Speaker: Delphine Lobelle (Univ. Utrecht)
Topic: Modelling the Lagrangian tracking of microplastic subject to biofouling, advection and mixing in 3D space and time

Lagrangian particle-tracking is a technique often used to investigate "things" in the ocean that are hard to observe, like microplastic transport. Our Lagrangian framework is Parcels (oceanparcels.org), which uses a set of customizable Python classes to simulate the "behavior" of active or passive particles. The flexibility of Parcels allows us to use any General Circulation Model as input data for advective fields and other hydrodynamic variables we require for our analysis. We are currently using the GCM output from NEMO-MEDUSA (1/12deg horizontal resolution, 5-day temporal resolution) for both hydrodynamic and biogeochemical components required to simulate microplastic subject to biofouling, advection and mixing. Biofouling is the attachment of algae to the surface of a particle, which is of concern when analyzing ocean microplastic transport since it can increase the density of the particle and therefore cause it to sink. We focus both on further developing both the physical and biological factors from Lobelle et al. 2021, that can influence microplastic biofouling. To simulate the biological dynamics, we use the Kooi et al. 2017 1D biofouling model in combination with the Yool et al. 2013 NEMO-MEDUSA equations to simulate the growth and death of algae on the particles. To simulate the physical dynamics (other than advection and temperature and salinity profiles), we model the unresolved vertical motions due to wind using a K-profile parameterization and due to tides using global tidal diffusivity maps. Initial results show that after 180 days, some of the smallest particles that we simulate (radius = 10 microns) continue to sink, whereas the larger ones (up to 1 mm radius) oscillate in the water column due to the growth and death of algae on the surface. The wind-mixing we include causes all of the smaller particles to start sinking immediately in the three locations we choose: North Pacific subtropical gyre, Equatorial Pacific and Southern Ocean.