Preprints
https://doi.org/10.5194/bg-2021-236
https://doi.org/10.5194/bg-2021-236

  27 Sep 2021

27 Sep 2021

Review status: this preprint is currently under review for the journal BG.

Modeling submerged biofouled microplastics and their vertical trajectories

Reint Fischer1,, Delphine Lobelle1,, Merel Kooi2, Albert Koelmans2, Victor Onink1,3,4, Charlotte Laufkötter3,4, Linda Amaral-Zettler5, Andrew Yool6, and Erik van Sebille1,7 Reint Fischer et al.
  • 1Institute for Marine and Atmospheric Research, Utrecht University, Utrecht, Netherlands
  • 2Aquatic Ecology and Water Quality Management Group, Department of Environmental Sciences, Wageningen University
  • 3Climate and Environmental Physics, Physics Institute, University of Bern, 3012 Bern, Switzerland
  • 4Oeschger Centre for Climate Change Research, University of Bern, 3012 Bern, Switzerland
  • 5Royal Netherlands Institute for Sea Research, Netherlands
  • 6National Oceanography Centre, Southampton, UK
  • 7Centre for Complex Systems Studies, Utrecht University, Utrecht, Netherlands
  • These authors contributed equally to this work.

Abstract. The fate of (micro)plastic particles in the open ocean is controlled by physical and biological processes. Here, we model the effects of biofouling on the subsurface vertical distribution of spherical, virtual plastic particles with radii of 0.01–1 mm. For the physics, four vertical velocity terms are included: advection, wind-driven mixing, tidally induced mixing, and the sinking velocity of the biofouled particle. For the biology, we simulate the attachment, growth and loss of algae on particles. We track 10,000 particles for one year in three different regions with distinct biological and physical properties: the low productivity region of the North Pacific Subtropical Gyre, the high productivity region of the Equatorial Pacific and the high mixing region of the Southern Ocean. The growth of biofilm mass in the euphotic zone and loss of mass below the euphotic zone result in the oscillatory behaviour of particles, where the larger (0.1–1.0 mm) particles have much shorter average oscillation lengths (< 10 days; 90th percentile) than the smaller (0.01–0.1 mm) particles (up to 130 days; 90th percentile). A subsurface maximum concentration occurs just below the mixed layer depth (around 30 m) in the Equatorial Pacific, which is most pronounced for larger particles (0.1–1.0 mm). This occurs since particles become neutrally buoyant when the processes affecting the settling velocity of the particle and the motion of the ocean are in equilibrium. Seasonal effects in the subtropical gyre result in particles sinking below the mixed layer depth only during spring blooms, but otherwise remaining within the mixed layer. The strong winds and deepest average mixed layer depth in the Southern Ocean (400 m) result in the deepest redistribution of particles (> 5000 m). Our results show that the vertical movement of particles is mainly affected by physical (wind-induced mixing) processes within the mixed layer and biological (biofilm) dynamics below the mixed layer. Furthermore, positively buoyant particles with radii of 0.01–1.0 mm can sink far below the euphotic zone and mixed layer in regions with high near-surface mixing or high biological activity. This work can easily be coupled to other models to simulate open-ocean biofouling dynamics, in order to reach a better understanding of where ocean (micro)plastic ends up.

Reint Fischer et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-236', Anonymous Referee #1, 07 Oct 2021
  • RC2: 'Comment on bg-2021-236', Anonymous Referee #2, 25 Oct 2021
  • RC3: 'Comment on bg-2021-236', Anonymous Referee #3, 31 Oct 2021

Reint Fischer et al.

Model code and software

OceanParcels / biofouling_3dtransport_2 Reint Fischer and Delphine Lobelle https://github.com/OceanParcels/biofouling_3dtransport_2

Reint Fischer et al.

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Short summary
Since current estimates show that only about 1 % of the all plastic that enters the ocean is floating at the surface, we look at subsurface processes that can cause vertical movement of (micro)plastic. We investigate how modeled algal attachment and the ocean’s vertical movement can cause particles to sink and oscillate in the open ocean. Particles can sink to depths of > 5000 m in regions with high wind intensity and mainly remain close to the surface with low winds and biological activity.
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