Microplastics (MP) data collection from the aquatic environment is a challenging endeavour that sets apparent limitations to regional and global MP quantification. Expensive data collection causes small sample sizes and oftentimes existing data sets are compared without accounting for natural variability due to hydrodynamic processes governing the distribution of particles. In Warnow estuarine sediments (Germany) we found significant correlations between high-density polymer size fractions (≥500 µm) and sediment grain size. Among potential predictor variables (source and environmental terms) sediment grain size was the critical proxy for MP abundance. The MP sediment relationship can be explained by the force necessary to start particle transport: at the same level of fluid motion, transported sediment grains and MP particles are offset in size by one to two orders of magnitude. Determining grain-size corrected MP abundances by fractionated granulometric normalisation is recommended as a basis for future MP projections and identification of sinks and sources.

Representative replicates (n = 3) were available for station S2 and S10 amounting to 78 ± 10 g −1 DW and 379 ± 28 g −1 DW, respectively. " now reads "Representative replicates (n = 3) were available for station S2 and S10 amounting to 78 ± 10 kg −1 DW and 379 ± 28 kg −1 DW, respectively. " In the legend for Figure 2 "The black line indicates the transition from LD to HD polymers determined by the density of water (~1 g m −3). " now reads "The black line indicates the transition from LD to HD polymers determined by the density of water (~1 g cm −3). " These errors have now been corrected in the PDF and HTML versions of the Article.