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Impacts of microplastics on marine soft sediment: Changes in bioturbating species and ecosystem functions
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This doctoral thesis investigated how microplastics disrupt ecosystem functioning in marine soft sediments by altering bioturbation activity of key macrofaunal species including a bivalve and a polychaete worm. Results showed that increasing microplastic concentrations reduced bioturbation, impaired nutrient cycling and sediment oxygenation, and interrupted feedback loops between macrofauna and sediment environments, with implications for long-term habitat degradation.
As a global issue, microplastics (MPs) are widely distributed in marine systems. These particles sink to marine soft sediments, exposing seafloor habitats to a high risk of microplastic contamination. Marine soft sediments cover most of the earth's surface, dominating the coastal and estuarine regions and delivering ecosystem services to humans. While these areas maintain many functions involving dynamic interactions among biological and environmental components, they are under the stress of climate change, pollution and habitat loss. It is urgent to understand how MPs disrupt ecosystem functions in marine soft sediment, which is also a key step towards risk assessment of MPs. I combine laboratory experiments with mechanistic models to investigate how MPs influence the ecosystem function by altering bioturbation. Bioturbation is a critical process in marine soft sediment that maintains key functions such as nutrient cycling, organic matter decomposition and sediment stability. I investigate the bioturbation of key macrofaunal species responding to increased concentration of MPs and upscale these impacts to ecological scales. In Chapter 1, I review the literature about the distribution and effects of MPs on macrofauna species. In Chapter 2, I investigate the changes in particle mixing of two functionally different species (Macomona liliana (tellinid bivalve) and Macroclymenella stewartensis (maldanid worm)) caused by MPs and estimate the broad39 scale shift in nutrient processing. I then conduct a parallel experiment to investigate how MPs influence the linkages of bioturbation to sediment environments and assess patterns of nutrient cycling under this stress (Chapter 3). In Chapter 4, I further analyze the hydraulic activity of M. liliana as a different mechanism of response to MPs and estimate the changes in sediment oxygenation. Finally, I use new knowledge from the previous chapters to explore the possibility of linking these mechanisms to risk assessment at a habitat level (Chapter 5). As I discuss the main findings of this thesis, I emphasize that MPs interrupt a feedback loop relationship between macrofauna and the sediment environment, with the risk of accelerating habitat degradation.
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