Elucidating the vertical transport of microplastics in the water column: A review of sampling methodologies and distributions

There have been numerous studies that have investigated floating microplastics (MPs) in surface water, yet little data are currently available regarding the vertical distribution in the water column. This lack constrains our ability to comprehensively assess the ecological effects of MPs and develop further policy controls. In this study, we reviewed current progress of sampling methodologies, the distribution patterns, and the physiochemical properties of MPs throughout the water column. Three sampling protocols were identified in this study: bulk, net and submersible pump/in-situ sampling. In different regions, the vertical patterns of MPs in the water column varied with depth, which is possibly related to the morphological characteristics, polymeric densities, and biofouling of the MPs. The results of this review revealed that fibrous and fragmented MPs comprised over 90% of the total MPs by quantity, of which fibrous MPs constituted the majority (43%-100%). In addition, polyethylene terephthalate, polyamide, polyethylene, polyvinyl chloride, and polypropylene have been widely identified in previous studies. To minimize the impact caused by various sampling protocols, the use of a volume gradient trail experiment and a unified mesh size of 60-100 μm for the initial concentration are recommended according to the results of this review. Given the limited knowledge regarding the vertical transport of MPs in the water column, harmonized sampling methods should first be developed. The mechanisms of this process can be separately considered for different water bodies, such as freshwater systems, coastal waters, and pelagic zones. The presence of these anthropogenic pollutants in the water column poses a threat to the largest but most vulnerable habitats of life on earth, and hence they merit further investigation.