Date of Award

Spring 2024

Project Type

Thesis

Program or Major

Biological Sciences

Degree Name

Master of Science

First Advisor

Gregg E Moore

Second Advisor

Alyson Eberhardt

Third Advisor

Easton White

Abstract

With an estimated 8.75 million metric tons of plastic entering the world’s oceans each year, marine plastic litter has become a fast-growing concern. Plastics have been found to have severe impacts on aquatic environments and marine organisms in which both physical and chemical effects have been described. Once plastics enter the oceans, the qualities that make them so desirable- their persistent and durable nature- also make them a substantial and persistent threat. Through natural processes (solar degradation, mixing, and biofouling) larger plastic pieces break apart, but never fully degrade, existing as smaller and smaller pieces called microplastics.

Microplastics, defined as plastic particles less than 5 mm in size, are prolific in marine environments, having been found in all major marine habitats. With an estimated 80% of marine microplastics originating from land-based sources, coastal habitats situated at the intersection of terrestrial and aquatic environments, are critical ecosystems of concern. However, established methodologies to detect microplastics from estuary systems are not well defined. The development and implementation of modified and uniform sample collection, processing, and reporting standards are needed to better assess and manage microplastics in estuarine environments.

In an effort to develop an improved sampling and processing methodology for estuarine surface waters, two common microplastic surface-water collection methodologies were compared: manta trawls and discrete grabs. Through assessing the success of each method across both field and lab protocols, we explored the accessibility, accuracy and comparability of both approaches in an estuarine environment. Findings indicated that in the field, discrete grab sampling provided clear advantages over manta trawl sampling in terms of accessibility. In the lab, discrete grabs provided advantages over manta trawls in terms of accessibility and accuracy. However, as the most common microplastic water sampling technique, manta trawl samples provided advantages over discrete grabs in terms of comparability. Modifications were made to further optimize lab protocols, such as integration of fluorescent staining and automated imaging, a more recently emerging technique to identify microplastics from environmental samples.

Using the modified field and lab approach, surface-water samples were collected from the Hampton-Seabrook Estuary (HSE) in Southern New Hampshire in 2021. Microplastics were found in nearly all samples collected, with average abundance greater than surrounding open ocean samples from the Gulf of Maine. Increased microplastic abundance was not found to positively correlate with the highest tidal heights, indicating that mobilization events, such as tidal flooding, are not the sole factor driving microplastic concentration in the system. Instead, a low tidal height was associated with elevated microplastic abundance, possibly indicating the role that reduced water volume plays in increasing microplastic density (MP/L). Microplastic abundance was also not found to differ significantly between upper and lower estuary sites. However, increased variability in microplastic abundance at upper estuary locations during spring tides likely reflects the impact that proximity to pollution sources has on localized microplastic concentration.

Through mapping average microplastic abundance with a range of anthropogenic sources, a trend emerged with sites of high microplastic abundance located adjacent to areas with high population density, significant impervious surface, and proximal to locations of frequent flooding. However, spatial analysis of quantitative anthropogenic variables indicated only one significant factor, suggesting that many other variables not assessed in this study are likely impacting microplastics entry, abundance, and movement into the estuary. It is therefore likely a complex interplay of environmental and anthropogenic sources, pathways of entry, and opportunities for mobilization which collectively determine microplastic presence and abundance within the HSE. Continuing to study and understand these factors, and their role in the movement of emerging pollutants, such as microplastics, into our marine systems is critical to manage both environmental and community resilience. However, this will only become more complicated as increased coastal development and climate change continue to shape and impact the factors that influence the entry and movement of these contaminants. Gathering comprehensive baseline data is the first step in this process, and one that we recommend is continued in an effort to monitor and manage our most critical ecosystems.

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