Date of Award

Fall 2026

Abstract

Global plastic pollution is increasing in part because more than 90% of plastic is discarded and not recycled. Plastic travels from rivers, shorelines, coastal soils, and estuaries, ultimately reaching the open ocean, where it now represents the most abundant form of marine debris. Degradation of plastic polymers has resulted in smaller particles <5mm in size termed microplastics (MP). Attempts to quantify the amount of MP, especially in the oceans, are stymied by the variable interpretations of MP in samples. Their ability to remain suspended in the water column for long periods, means marine species are likely to accumulate MP over time in their muscles and organs and thus, marine MP have been reported to have multiple lasting effects in marine organisms around the world. The aim of this dissertation was to build a baseline of microplastic (MP) concentrations for Great Bay Estuary (GBE) and evaluate MP distribution patterns in the GBE water column and living resources.Surface water samples were collected May to November, 2018-2022 at multiple sites. The body burden of MP in three ecologically important taxa of Great Bay Estuary, NH (GBE) was determined from 310 specimens of each collected through the months of May through October in 2021 and 2022. and oysters were collected throughout the months of June and August of 2022 from two regions, Rehoboth Bay, Delaware, and Casco Bay, Maine. Microplastic burdens were quantified in gut content from European green crabs, Carcinus maenas, whole body preparations of Eastern oysters, Crassostrea virginica, and gut, gill, and muscle tissues from Atlantic silverside fish, Menidia menidia. Oysters collected throughout the months of June-August 2022 from two regions, Rehoboth Bay, Delaware, and Casco Bay, Maine, were analyzed for MP content to 1 determine if there were consistent predictable differences between farmed and wild. All three studies used standardized methods of separation, chemical digestion of biogenic matter, and filtration of MP, after which MP were quantified and analyzed using laser direct infrared (LDIR) spectrometry on all samples and specimens. Across 389 water samples, MP were detected in GBE waters at an average of 248±23 particles m-3. Particle quantity and morphology varied significantly across sites on any particular sampling day. On average, MP shapes were nearly circular. The most commonly identified particles were polyvinyl chloride (the third most common synthetic plastic used for packaging and construction), polypropylene (the most used thermoplastic polymer in food containers, automotive parts, and packaging), and polyamide (mostly known as nylon and used for automotive and machinery parts). Simulations using velocity fields obtained from hydrodynamic modeling and offline particle tracking algorithms showed significant spatial variation of MP particles sampled high and low tide, and particle distributions changed rapidly at all sites and at most sampling stations, usually by day two of a simulation. Most MP extracted from tissues were approximately 10-50 μm in diameter and had aspect ratios close to 1-1.5, indicative of rounder shapes. Most polymers detected (88±2%) were biogenic as opposed to anthropogenic, and 40±5% of biogenic particles were rubber. In living resources, the predominant anthropogenic MP among 14 total MP polymer types detected across all 40 specimens were polyurethane and polypropylene. European green crabs, C. maenas (n=20), had a gut MP range from 0-3 ́104 MP. Eastern oysters, C. virginica (n=10), had a whole-body range of 257-8 ́103 MP. Tissues from Atlantic silverside fish, M. menidia (n=10), had MP ranging from 725 to 11 ́104 in their guts, MP ranging from 11 ́104 to 32 ́104 in their gills, and MP in muscle tissue ranging from 2 ́103 to 3 ́103 MP. 2

Microplastics were found in 100% of farmed and wild oysters collected from Rehoboth and Casco Bays. The MP were predominantly circular, £ 50 μm in size, round (very few fibers), and consisted of many types of polymers. Numbers of MP g-1 wet tissue in Casco Bay oysters ranged from 17-2456 MP, and in Rehoboth Bay the range was 60-7969 MP. Average MP in farmed oysters was 904±229 g-1 and 501±185 g-1 for Casco and Rehoboth, respectively, whereas wild oyster averages were 414±261 g-1 and 2067±874 g-1 in Casco and Rehoboth, respectively. Rehoboth Bay oysters had three times the average MP g-1 count as Casco Bay oysters, but the difference was not significant (p=0.27). Even though age was roughly 2 ́ lower for farmed than wild oysters collected from both estuaries, no significant relationship was observed between the MP of farmed versus wild oysters.The data presented provide evidence of the growing concerns surrounding MP pollution. Together, these three species offer a multi-tiered insight of MP exposure across the estuary: oysters as open circulatory system filter feeders integrate MP from the water column into their tissues, green crabs as opportunistic omnivores, reflect benthic accumulation, and silversides serve as a critical trophic link transferring MP to opportunistic feeders (e.g., green crabs), larger fish, birds, and marine mammals. This cross-taxa baseline equips oyster farmers, restoration ecologists, and policymakers with the evidence to assess risk, prioritize mitigation, and protect both ecosystem and human health.

Document Type

Dissertation

First Advisor

Bonnie Brown

Second Advisor

Bonnie Brown

Third Advisor

Raymond Grizzle

Department or Program

Biological Sciences

Degree Name

Doctor of Philosophy

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