Fig. 4

The three different types of measurement frames loosely based on ISO 13322–1:2014 [52]. Principally, the location of a plastic particle inside a measurement frames defines whether it is measured. The margin of the measurement frame is represented by dark blue, while the margin of the field of view is colored in light blue. Microplastics that are going to be measured are orange, whereas excluded plastic particles are red. A Only microplastics whose centroid lies inside the measurement frame are considered for particle size and shape measurement. B All particles touching the right and upper image margins are measured, whereas microplastics at the left and bottom are ignored. For both, A and B, the measurement frames are separated. C In the third type, the measurement frames are neighboring, since each measurement frame is as large as the respective field of view. Microplastics that are cut off by the margins are all excluded from particle measurement, which makes it of necessity to correct the raw counts. Created with biorender.com