Differences in microplastic distributions on the surface freshwater collected using 100– and 355–<i>μ</i>m meshes

Microplastics have recently been considered anthropogenic pollutants. Of the arguments to describe microplastic distributions is what mesh size should be employed. Many researchers have reported that the use of different mesh sizes causes naturally generated microplastic quantity differences. However, studies on how much specific microplastic distribution is overlooked in a large mesh remain insufficient in the aquatic environment, particularly in freshwater. Therefore, this study demonstrated qualitative and quantitative differences in microplastic distributions between 100– and 355– μm meshes from five perspectives: numerical/mass abundances, distributions along the flow direction, and microplastic features (size, shape, and polymer type). As observed, median values of numerical and mass abundances were 13.9 particles/m3 and 6.0 μg/m3, respectively, in the 100 μm mesh, then 0.4 particles/m3 and 1.0 μg/m3, respectively, in the 355 μm mesh. Although differences in mass abundances were six times between both meshes, for pristine river characteristics, the difference was ignored in this study. Results also showed that accidental irregularities discovered in the sampling analysis step affected the distribution tendency along the flow direction using the 355 μm mesh. Moreover, the 100 μm mesh showed the highest abundances in the lower sampling station, thereby reflecting the adjacent urban and its tributaries. A gradual increase in numerical fragment abundance toward a smaller size was observed with the 100 μm mesh. Additionally, results showed that cumulative probabilities relating to the minimum Feret diameter of films and fragments were divided into three parts. This division showed a 97% and 67% potential underestimation in the 355– and 100–μm meshes, respectively. Besides, although films, fibers, and fragments having seven polymers were observed in the 100 μm mesh, few shapes and polymer types were revealed in the 355 μm mesh. This finding made it was challenging to trace microplastic origins and presume bioaccumulation potentials using the 355 μm mesh. In conclusion, since the 100 μm mesh revealed completely different distributions from the 355 μm mesh, it was recommended in this study. However, viewpoints that the 355 μm mesh has an advantage in clogging the mesh exist. Therefore, a proper sampling method should be employed by establishing strategic research plans.