Feret diameter and microplastic type were documented from the photographs using ImageJ software (Schneider, Rasband, and Eliceiri 2012). Photographs were analysed using a custom macro that allowed tracing of individual items of plastic, calculation of the Feret diameter, surface area and recording of the microplastic type. (See Supplementary Information for the custom macro). Microplastic types were classified as three main categories based on shape and form: rigid, sheet and fibre. Rigid was described as pellets, balls of entangled fibres, or foam like fragments resembling pellets or beads. Sheet was described as films or ribbons of plastic.