Widespread occurrence of microplastic pollution in open sea surface waters: evidence from the mid-North Pacific Ocean

Microplastic pollution has become a pressing worldwide environmental crisis due to its ubiquitous occurrence and distribution across the globe and potential impacts on ecosystems and human health. An increasing amount of studies on microplastic pollution worldwide have been reported. Nonetheless, knowledge of microplastic abundance, distribution, and characterization in the mid-North Pacific Ocean remained underexplored. Here, we collected microplastic samples from surface waters at 15 field stations in the mid-North Pacific Ocean using a Manta trawl from September 19, 2019, to October 30, 2019 and from a ∼1500 km latitudinal transect (15-25°N) and a ∼4300 km longitudinal transect (150°E-168°W). The abundance, distribution, and characterization of microplastics were investigated. Our results showed that microplastic abundance ranged from 0.06 to 1.23 items m-3, with a mean of 0.51 ± 0.36 items m-3. The most dominant polymer composition, color, and shape of microplastics were polypropylene (PP, 53%), light colors (white and clear, ∼60%), and irregular fragments (∼31%), respectively. A novel diversity index analysis of microplastic characteristics was carried out to identify multiple sources of microplastic pollution, providing a robust tool for inferring sources of microplastic pollution in the open sea. The K-mean cluster analysis suggested that the 15 sampling sites may be divided into a total of 6 clusters with similar polymer composition. The quantitative results from the field survey indicated the widespread occurrence of microplastic pollution in the open sea, which will allow us to better grasp the rate and extent at which the mid-North Pacific Ocean is increasingly polluted with microplastics.