All statistical analysis and figures were computed using R software v4.4.0 The packages 'ggplot2', 'dplyr', 'tibble', 'stringr', 'tidyr' from the 'tidvverse' and 'ggpubr' together with 'patchwork' were used for data manipulation and production of figures. Microplastic concentration and driver variables were modelled using the "lm" function of the 'vegan' package . nMDS was calculated for the difference in community composition of polymer type in surface water and sediment using the 'vegan' package and PERMANOVA was done to test the statistical significance between surface water and sediment. For the comparison between the community composition of polymers, two samples (one surface sample from Takehara and one sediment sample from Tateyama) were removed from the analysis as they consisted of only 1 and 2 particles, respectively, identified as "other plastics". The difference in the size distribution of feret diameter among the different matrices was computed using the 'glmer' function in the 'lme4' package and "lmer" function was used to assess the relationship between feret diameter and distance from river.