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

Abstract

Microplastics (< 5 mm) have long been a concern in marine debris research, but quantifying the smallest microplastics (< 333 μm) has been hampered by appropriate collection methods, like net tows. We modified standard epifluorescence microscopy methods to develop a new technique to enumerate < 333 μm 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 μm 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.