A spatially variable scarcity of floating microplastics in the eastern North Pacific Ocean

Abstract

Plastic waste accumulating in the global ocean is an increasingly threatening environmental issue. To date, the floating and thus most visible fraction of ocean plastic pollution has been mapped at global scale. Yet, large knowledge gaps exist in our current understanding of the transport and transformation processes of positively buoyant plastic debris at the sea surface. Observations at sea typically report an apparent scarcity of microplastics (<5 mm) relative to the expected abundance-size distribution based on fragmentation of larger plastic objects. Here, we provide a comprehensive study on the relative abundance of microplastics (>500 µm) and mesoplastics (0.5-5 cm) in the surface waters of the eastern North Pacific Ocean using data from 1136 040 plastic fragments collected by 679 neuston trawl deployments between 2015 and 2019. Our results reveal that the apparent microplastic scarcity is not uniformly distributed across the region. Instead, we show that the relative abundance of floating microplastics increases from the outside to the inside of the North Pacific Garbage Patch. We hypothesize that this observation could be explained by (i) a spatially variable microplastic removal due to spatial differences in ocean productivity, (ii) a differential dispersal of micro- vs. mesoplastics with a preferential accumulation of microplastics in the subtropical gyre, and/or (iii) the timescales associated with transport and fragmentation of plastic objects at the ocean surface with older, more degraded, floating plastic accumulation in subtropical gyres. The results presented here highlight that global estimates of the accumulation and removal of positively buoyant microplastics need to consider spatial aspects such as variations in ocean productivity, the dominant physical transport processes in a given area, as well as the time needed for a plastic object to reach the specific offshore location.