Investigating the exposure and potential impacts of microplastics in the Galápagos marine food web

The Galápagos Marine Reserve is a global conservation priority, representing a vulnerable ecosystem experiencing growing anthropogenic pressures. The aim of this thesis is to investigate the sources of plastic contamination, to explore pathways and fates in coastal habitats at a variety of geographic scales (Archipelago, island and single beach) and to investigate potential impacts of microplastic contamination on marine species. Using San Cristóbal island as a case study, I collected field data (via seawater tows, benthic sediment grabs and beach survey) to test the hypotheses that (i) accumulation hotspots are present on the exposed eastern coast, (ii) beach plastic contamination is primarily from external sources to Galápagos and (iii) microplastic is present in the food web. Analysis of beach plastics > 5 mm showed a pattern consistent with the prevailing Humboldt Current as a possible driver for accumulation (> 2,500 particles m-2 at the most contaminated site, the east-facing Punta Pitt) with just 2% of items identified as from local sources. Evidence of microplastic uptake was observed in the digestive systems of seven marine invertebrate species including filter feeders, grazers and deposit feeders (52% of 123 individuals, mean 0.5 - 1.7 particles per individual) demonstrating entry into the food web across a range of species with different feeding modes and habitat preferences. Using comparable environmental sampling methods, sites across two biogeographic zones in the Galápagos Marine Reserve were surveyed to test the hypothesis that contamination would be greater in the populated South-Central Zone (that has greater continental connectivity), compared to the upwelling Western Zone. The most significant differences were evident in microplastic, with concentrations in seawater, benthic sediment and sand six to ten-fold higher in the South-Central Zone suggesting a differing profile of exposure risk for wildlife across the Archipelago. As it is not ethically nor logistically possible to sample across the entire food web, a priority scoring method was developed and applied to enable the rapid assessment of potential risks from plastic interactions for 3,159 vertebrate and invertebrate species. Data on endemism, conservation status, commercial importance (for invertebrates) and literature evidence of harmful effects were incorporated into a risk ranking system. This identified 27 vertebrate and 15 invertebrate species to be at higher risk and therefore priorities for future research and mitigation action. Finally, analysis of data collected through citizen science beach surveys showed that visual identification of suspected microplastics using a standard unit quadrat method was 93% accurate, as verified by Fourier-transform infrared spectroscopy polymer analysis (synthetic vs organic particles), providing a reliable indicator to support increased spatiotemporal resolution of beach monitoring. Overall, this work contributes to our understanding of marine plastic contamination distribution and composition in this data-poor geographic area of high conservation importance, in addition to providing several tools to support management