Plastic marine debris has been a cause of concern for decades, especially in the Northeast Pacific Ocean. Recent research has shown that the vast majority of plastic in the ocean is not easily noticeable large pieces, but microplastic (< 5 mm). The distribution and abundance of microplastic in the ocean is relatively understudied, especially the subcategory nanoplastic (< 333 μm), because its size makes it difficult to accurately sample and enumerate. The goal of this dissertation is to better quantify the distribution, abundance, and ecological impacts of microplastics in the Northeast Pacific, and to assess utilization of suspended plastic particles by planktonic consumers. I examined the degradation of microplastic over time, the oceanic distribution of nanoplastic particles, and the chronology of microplastic deposition in coastal sediments throughout the past century.

I performed a laboratory weathering experiment and compared the chemical degradation of laboratory and open ocean samples to determine how long microplastic had been aging and degrading in the ocean. I found that carbonyl, hydroxyl, and carbon-oxygen bonds change progressively with weathering, but in a nonlinear manner. Weathering time can nevertheless be roughly approximated over large time periods. I found that microplastics from the North Pacific Subtropical Gyre had weathered longer than coastal plastics.

I introduced the use of epifluorescence microscopy to enumerate nanoplastics. By sampling surface waters in a transect from Seattle to Honolulu, I found concentrations of nanoplastics to be ~5-7 orders of magnitude higher than previous studies that analyzed larger microplastics. Nanoplastics are most likely to affect suspension-feeding zooplankton at the surface of the ocean. I analyzed multiple species of salps sampled in oceanic waters to determine whether they had ingested nanoplastics; every salp analyzed had ingested nanoplastics, regardless of species, life history stage, or oceanic region sampled in. The average plastic they had ingested was significantly smaller than the average ambient surface nanoplastic available to them.

I examined the temporal accumulation of microplastics in sediments over the past 72 years. By analyzing a box core from anoxic bottom waters in the Santa Barbara Basin off the coast of California, I found an exponential increase in plastic deposition in sediment since 1945. This increase in plastic deposition rate was tightly correlated with exponential increases in Southern California population and worldwide plastic production over the same period.

This dissertation found that microplastics and nanoplastics are more abundant, temporally persistent, and spatially widespread than previously thought. These plastics are being consumed by suspension-feeding pelagic tunicates, and thus could be entering much of the pelagic food web.