Patterns of suspended and salp-ingested microplastic debris in the North Pacific investigated with epifluorescence microscopy

Scientific Significance Statement Marine debris is a worldwide ocean pollution problem and evidence suggests that the tiniest pieces of plastic in the ocean, microplastics, may be the most abundant forms of plastic pollution in the ocean; however, the smallest microplastics have yet to be accurately quantified due to methodological limitations. We successfully developed and tested a new method for collecting and counting the smallest microplastic pieces in seawater and ingested inside the guts of salps, a planktonic species at the base of food webs and key to transport of carbon and particles from the sea surface to the deep sea. We determined that the true abundance of these tiniest microplastics far outnumber previously reported counts, and that every salp we examined had ingested plastic. Microplastics (< 5 mm) have long been a concern in marine debris research, but quantifying the smallest microplastics (< 333 mu m) has been hampered by appropriate collection methods, like net tows. We modified standard epifluorescence microscopy methods to develop a new technique to enumerate < 333 mu m microplastics (mini-microplastics) from filtered surface seawater samples and salp stomach contents. This permitted us to distinguish mini-microplastics from phytoplankton and suspended particles. We found seawater mini-microplastic concentrations that were 5-7 orders of magnitude higher than published concentrations of > 333 mu m microplastics. Mini-microplastics were the most abundant in nearshore waters and more evenly distributed from the California Current through the North Pacific Subtropical Gyre. Every salp examined had ingested mini-microplastics, regardless of species, life history stage, or oceanic region. Salps ingested significantly smaller plastic particles than were available in ambient surface seawater. The blastozooid stage of salps had higher ingestion rates than oozooids.