Subsurface observations of ocean plastics are very scarce but essential for adequate estimates of marine plastic pollution levels. We sampled plastics from the sea surface to a depth of 5m, at 0.5m intervals. Vertical mixing was dependent on sea state and affected the abundance, mass, and sizes of plastic debris floating at the sea surface. This has important implications for studies assessing at-sea plastic load, size distribution, drifting pattern, and impact on marine species and habitats.