28.06.2021: Online Ocean Circulation and Climate Dynamics Colloquium

online 11 am

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Passcode: 414986
Meeting URL: https://geomar-de.zoom.us/j/82311283560?pwd=dS9ZY2lDRjRlaXl3eWQyOHZ1NTNBQT09

Abstract:
The fate of microplastics in the open ocean is controlled by physical and biological processes that interact with these particles. Simulating these interactions requires a coupled physical-biogeochemical framework. Here, we develop a Lagrangian model that includes both biological as well as physical forcing on virtual microplastic particles with radii of 0.01-1 mm. For the physics, we include three-dimensional advection from 1/12° NEMO, mixing parameterisations from computed wind-derived vertical diffusivity, and a background full-depth term from tidal mixing. For the biology, we model the interaction between ambient diatoms from MEDUSA2.0 and microplastic to simulate the attachment and growth of a biofilm that can cause the particles to sink. A key novelty applied here is our assumption that attached algal cells that cause microplastic to sink below the euphotic zone can remain attached to a particle and continue to affect its density until the dead cells have completely dissolved. We explore our open-source model by running simulations for 2 years, releasing 10,000 particles in each of the three different regions in this study. The choice of the regions is based on different physical and biological water 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. By mapping the vertical distribution of particles released from the surface after 2 years, we see that the maximum depth reached across all simulations is 4000 m. The growth of algae in the euphotic zone, and death of algal cells below the euphotic zone result in varying oscillations of microplastics particles, where the larger (0.1-1.0 mm) particles have much shorter average oscillation lengths (5 to 25 days) than the smaller (0.01-0.1 mm) particles (25 to >200 days). When the biological and physical processes that can cause vertical movement of the particles are in equilibrium, the particles obtain neutral buoyancy and can accumulate subsurface. A subsurface maximum concentration for the largest particles we simulate occurs between 100 and 250 m in the North Pacific Subtropical Gyre and at 50 m in the equatorial Pacific.

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