Buoyant plastics at sea: concentrations and impacts

Millimetre-sized plastics are a predominant type of marine debris floating at sea. These small macroscopic particles are numerically abundant in some marine environments, but little is known about their spatial distribution and environmental impacts. The goals of this thesis were to investigate how buoyant plastics are distributed in sea surface waters (both horizontally and vertically), and characterise organisms and textures on the surface of millimetre-sized marine plastics. This work is the first to (1) quantify plastic contamination levels in Australian waters, (2) characterize the biodiversity of organisms living on millimetre-sized plastics from waters around Australia, and (3) obtain highresolution depth profiles (0 – 5 m) of plastic pollution in an oceanic accumulation zone. I collected 839 pieces of plastic in 171 surface net tows from surface waters around Australia, and 12,751 pieces of plastic in 12 multi-level tows from an oceanic accumulation zone in the North Atlantic. Plastics were mostly fragments resulting from the breakdown of larger objects (e.g. packaging and fishing gear) made of polyethylene and polypropylene polymers. Contamination levels in waters around Australia were similar to those in other marine regions (e.g. Caribbean Sea and Gulf of Maine), but considerably lower than those found in plastic pollution hotspots within subtropical gyres and Mediterranean Sea. There was a wide range of microbes and a few invertebrates on the surface of floating plastics from Australia-wide sample collections. Diatoms were particularly diverse, represented by 14 genera, 11 of which are new records of ‘epiplastic’ organisms. Plastic pollution levels in the North Atlantic accumulation zone decreased exponentially with water depth, with decay rates decreasing as wind strength increased. Plastic mass per cubic metre of water decreased more rapidly with depth than the number of plastic pieces per cubic metre, as the smaller plastic pieces were associated with lower rising velocities and were more susceptible to vertical mixing. This thesis contributed towards the global efforts of quantifying plastic contamination levels and impacts in surface waters. It highlights the widespread distribution of anthropogenic polymers, which has created a new pelagic habitat for microorganisms and invertebrates. Plastic inhabitants seem to be invading non-native marine regions by plastic transport, and playing an important role on ocean plastic degradation.