Fall 2022

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)


Moss Landing Marine Laboratories


The widespread use and subsequent reuse of plastics in the agriculture industry increases the risk of improper disposal, posing a threat to important wetland habitats. When plastics degrade, they break up into smaller pieces that pose serious threats to organisms that ingest them and to habitats they settle in. This study quantifies the estimated concentration, types, and lengths of microplastics (< 5mm plastic particles) in the marsh environments of Elkhorn Slough, California’s second largest estuary. Replicate samples of marsh soil samples were extracted from seven Elkhorn Slough marshes at varying distances from the head and the mouth of the estuary and potential sources of agricultural plastic. Using a safe and cost-effective density separation technique, microplastics were separated from the soil, identified, and counted on micro-filters using a dissecting microscope, then further analyzed with a Scanning Electron Microscope equipped with an Energy-Dispersive Spectrometer (SEM/EDS) to analyze surface microstructures and the elemental compositions of the particles. Two main microplastic morphotypes, fragments and fibers were observed. The average concentration of microplastics estimated by this study is ~1600 particles per kg of wet soil. Fragments are shorter but more abundant (making up 85% of microplastics found) than fibers and have an average length of ~85 μm and ~500 μm respectively. All microplastics found in collected samples exhibit signs of weathering, including pitting and fractures on the surface. I studied the particle size distributions of microplastics and fine-grained estuarian soil to highlight the similarities in the physical and hydrological influences controlling their distributions. This study helps to make conclusions about potential sources of microplastics to Elkhorn Slough and the Monterey Bay National Marine Sanctuary with an emphasis on California’s agriculture industry and watershed dynamics. The microplastic particle size distributions reflect hydrological influences on the suspension and deposition of microplastics in the Elkhorn Slough watershed. This finding assisted in identifying plastic sources as non-local agriculture plastics immigrating from outside of this watershed due to the removal of a natural barrier and the unique hydrological dynamics of Elkhorn Slough.