Investigating the environmental partitioning of microplastics in two contrasting marine ecosystems.

Plastic is a pervasive pollutant of marine ecosystems globally, found throughout the water column, in sediments, and biota. Small plastic particles, or microplastics, are numerous and readily ingested by marine organisms. However, these plastic particles are distributed unevenly throughout marine environments and the physical properties of the particle can influence how they are transported, and ultimately where they are found. In this thesis I review the current literature to explain how plastic particles behave in the marine environment according to their physical attributes, and how this might influence the number and types of plastic to which organisms are exposed. I then explore two cases of plastic partitioning across compartments of an Arctic fjord (Kongsfjorden, Svalbard), and in rocky shore habitats of Devon and Cornwall, UK via an extensive field sampling campaign. Using a boat based sampling programme, seawater microplastic contamination for two different water bodies, local Arctic and Atlantic, within an Arctic fjord was assessed via sampling at two different depths of the water column. Salinity-temperature-depth (CTD) profiles were acquired, and microplastic particles collected from sea surface and 160 m depth at three different locations in Kongsfjorden, Svalbard, using surface plankton net trawls and niskin bottles. The isolated microplastic particles were counted and analysed by FTIR spectroscopy. The parameters defining Atlantic water were not detected, however the mean microplastic concentration in deep waters (2.9 (± 1.7) x 104 particles m-3) was significantly greater than surface waters (112 ± 53 particles m-3). The most common polymers identified were polyester (18%), ethylene-propylene copolymer (11.8 %), and polyacrylic acid and polyethylene (10 % each). Particles at the surface were significantly larger than particles at 160 m, fragments were on average 5430 µm larger, and fibres 850 µm longer. Significantly greater proportions of white fragments and blue fibres were found at the surface compared to 160 m, and black and blue fragments at 160 m compared to surface water. The environmental partitioning of macro-, meso- and microplastics across surface sediment, seawater, and mussels Mytilus edulis were then analysed from 9 intertidal locations in the South West of England. Micro- and mesoplastic-like particles were found in 88.5% of the 269 mussels sampled, ranging from 1.43 to 7.64 items per mussel. Of these plastic particles, 70.9% were identified as semi-synthetic (mainly modified-cellulose). Mussel microplastic abundance, but not polymer type, was correlated with that of their surrounding sediment, but not with sea-surface microplastic concentration or mussel size. Significant differences were present in the relative abundance of polymers and particle sizes between seawater, sediment, and mussels, with mussels containing a greater abundance of cellulose fibre but less polyvinyl polymer. The particle characteristics of mussel microplastic contamination are not directly proportional to that of the microplastics in their surrounding environment. The data from these two contrasting ecosystems both add to the growing evidence that microplastics are not just a sea surface problem and partition across marine ecosystems with particle characteristics such as polymer type (density), shape, and size all likely playing a role. Although uptake of particles by organisms may be subject to processes of selection, ultimately, the distribution of plastic particles governs the particles to which organisms are exposed and might ingest. Therefore, understanding particle characteristics and dynamics will play a role in determining the biological consequences of microplastic pollution in marine biota.