Date of Award
2024
Degree Type
Dissertation
Degree Name
Doctor of Philosophy in Oceanography
Department
Oceanography
First Advisor
J. P. Walsh
Abstract
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 also measured MPs in sediment cores and found exponentially increasing levels of MP pollution from 1950 to the present day throughout the estuary. Remarkably, RI marsh sediments trap up to 50 times more microplastics than the nearby seabed, raising concerns for benthic biota inx these ecosystems.
Plastic pollution is a global problem, and local conditions and social norms are anticipated to play a large role in the amount of plastic debris present on shorelines. A four-month timeseries of shoreline plastic debris was conducted in the coastal city of Nha Trang, Vietnam (Chapter 3). High plastic deposition and removal was found at all sites, with highest plastic concentrations on sandy shores and lowest on hardened shorelines. While sandy and vegetated shorelines have more plastic accumulation potential, urbanized areas with hardened shorelines typically are unable to accumulate debris. Thus, hardened shorelines act to funnel plastics down the system and potentially augment loads to natural shorelines and the flux to the open ocean. Comparison of debris concentrations with environmental forcings highlighted how river discharge is an important driver for shoreline deposition. Storm waves and winds, as well as tidal cycles, clearly impacted removal of debris from shorelines. This timeseries study demonstrated the complexities of shoreline debris deposition and removal and reinforces how source identification is key to reducing the buildup of plastic debris in the environment.
Plastic debris, including MPs, can be exported from the coastal ecosystems into the open ocean, but ocean dispersal and resulting distributions remain poorly quantified. To help understand the marine plastic problem, The Ocean Race collected samples during the 2023 around-the-word sailing competition. This work provided a snapshot of the current ocean MP problem and filled crucial data gaps where few MP samples had been taken previously (Chapter 4). MPs were found in every sample, with high concentrations not only near coastal regions, but also along some remote areas of the Southern Ocean. This work highlighted the complexity of MP transport and the ubiquity of these pollutants in the ocean, and the data can be used to refine global forecasting models of microplastic distributions and fate in the ocean.
Once MPs have accumulated in an environment, they are difficult to clean up due to their small size. The small size of MPs also poses another problem; they are easily ingested by marine biota. The last portion of this dissertation focused on MP ingestion by two microzooplankton species to determine physiological impacts (Chapter 5). Marine heterotrophic dinoflagellates make up the base of the marine food web and control the transfer of energy between the microbial food web (the phytoplankton upon which they graze) and the classic marine food web (the zooplankton and larvae which feed on them). The species studied are found in temperate coastal regions of the Atlantic and Pacific Oceans, including on the Northeast Atlantic Shelf. When microzooplankton ingested microplastics, reduced growth rates and reduced ability to graze on algae were measured. These negative effects led to up to a 50% decrease in secondary production. The results of this study point to the dramatic negative impact MP pollution could have on marine productivity in the future.
The field of plastic pollution combines knowledge of anthropogenic pollution sources, social science, coastal and oceanographic processes, sediment transport, and marine ecology. This dissertation touched on many of these topics and expanded the body of knowledge on the plastics problem from localized coastal ecosystems to the global surface ocean, with widely applicable results. Altogether, the insights from this dissertation enhance understanding of macroplastic and microplastic transport and fate, and the potential consequences for the marine ecosystems.
Recommended Citation
Fulfer, Victoria Madison, "ASSESSING PLASTIC POLLUTION ON SHORELINES AND IN COASTAL SEDIMENTS" (2024). Open Access Dissertations. Paper 1632.
https://digitalcommons.uri.edu/oa_diss/1632