Microplastics Affect Organisms across Multiple Levels of Biological Organization: Informing Individual and Ecological Rsk

Demonstrating the ecological consequences (or risk) of microplastics is critical to inform policies that will mitigate their presence in the environment. However, assessing the risk of microplastics is made difficult by their complexity: microplastics are a suite of contaminants varying in size, shape, polymer type, and associated chemical cocktail. In the first chapter of my thesis, I synthesize studies testing hypotheses about the effects of microplastics to measure the weight of scientific evidence regarding the effects of plastic pollution at every level of biological organization. By systematically reviewing the literature, I found that macroplastics are undoubtedly causing ecosystem-level effects, but that more research is needed to fully understand the ecological consequences of microplastics because effect mechanisms are likely driven by their diverse characteristics. By finding patterns in how particle characteristics relate to effects, I show that particle shape, size, and polymer type are all relevant to toxicity. In my second and third chapters, I conducted laboratory experiments designed to better understand the effects of environmentally relevant microplastics, comparing plastics that I purchased from a manufacturer and plastics that I collected from the environment. In my second chapter, I showed that microplastics are both a physical and a chemical stressor. Environmentally sourced microplastics were not only more harmful to developing larval fish than pristine microplastics, but they also caused impacts when the fish were exposed to the leachates alone, while the pristine microplastics did not. In my third chapter, I investigated the long-term effects of microplastics on two generations of fish. Here, I showed that both pristine and environmentally sourced microplastics caused physical stress resulting in decreased adult growth, lipid storage, and a visible dermal stress response. However, only the environmentally sourced microplastics had effects on reproduction and offspring viability. Finally, in my fourth chapter I proposed a risk and management framework to assess the harm of microplastics in the ambient environment. This framework can be used by decision-makers to inform microplastic concentration thresholds that will trigger management decisions. Throughout my thesis, I highlight the need to consider microplastics as a complex contaminant, demonstrate their complexity through laboratory studies, and suggest how their complexity can be incorporated into risk assessment.