Microplastics are small piece of plastic debris < 5 mm in diameter, which arise and enter the environment either as a consequence of the direct release of small pieces of plastic debris or as a consequence of the fragmentation of larger items. They are widely distributed in the marine environment, on shorelines, at the sea surface, on the seabed. They are also ingested by a variety of marine organisms, and there is some evidence that this may cause adverse effects. This thesis investigates the potential sources, distribution and fate of this contaminant. Deterioration of plastics in a variety of environments was examined as a potential source of microplastics. Plastics were exposed in light and shade, in air, fresh water and sea water. Their deterioration was monitored over six hundred days. The fastest rate of deterioration, by far, was observed in air, and this was further accelerated by natural sunlight. Degradation in water was much slower and it was therefore concluded that plastic debris only fragments slowly into microplastics while they are in the water, and much faster when they are washed ashore and/or remain exposed to air, for example on a beach. A method to assess the contamination of beaches by microplastics' likely predecessors - small items of plastic debris - was therefore developed. High spatial variability, which can impede comparison between surveys undertaken by random sampling was overcome by assessing deliberately selected debris accumulations. An attempt to utilise citizen science using this rapid survey approach returned too much variability between volunteers to obtain consistent results. Nevertheless, the survey method was successfully used to identify narrow opening towards the sea, small tidal range and several weeks of high on-beach waves as predictors for high contamination. Evidence of harmful effects of microplastics on animals is primarily from laboratory studies and could be substantiated by exposure experiments \emph{in-situ}. Cockles were exposed to fluorescent plastic particles in an intertidal mudflat. After two weeks, particles were still present in the sediment. However, the test material could not be identified in the cockles with enough certainty to validate the method. The thesis concludes that fragmentation of larger items of debris on land is an important potential source of microplastics. Therefore, preventing plastics that are prone to deteriorate from entering the environment is likely the most effective way to prevent the generation of microplastics. Measuring the success of such preventive measures in the field returns highly variable results, as small plastic debris are elusive. Measuring effects in the environment demands large amounts of data, and highly specific methods are required to detect microplastics in animals. The shift of environmental plastic contamination towards small particles demands that plastic debris research adjusts to the elusiveness of these particles. Large volumes of variable data on plastics density and precise biological essays will be required.

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