Marine microplastic pollution: An interdisciplinary approach to understanding the effects on organisms, ecosystems, and policy

Thesis (Ph.D.)--University of Washington, 2020Microplastics (plastic < 5mm) are ubiquitous in marine environments, from surface waters to benthic sediments. Microplastic in the oceans was first documented in 2004 and our current knowledge of potential biological implications is limited and rapidly growing. Thus far, we know marine organisms are exposed to microplastics in natural settings, ingest microplastics, and experience negative physiological impacts. Many aspects of microplastics such as ingestion fate, extent of trophic transfer, and effect on marine ecosystems remain unknown. Motivated by the need to understand the impact microplastic pollution has on our environment and our lives, I investigated three aspects of the marine microplastic problem: impacts on marine organisms, the ecosystem they support, and the linkages between scientific research and public policy. In nature, mussels experience a wide range of particle types and concentrations, readily filtering microalgae and abiotic particles other than microplastic. Mussel clearance rate is sensitive to stress, making it a good indicator of stressful conditions and polluted environments. In Chapter 1, I compare mussel (Mytilus trossulus) clearance rates when exposed to two different abiotic particles, microplastic and silt, across multiple concentrations. I measure the clearance rates of mussels exposed to increasing concentrations of three particle treatments: Algae, microplastic + algae, and silt + algae. I found that mussel clearance rate was inhibited by high concentrations of microplastics but not silt. In the absence of microplastic, mussel clearance rate was not dependent on the addition of silt, total particle concentration, or algal concentration. Mussels readily ingest microplastics in natural and laboratory settings, raising concerns about particle fate. Mussels are key benthic-pelagic couplers, concentrating particles from the water column into dense and nutrient rich biodeposits. In Chapter 2, I evaluate how microplastic changes the benthic-pelagic coupling role of marine mussels (M. trossulus). I expose mussels to feeding regimes with and without microplastic and measure four attributes of biodeposits: morphology, quantity of algal and microplastic particles, sinking rate, and resuspension velocity. I found biodeposits from the algae treatment contained more algal cells on average than those from the microplastic treatment. Further, biodeposits from the microplastic treatment sank slower and resuspended at slower water velocities than biodeposits from the algae treatment. To combat plastic pollution, there is sufficient evidence that policies can lead to reduced plastic production and consumption both locally and globally. In Chapter 3, I examine global growth and spread of the marine microplastic field in conjunction with growth and spread of national plastic policies using scientometric and diffusion methods. I conduct systematic literature reviews of marine microplastic papers and national plastic policies through 2019. At a global level, marine microplastic research and national plastic policies have grown exponentially and remain in the early phases of growth and spatial diffusion. Marine microplastic publication spread at the institution level was best explained by a hybrid of expansion and relocation diffusion, while national plastic policy spread was best explained by expansion diffusion. Taken together, findings from Chapters 1 and 2 indicate mussels readily filter, ingest, and egest microplastics, demonstrating their ability to transport particles between benthic and pelagic habitats. When exposed to microplastics, decreased clearance rate may result in fewer particles removed from the water column and subsequently available to benthic organisms. Further, decreases in sinking rate and resuspension velocity of biodeposits containing microplastic may increase dispersal distances, thus leading to increased transport of both algal cells and microplastic particles away from mussel beds. While extent of marine microplastic research is not a good indicator of national plastic policies, both the scientific field and national efforts to reduce plastic pollution are spreading globally at exponential rates. Marine microplastic pollution is a local, regional, and global issue that require cross disciplinary attention from researchers and policy makers around the world