Gillanders, Bronwyn M.Williams, Mike (CSIRO)Pring, Nikita Jade2025-07-232025-07-232025https://hdl.handle.net/2440/146274Globally, marine debris is a growing problem and cause for concern in our environment. Plastic is a major contributor to the millions of tonnes of debris that enter our oceans annually as we continue to produce more and more due to its usefulness, durability and longevity. The durability and longevity of plastic cause problems within the marine environment where it eventually breaks down into microplastic. Here I examine, the presence and abundance of marine debris including macroplastic and microplastic in vegetated and non-vegetated nearshore sediments along the coast of South Australia. In the first chapter, I investigated the abundance, characteristics, and spatial distribution of microplastics in nearshore sediments off the Adelaide metropolitan coast, offering insights into how wastewater and river outfalls influence their distribution in the marine environment. The spatial distribution and abundance of microplastics were likely influenced by wastewater treatment plant outlets and river outfalls, with local hydrodynamics also playing a key role through processes such as vertical mixing and sediment transport. My next two data chapters explore whether vegetated habitats, such as mangroves, saltmarshes, sand dunes, along with seagrass meadows, function as traps for marine debris, including microplastics, compared to nearby non-vegetated soft sediment habitats. A total of 770 pieces of marine debris were recorded, 40% of which comprised macroplastic. There was average of 1 microplastic piece per gram sediment sample. Vegetated habitats tended to have a higher abundance of marine debris and microplastics compared to non-vegetated habitats. Our results suggest that coastal habitats vary in their capacity to trap marine debris and microplastics. Understanding the trapping mechanisms of these coastal ecosystems is crucial for effective monitoring and developing protection measures against marine debris and microplastic pollution. In the final chapter, I investigate seagrass leaves and their epibiont communities which may collect microplastics potentially enabling trophic transfer of microplastics through herbivores. Throughout South Australia, seagrass leaves of Posidonia sinuosa and Zostera spp. were collected from coastal areas. Posidonia sinuosa had significantly more microplastics on leaves compared to Zostera spp, but only when the surface area of the leaf was considered. Microplastics were not correlated with the amount of epibionts suggesting other factors influence abundance of leaves. This study demonstrates the presence of microplastics on seagrass leaves, emphasising their distribution throughout the marine environment. In this thesis, I confirm the presence of microplastic in coastal sediments across South Australia. Throughout the entire study, marine debris was abundant across all habitats, with clear microplastic fibres the most dominant colour and type of microplastic closely followed by blue and black. The awareness of plastic pollution and its harm on the environment is increasing globally. This thesis provides evidence of marine debris and microplastic presence in vegetated and non-vegetated habitats along the South Australian coastline, thereby providing a substantial contribution to the scientific body of plastic pollution work.enMarine DebrisMicroplasticsCoastal HabitatsSedimentMangrovesSeagrassesThesis