Sources, fate, and biological impacts of microplastic debris in the Delaware Estuary
Date
2025
Authors
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Publisher
University of Delaware
Abstract
The goal of this project was to determine the sources, distribution, composition, and biologic impact of microplastic debris within the Delaware Estuarine system. Specifically, this project (1) provides the methodology to quantify mismanaged plastic debris entering the coastal Delaware Bay from terrestrial sources through watershed raster modeling; (2) quantifies and characterizes microplastic debris throughout the water-column in the Estuarine Turbidity Maximum; and (3) seeks to determine the biological impact plastic has to the estuary through body burden analysis of ecologically important species and physiological impacts to important zooplankton species. ☐ I developed a geospatial hydrologic model quantifying mismanaged plastic debris based on land-use characteristics to determine areas of high plastic output along the coastal Delaware Bay watersheds. This model, along with past observations, aids in interpreting plastic concentrations throughout the Delaware Bay with a focus on previously recorded high plastic zones. The model will be constructed through aggregation of population density, industrial, commercial, and institutional (ICI) land-use, as well as municipal and mismanaged waste data. Utilizing reported United States litter statistics I calculate that an additional 1.8% of municipal solid waste is released into the environment as plastic litter. Given land-use characteristics I find that residential land-use litter between 6 x 10-7 tons m-2 to 0.001 tons m-2 given population density. Additionally, ICI wastes 6 x 10-6 tons m-2. Using these spatial statistics, a modeling case study of the St. Jones River Watershed find that 2.7 tons of plastic is exported to the Delaware Bay annually by the St. Jones River watershed. This model was used to determine that northern Delaware watershed had the potential to contribute more plastic litter into the environment than southern Delaware watersheds. ☐ Marine microplastics are a pervasive pollutant, with estuaries considered a sink for plastic debris. Due to dynamic physical influences in an estuarine system, residence times of particulate debris – including plastic material – increase, thus aggregating material at higher rates than the open ocean. The goal of Chapter 3 was to quantify, characterize, and contextualize microplastic debris in the Delaware Bay’s Estuarine Turbidity Maximum (ETM) and the surrounding region. In the Delaware Estuary it was previously found that the ETM region accumulates microplastics at higher concentrations relative to the remainder of the bay. Previous research in this region has focused on the characterization of surface material. In this study, I characterize the morphology, size, and polymer identities of microplastic material throughout the water column within the Delaware Bay ETM region. Microplastics were found at all stations throughout the sampling region, with average ETM concentrations of 1.4 MP/m3, 0.5 MP/m3, and 0.8 MP/m3 found at the surface (0.5m), mid-water (7m), and bottom (12m) depths, respectively. Fragments were the most abundant plastic type, followed by fibers, across all depths, whereas beads were found more commonly at depth. µFTIR analysis suggests fragments were predominately polyethylene, fibers were commonly polyester or rayon (a semi-synthetic material), and beads were primarily polystyrene. These analyses establish that microplastics are not simply a surface water feature and provide a comprehensive understanding of common microplastic characteristics. In doing so, we further contextualize the microplastic problem in the Delaware Estuary. ☐ Plastic concentrations in the water column were then related to the occurrence of plastic material within estuarine fish and invertebrate digestive tracts and gills, with a focus on the economically important blue crab, Callinectes sapidus. The aim of Chapter 4 is to investigate whether such plastic aggregations correlate with plastic body burdens in estuarine organisms, such as the Atlantic Blue Crab (Callinectes sapidus) and other abundant and ecologically important species. Estuarine organisms were sampled across 4 stations within the Delaware Bay throughout the spring of 2021, in regions of high and low plastic concentrations. Additionally, monthly mature female blue crab stomach samples were collected throughout 2021 and summer of 2023, along with mature male blue crabs in June of 2024. While 12% of estuarine organisms contained plastic in their stomach or gills, 26% of C. sapidus females, and 30% of C. sapidus males containing plastic. Plastic identified were predominantly fibers with an average of 1-2 pieces found per organism. Seasonal analysis reveals C. sapidus collected in the spring (April and May) contained more plastic than other seasons. Plastic body burdens were prevalent across sampling locations and species, with no significant differences observed based on uptake method, age, size, or habitat. These analyses provide insight into spatial patterns of microplastic body burdens within an array of estuarine organisms and C. sapidus. ☐ Finally, zooplankton physiological condition in response to environmental plastic was assessed for the mysid shrimp, Neomysis americana, a key component to the estuarine food web. Using thermal performance curves for metabolic rate, coupled with O:N ratios as an indicator of metabolic substrate, I tested for seasonal (summer, winter) and location (low and high plastic load) effects on these metrics of physiological performance. The estuarine mysid species, Neomysis americana, was collected from high microplastic and low microplastic locations within the Delaware Bay. Mysids were assessed for their respiration and excretion rate when acutely acclimated in the laboratory to a range of temperatures, representing a multi-stressor scenario. No difference was observed between respiration rates, nitrogen excretion or O:N between locations. Although, an increase in respiration rate and nitrogen excretion rate was observed from winter to summer at the high microplastic location. Specifically, during winter, we see a drastic increase in respiration rate at 10°C, with a high Q10 (11.6) between 2°C and 10°C. These results may indicate an initial stress response at acute temperatures of 10°C in the winter as opposed to a larger thermal window in the summer. Furthermore, no metabolic differences were detected at the current concentrations of microplastic in the bay. However, future studies exploring theoretical microplastic exposure limit or ingestion may reveal additional trends. ☐ Overall, this dissertation identifies sources and hotspots of terrestrial plastic waste through land use modeling, the accumulation and identification of microplastic in the Delaware Bay, and highlights potential ecological implications. This work lays the foundation for targeted mitigation and management strategies through identifying body burdens of estuarine species, metabolic effects on zooplankton, and identifying regional plastic litter aggregations. Ultimately, this work serves to address the pervasive issue of plastic pollution and its threat to ecosystem health.
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Keywords
Delaware Bay, Microplastic debris, Estuarine Turbidity Maximum, Estuarine organisms, Zooplankton