The effect of particle properties on the depth profile of buoyant plastics in the ocean

Abstract

Most studies on buoyant microplastics in the marine environment rely on sea surface sampling. Consequently, microplastic amounts can be underestimated, as turbulence leads to vertical mixing. Models that correct for vertical mixing are based on limited data. In this study we report measurements of the depth profile of buoyant microplastics in the North Atlantic subtropical gyre, from 0 to 5 m depth. Microplastics were separated into size classes (0.5–1.5 and 1.5–5.0 mm) and types (‘fragments’ and ‘lines’), and associated with a sea state. Microplastic concentrations decreased exponentially with depth, with both sea state and particle properties affecting the steepness of the decrease. Concentrations approached zero within 5 m depth, indicating that most buoyant microplastics are present on or near the surface. Plastic rise velocities were also measured, and were found to differ significantly for different sizes and shapes. Our results suggest that (1) surface samplers such as manta trawls underestimate total buoyant microplastic amounts by a factor of 1.04–30.0 and (2) estimations of depth-integrated buoyant plastic concentrations should be done across different particle sizes and types. Our findings can assist with improving buoyant ocean plastic vertical mixing models, mass balance exercises, impact assessments and mitigation strategies.

Role of Indian Ocean Dynamics on Accumulation of Buoyant Debris

“…It is unclear how plastics relate to the drifters. Depending on their rising velocity, and wind and wave conditions (Brunner et al, ; Kukulka & Brunner, ; Kukulka et al, ), plastics drift both on the sea surface and submerged in the upper ocean (Kooi et al, ; Reisser et al, ). As a result, both types of drifters are possible valid proxies.…”

“…We also need to relate material properties of plastics to usuable transport properties, such as windage coefficient or rising velocity. Several studies have reported a correlation between the size of plastics and their rising velocity (Kooi et al, 2016;Lebreton et al, 2018;Reisser et al, 2015). Chubarenko et al (2016) determined the windage coefficient of plastics based on size and density, and Lebreton et al (2018) derived a windage coefficient for plastics in the North Pacific garbage patch by using it as a calibration parameter for their numerical model.…”

Using Numerical Model Simulations to Improve the Understanding of Micro-plastic Distribution and Pathways in the Marine Environment

“…Infrequent surveys of the standing stock of litter on beaches provide crude estimates of debris types and abundance, but are biased by differential removal of litter items by beachcombing, clean-ups and beach dynamics. However, there is increased sampling of and analyses of micro-plastics on the ocean's surface (Reisser et al, 2013;Cózar et al, 2014;Eriksen et al, 2014;Isobe et al, 2015;Ryan, 2015;van Sebille et al, 2015) with fewer studies reporting on sub-surface micro-plastics (but see Reisser et al, 2015;Kooi et al, 2016).…”

Plastic Debris Occurrence, Convergence Areas and Fin Whales Feeding Ground in the Mediterranean Marine Protected Area Pelagos Sanctuary: A Modeling Approach

“…All data were corrected according to weather and sea conditions considering the possible "wind stress" effect, considering the wind speed (m/s) and wave height (m) applying the correction factors adopted by Kooi et al (2016) and Kukulka et al (2012).…”