Plastics, including those of buoyant polymers are increasingly found in high concentrations in benthic sediments where they have the potential to be ingested and interact with a diverse range of benthic marine species. This thesis combines field work and laboratory mesocosm experiments to look at the presence and characteristics of synthetic particles found in situ within estuarine benthic habitats, and then investigates experimentally how interactions with benthic species might influence the fragmentation and movement of plastics within a test benthic ecosystem.

Firstly, a field study was undertaken to determine the abundance and particle characteristics of synthetic particles in benthic sediment and within natural populations of the benthic-dwelling polychaete, Hediste diversicolor. Sediment (10 samples per site) and worm (30 individuals per site) were collected from three locations of differing anthropogenic influence across South Devon, UK. Samples were analysed for synthetic particle content using density separation for sediments or tissue digestion, followed by particle identification using Fourier-transform infrared spectroscopy (FTIR). All sediment samples analysed contained synthetic (i.e. plastic and/or modified cellulose) particles, but with significantly higher abundance in sediment from the Plym estuary (mean 116 synthetic particles kg-1 +/- 18.09 SE) compared to Kingsbridge (mean 62 synthetic particles kg-1 +/- 13.55 SE; P < 0.001). of the H. diversicolor sampled, 48% of the worms contained synthetic particles with a mean of 0.73 synthetic particles per worm +/- 0.15 SE and no difference in particles per worm by site (P = 0.30). The dominant polymer type found within the sediment was polypropylene, accounting for 30 % of the total synthetic particles across the three sites. Semi-synthetic cellulose fibres and polystyrene particles were the most prevalent particle types found within H. diversicolor.

The potential role of benthic invertebrates in the fragmentation and movement of plastic litter within a test benthic ecosystem was investigated via a mesocosm study. Biofouled polyethylene (PE) crates (half a crate per tank) were used as the test macroplastic. Mesocosms comprising three benthic species with different functional roles; the purple sea urchin Paracentrotus lividus, the blue mussel Mytilus edulis and the sediment-dwelling polychaete, Alitta virens were maintained for 13 days under four treatment scenarios (3 tanks per treatment); 1) a no plastic control comprising all species and macroalgae but no added crate, 2) plastic crate, all organisms but macroalgae absent, 3) plastic crate, all organisms and macroalgae present and 4) plastic crate, urchins and ragworms (no mussels) with macroalgae present. In every tank containing a plastic crate and urchins, small plastic fragments were recovered from the water and from the sediment. PE fragments were present within 100 % of urchins (6.3 +/- 1.6 (SE) particles per individual), 62 % of mussels (3.5 +/- 0.6 (SE) particles per individual) and 65 % of ragworms (2.2 +/- 0.5 (SE) particles per individual), confirming urchins are effective at generating plastic fragments (size range 10.2 μm to 5816.6 μm) that are subsequently bioavailable for uptake by benthic-dwelling organisms. In this experimental system, a combination of urchin food availability and mussel presence acted to increase the bioavailability and uptake of plastic fragments into a sediment-dwelling polychaete with a ~ four-fold increase in PE fragments found within ragworms when both macroalgae and mussels were present (3.6 +/- 0.5 (SE) particles per individual) compared to mussel absence (0.9 +/- 0.2 (SE) particles per individual), and a ~ 1.6-fold increase compared to macroalgae absence (2.2 +/- 0.7 (SE) particles per individual).

Overall, this work advances the understanding of how organisms alter the distribution, accumulation and fate of microplastic in the benthic ecosystem while also highlighting the prevalence of synthetic particle contamination in benthic sediments.