Distributions, Characteristics, and Impacts of Microplastic Pollution in Rhode Island’s Waterways

Microplastics (MPs, plastic pieces < 5 mm) are ubiquitous pollutants and research on their presence and impacts within the environment has sharply increased in recent years. Despite sharing a classification, MPs are immensely diverse with variable characteristics across size, morphology, and chemical composition. These variations make predicting MP behavior and fate within the environment challenging. As such, there is a growing need for robust investigations into MP distributions and assemblages, likely sources, and effects on ecosystems determined by applying realistic experimental scenarios. This dissertation presents three chapters aimed at filling critical knowledge gaps about the assemblages of MPs within coastal aquatic ecosystems to inform pollution mitigation efforts and provide novel methodological improvements to advance future research. Chapter 1 assessed New England’s largest estuary, Narragansett Bay, for spatial and temporal variability in MP concentrations within surface waters along a human population gradient. Sampling results demonstrated high variability in MP recovery, ranging from 0.05 to 2.74 particles m-3, strongly influenced by short-term (tidal) and longer-term (seasonal) conditions. Sampling locations proximal to areas with high coastal populations tended to yield higher MP concentrations, but with spatial and temporal variability driven by seasonal river flow and wind conditions unique to the estuary. Sampling efforts were coupled with predictive oceanographic modeling, which identified that, while modeling cannot effectively replace field sampling, it can be used supportively to predict future pollution scenarios and to understand the hydrodynamic processes mediating MP movement. Chapter 2 investigated concentrations and characteristics of MPs in freshwater areas with varying levels of surrounding anthropogenic activity across the Rhode Island coastal watershed. To identify potential MP sources, sampling results were compared to land use around each site and historic water quality metrics linked to anthropogenic activities, including turbidity (previously associated with elevated MP concentrations), chloride (associated with road salts applied in winter), and elevated nutrients (chlorophyll a, nitrate, nitrite, and phosphorus; associated with fertilizers from agricultural and residential runoff and atmospheric deposition). Survey results yielded variable mean (± SE) MP concentrations, ranging from 1.99 ± 0.9 particles L-1 to 5.06 ± 0.5 particles L-1, with the highest concentrations identified in a rural lake with the lowest proportion of surrounding anthropogenic pressure compared to other sites, suggesting that variation in MP assemblages cannot be solely predicated by differences in surrounding land use. Results also indicated that freshwater areas are likely vulnerable to multiple sources of MPs with similar characteristics (such as microfibers), which confounds