Abundance and ecological implications of microplastic debris in the North Pacific Subtropical Gyre

Plastic pollution in the North Pacific Subtropical Gyre (NPSG), dubbed the "Great Pacific Garbage Patch," has been the subject of substantial public concern. However, there is relatively limited scientific understanding of how microplastic affects pelagic ecosystems. The motivation for this dissertation is to provide scientific information on the extent and impact of microplastic in the NPSG. The dissertation is organized around two central questions : 1) What are the abundance, distribution, and characteristics of plastic microdebris in the NPSG? 2) What is the impact of this microplastic on the neustonic zooplankton and plastic-associated rafting communities?. I documented widespread, though spatially and temporally variable, plastic pollution in the NPSG and adjacent water masses. The numerical majority of objects are small particles, but the majority of debris surface area is found in large objects. While plastic was highly variable on the submesoscale, an analysis of all available data showed that overall NPSG microplastic concentrations increased by two orders of magnitude between 1972-1988 and 1999-2010. I performed a laboratory weathering experiment on plastic pre-production pellets that suggested that changes in microplastic composition over the eastern North Pacific may be explained by differential rates of weathering between plastic types, and that carbonyl formation may be a proxy for the length of time a plastic object has weathered in the ocean. Microplastic interacted with marine life through its direct ingestion and by providing a hard substrate for oviposition and settlement. Thirty-three percent of lepadid barnacles collected in 2009 contained microplastic in their gastrointestinal tract. In contrast, neustonic zooplankton did not show significant ingestion of plastic microspheres during a series of at-sea incubation experiments. Oviposition in the oceanic insect Halobates sericeus was positively correlated with microplastic abundance. Most plastic- associated macroinvertebrates were known members of the rafting assemblage, but several potentially invasive taxa were also associated with debris. The diversity of taxa in the rafting assemblage increased with debris surface area, as predicted by the concept of island biogeography. This dissertation demonstrated that microplastic pollution is pervasive at the surface of the NPSG, and that ecological impacts include direct ingestion, release from substrate limitation, and enhanced dispersal. The introduction of microplastic to the NPSG may therefore represent a widespread alteration of the pelagic ecosystem