine environment. Narragansett Bay, RI was used as a case study site (Chapters 1 and 2) as it is an estuary with a long history of environmental pollution research upon which to build, and with a clear urban-to-rural gradient down the system along which pollution levels could be measured. Using shoreline transect and seabed grab sampling, high concentrations of microplastics were measured, with a down-system decreasing trend away from the city of Providence on the shorelines and the seabed. A diverse range of polymer types were identified in the samples using FT-IR spectroscopy, dominated by polyethylene, polypropylene, and polystyrene. The diverse range of plastic polymer densities complicates particle transport. The transport of MPs is likely related to the hydrodynamics, particle attributes and biofouling; the final fate of all plastics is either along shorelines, in the seabed, and a very small portion is removed by cleanups. Most MP particles are believed to become part of the sediment record, allowing study of their accumulation over time. This doctoral research