The vertical distribution of buoyant plastics at sea: an observational study in the North Atlantic Gyre
- 1School of Civil, Environmental and Mining Engineering and UWA Oceans Institute, University of Western Australia, Perth, Australia
- 2The Ocean Cleanup Foundation, Delft, the Netherlands
- 3Roger Williams University, Bristol, USA
- 4Pangaea Exploration, Miami, USA
- 5Instituto de Oceanografia, Universidade Federal do Rio Grande, Rio Grande, Brazil
- 6Centre for Microscopy, Characterisation and Analysis, University of Western Australia, Perth, Australia
- *These authors contributed equally to this work.
Abstract. Millimetre-sized plastics are numerically abundant and widespread across the world's ocean surface. These buoyant macroscopic particles can be mixed within the upper water column by turbulent transport. Models indicate that the largest decrease in their concentration occurs within the first few metres of water, where in situ observations are very scarce. In order to investigate the depth profile and physical properties of buoyant plastic debris, we used a new type of multi-level trawl at 12 sites within the North Atlantic subtropical gyre to sample from the air–seawater interface to a depth of 5 m, at 0.5 m intervals. Our results show that plastic concentrations drop exponentially with water depth, and decay rates decrease with increasing Beaufort number. Furthermore, smaller pieces presented lower rise velocities and were more susceptible to vertical transport. This resulted in higher depth decays of plastic mass concentration (milligrams m−3) than numerical concentration (pieces m−3). Further multi-level sampling of plastics will improve our ability to predict at-sea plastic load, size distribution, drifting pattern, and impact on marine species and habitats.