The movement of plastics through marine ecosystems and the influences on bioavailability and uptake into marine biota

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