Date of Award

2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Biological and Environmental Sciences

Department

Fisheries, Animal and Veterinary Science

First Advisor

Coleen C. Suckling

Second Advisor

Andrew J. Davies

Abstract

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 efforts to precisely identify sources. High concentrations of tire wear particles were identified in a lake with comparatively low density of surrounding roadways to other locations, but which historically experiences spikes in chloride concentrations. This suggested that the presence of road-associated pollutants in freshwater systems may be a better indication of vulnerability to roadway runoff than simply measuring proximity to roadways itself. Chapter 3 evaluated the retention time and accumulation potential of MPs by predatory Jonah crabs (Cancer borealis), an emerging fishery in New England, through trophic transfer after predation on bivalves exposed to MPs. This study improved on previously published trophic transfer experimental methods by evaluating the use of a novel experimental tool to consistently dose shellfish prey tissues with environmentally realistic MP types (polyester microfibers) and quantities through targeted injection of a well-characterized MP stock solution. C. borealis was found to ingest ~70% of PES microfibers dosed in an Crassostrea virginica prey item, with 99% of ingested fibers egested after 24 hours from the stomach, highlighting efficient removal of MPs by the crabs’ digestive system. Crabs were identified to take in ~1% of MPs in prey tissues onto respiratory tissues (gills), likely due to messy prey handling that which released MPs from prey tissues into the ambient water column, making them bioavailable for uptake. Trace MPs were identified in hepatopancreas and reproductive tissues (individual), though none were identified in hemolymph, suggesting that internal translocation of MPs within this study’s spike MP size range (650 µm x 16 µm) was minimal. However, an analysis of environmental MPs > 100 µm in a subset of crabs found significantly more MPs in the gill (5.23 ± 1.1) and reproductive tissues (3.81 ± 1.5) than in the stomach (2.23 ± 0.8) and hepatopancreas (0.62 ± 0.6). These results demonstrate the need for additional research to identify the physiological mechanisms mediating uptake and internal movement of MPs by edible decapod crustaceans to inform ecosystem management strategies and protect consumer health.

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