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The metal-oxygen battery is the pinnacle of beyond Li-ion battery technologies, providing a very high theoretical specific energy relative to current battery chemistries. However, the practical use of alkali metals (Li and Na) in the presence of oxygen species and carbon cathodes provides substantial problems in the development of this technology. The highly nucleophilic and reactive nature of the reduced alkali-oxygen species causes degradation of the electrolyte and cathode. In order to overcome these factors, the fundamental reaction mechanism taking place at the solid/electrolyte interface need to be understood. In this thesis, a range of electrochemical and spectroscopic techniques have been used to help elucidate the reaction pathways and products. Initial studies have been conducted using shell isolated nanoparticles for enhanced Raman spectroscopy (SHINERS) to allow surface enhanced Raman studies on previously inaccessible electrode materials. The use of SHINERS utilised the localised surface plasmon from the gold nanoparticles without any additional effect from the particle surface, due to the Raman inactive silica shell. This allowed the vibrational modes of nearby molecules adsorbed on the surface to be enhanced. A comprehensive characterisation and analysis of SHINERS, as well as their functionality on a range of different electrode morphologies has been presented. SHINERS have been used to compare the oxygen reduction reactions (ORR) with and without the presence of lithium on a variety of polycrystalline electrodes. The behaviour of superoxide (O2 .- ) without the presence of lithium was shown to be highly dependent on the electrode surface. The ORR in the presence of lithium was demonstrated to be both surface and solvent dependent on a range of carbon electrodes. SHINERS were also used to study more practical battery electrodes, with the detection of lithium oxides and side products on the electrode surface after discharge and charge. The use of single crystal electrode surfaces were also studied, in order to provide an ideal surface to understand the reaction mechanisms. The effect of moving from an aqueous to a non-aqueous electrolyte on a gold (111) surface was studied using voltammetry and SHINERS. The inhibition of the electrode surface with trace amounts of MeCN was detected. The degradation of MeCN was also observed with trace amounts of water. Finally the behaviour of the sodium-oxygen system was studied on the three low index basal planes of platinum (111), (110) and (100). The electrochemical behaviour was shown to be facet dependent, with the addition of a supporting salt also affecting the reaction mechanism taking place.

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.

The first reports of small plastic debris floating at the ocean surface were recorded in the 1970s, but it is only in the last decade that scientific and media attention has soared. Microplastics (plastic 1 μm – 5 mm) have since been acknowledged as a global marine contaminant, raising concerns about the interactions between anthropogenic debris and natural biological processes. In this thesis, I explore the hypothesis that microplastics can be transported via biotic-driven mechanisms through the water column and into coastal sediments, leading to adverse impacts on the health and functioning of marine fauna and ecosystems. In Chapter 2, I demonstrate that a key pelagic species, the copepod Calanus helgolandicus, alter their prey selection dependent upon the size or shape of the plastic in their ambient surroundings, with the capacity to reduce feeding. I also establish that C. helgolandicus faecal pellets sink slower when contaminated with low density polyethylene (PE), whereas sinking rates increase when contaminated with high density polyethylene terephthalate (PET), highlighting potential impacts to marine nutrient flux. In Chapter 3, I develop a method utilising the differential density of sediment and plastic to isolate and recover microplastics from sediments; I apply this method in Chapter 4, and latterly discuss harmonisation of microplastic estimates between studies and its use across the wider international field (Chapter 5). In Chapter 4, I employ a multi-faceted study to explore the role that benthic fauna play in the uptake of microplastics by the seabed. My environmental data demonstrate that microplastics are being permanently buried in coastal sediments, and that this process is ubiquitous across sampled sites and seasons. I further identify that benthic faunal functional groups that move sediment vertically (“conveyors”) and randomly (“biodiffusers”) influence sediment plastic loading differently, affecting ultimate burial and deep sediment loading. Furthermore, experimental data indicate that a key benthic species, the brittlestar Amphiura filiformis, buries nylon fibres along its burrow structure and that burial activity deep in the burrow is impaired when plastic is consumed. Collectively, my research contributes to our understanding of the mechanisms governing microplastic transport through the water column and into the sediment matrix, highlights risks posed to marine fauna and ecosystems and provides evidence that coastal sediments are final sinks for microplastics.

