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Microplastic pollution has been identified as an ever increasing proportion of marine litter. Despite an increase in microplastic awareness over the last decade, it represents an as yet unquantified threat to the marine environment. The relatively few studies that monitor its distribution and impact have illustrated a range of worrying effects on marine habitats and communities. The Clyde Sea Area (CSA) is subject to many sources of terrestrial and maritime plastic input. The use of plastics in recreational and commercial vessels throughout the CSA is believed to result in large levels of microplastic fibres, which have previously been seen to be ingested by a range of marine organisms. In a study of the breakdown of commonly used polymers in benthic environments, it was found that ropes of 10 mm diameter in sub-tidal conditions release between 0.086 and 0.422g of microfibers per meter per month in the early stages of degradation. This rate would be expected to increase over subsequent months, releasing substantial amounts of fibres into the CSA environment. In addition to the presence of numerous sources of microplastics, the CSA is relatively enclosed, and may accumulate high levels of debris as a result. Monthly sampling of the water and sediment in the CSA revealed contamination similar to that observed in other near-shore environments. Thus, it is expected that the potential threat to organisms in other areas will be similar to that observed in the CSA. One organism known to take up microplastics is the Norway lobster, Nephrops norvegicus, the target of the main fishery in the CSA. In this work we examined the levels of microplastic in the gut of N. norvegicus from the Scottish waters. Examination of individuals from the CSA revealed both a high occurrence and high accumulation of microplastic. This was found to be much greater than in N. norvegicus sampled from more remote Scottish waters. As a result, N. norvegicus from the CSA are most likely to suffer from the negative impacts associated with microplastic ingestion than those in offshore or in areas of low anthropogenic activity. In order to determine the potential impacts of microplastic ingestion on N. norvegicus, we first examined the mechanism by which N. norvegicus retain and egest microplastic. The position of microplastic aggregations in the foregut indicates that the gastric mill is the main obstacle to microplastic egestion. Inducing moult in microplastic-fed individuals demonstrated that expulsion of the gut lining during ecdysis enables N. norvegicus to reduce their plastic load, limiting plastic aggregation to the length of a single moult-cycle. In an 8 month controlled-feeding experiment retained plastic was seen to have a range of impacts on N. norvegicus. Feeding rate and body mass was seen to decrease in plastic loaded N. norvegicus, and a reduction was observed in a number of indicators of nutritional state. The results presented in this thesis have a number of implications to the CSA and wider marine environment. The similarity in the level of microplastic observed in the CSA to that of other studies of inshore waters indicates the potential for high microplastic uptake by crustaceans in those areas. The high variability in observed microplastic abundance suggests that small-scale monitoring is unsuitable for monitoring marine microplastic debris, and that use of an indicator species may provide a more reliable method of monitoring that is not subject to small-scale heterogeneity in distribution. The seasonal retention of microplastic by N. norvegicus indicates that crustaceans may provide a suitable indicator of local contamination. However, in the CSA, the high level of fibre aggregation and observed impacts of prolonged retention indicate that microplastic may be causing further pressure on an already exploited resource, reducing the stability of the valuable N. norvegicus population. |
