Abstract
The whereabouts of the overwhelming majority of plastic estimated to enter the environment is unknown. This study’s aim was to combine information about the environmental occurrence and physicochemical properties of widespread polymers to predict the fate of aquatic plastic litter. Polyethylene and polypropylene are common in the surface layer and shorelines; polyester and cellulosic fibres in sewage treatment works, estuarine and deep-sea sediments. Overall, non-buoyant polymers are underrepresented on the ocean surface. Three main explanations are proposed for the missing plastic. The first is accumulation of both buoyant and non-buoyant polymers in sewage treatment works, river and estuarine sediments and along shorelines. The second is settling of non-buoyant polymers into the deep-sea. The third is fragmentation of both buoyant and non-buoyant polymers into particles smaller than captured by existing experimental methods. Some isolation techniques may overrepresent larger, buoyant particles; methodological improvements are needed to capture the full size-range of plastic litter. When microplastics fragment they become neutrally-buoyant, thus nanoplastics are potentially widely dispersed in aquatic systems, both horizontally and vertically. Ultimately, over decades or longer, plastics are potentially solubilized and subsequently biodegraded. The rates at which these processes apply to plastic litter in different environmental compartments remain largely unknown.