Microplastics are a diverse array of contaminants comprising a suite of sizes, shapes, and polymer types. Here I present a body of work investigating the distribution and movement of microplastics through the marine ecosystems via transportation and transformation pathways. First, I look at litter items of beaches of the Cornish coast, demonstrating that 41% of litter was plastics fragments unattributable to source and that this litter was continually re-stocked such that it was always present despite cleaning efforts. Then I took to the seas to conduct sea surface trawls in the North East Atlantic to investigate the floating proportion of marine plastic debris. Microplastics were found in every sample, yet were highly variable in concentration over geographic space ranging from 0.038 to 0.45 particles m-3. Counter to the prevailing trends, plastic fragments (84 μm - 21.8 mm) were the dominant shape (63%), with fewer fibres present. The likelihood of encounter and therefore risk of plastic to plankton was calculated and it was found that for every 1 plastic particle, there were between 500 and 1000 plankton, suggesting very low risk of biological uptake for this region. Plastics are not just found on the sea surface and are increasingly found in benthic sediments and biota. I tested whether marine snows would act as a transport mechanism of plastics from the surface to the seafloor. I demonstrate that under experimental conditions a range of plastic particle sizes, shapes, and polymer types, all readily incorporated into marine snows. This incorporation into marine snows both overcame the buoyancy of floating particles but also increased the sinking rate of dense particles. Buoyant polyethylene went from floating as a free particle to sinking at 818 m day-1.

Over the last sixty years, the development of synthetic and durable materials, namely plastic, coupled with a growing human population, has resulted in a rapid increase in the levels of anthropogenic debris in rivers, along coastlines and in the wider marine ecosystem. Currently, an estimated 4.8 to 12.7 million tons of plastic enter the oceans every year but this is expected to increase to between 9.6 and 25.4 million tons by 2025. As such, it is one of the most widely recognised pollution issues facing the planet due to its wide-ranging ecological and socio-economic implications. The main aims of this thesis were to i) examine citizen-science beach clean data to better understand the composition of anthropogenic litter deposited on British beaches by determining the most common items, materials, sources and pathways, and exploring the data for spatial patterns and temporal trends in litter density; ii) investigate an indirect pathway (trophic transfer) of microplastic (< 5mm in size) ingestion in marine top predators by analysing scat (faeces) from captive grey seals (Halichoerus grypus) and the wild-caught fish they were fed upon; iii) explore the extent to which wild marine mammals ingest microplastics and consider the potential implications by examining the digestive tracts of 50 marine mammals from 10 species that stranded around the British coast; iv) develop a method of investigating dietary exposure of marine mammal top predators to microplastics, by combining scat-based molecular techniques (metabarcoding) with a microplastic isolation method. The research carried out for this thesis reveals that i) plastic is the main constituent of marine litter on British beaches and the majority of traceable items originate from land-based activities, such as public littering. The coasts of the southwest England and south Wales have the highest litter levels and certain items - small plastic fragments, plastic food packaging, wet wipes, polystyrene foam, balloons and large fishing net - are increasing; ii) trophic transfer is an indirect and under-studied, but potentially major, route of microplastic ingestion for marine top predators; iii) microplastics are ubiquitous within the digestive tracts of wild marine mammals stranded around the British coast but the overall low abundance suggests they may be egested; iv) the rate of microplastic ingestion by marine top predators may be related to the type of prey they consume but further work is needed to assess the impacts of this omnipresent pollutant.

Plastic debris is entering into the marine environment at an accelerating rate, now becoming one of the most ubiquitous and long-lasting changes in natural systems. Marine turtles are large marine vertebrates with complex life histories and highly mobile behaviour that may make them particularly vulnerable to its impacts. The main goals of this thesis were to i) evaluate the potential implications of the presence of plastic pollution in the environment to marine turtles by reviewing current literature ii) provide a global summary of the issue of entanglement in this taxon, utilising a global network of experts iii) explore the drivers of key interactions between marine turtles and plastic ingestion and develop novel additions classification methodologies to explore selective ingestion of plastics iv) develop a methodology for investigating and isolating the presence of microplastic ingestion in marine turtle gut content and v) examine plastic pollution on a key habitat for marine turtles e.g. nesting beaches. Major findings of the thesis include i) the issue of entanglement with plastic debris, the majority in ghost fishing gear, is both an under-reported and under-researched threat ii) a clear display of strong diet-related ingestion towards plastic debris that resemble natural food items, utilising a case study of green turtles in Northern Cyprus iii) a method development that allowed the identification and isolation of a suite synthetic particles in gut content residue samples, providing evidence of ingestion of synthetic debris at the microscopic size class iv) a more comprehensive viewpoint on plastic concentrations on nesting beaches, in the form of 3D sampling to investigate subsurface plastic densities, showing microplastics present down to turtle nesting depth of both loggerhead and green turtles in Northern Cyprus. In conclusion, this thesis forms the most detailed and comprehensive investigation to date on the impacts of this pollutant on the taxon of marine turtles; contributing to knowledge into macro and microplastic ingestion, entanglement and key habitats through method development and integration of marine turtle feeding ecology and developmental biology.