State-of-the-art modeling the interaction between microplastic and the low-trophic level marine biota

Starting our work on model-based coupling microplastic (MP) and the low-trophic level marine biota, we present a contemporary overview of existing models, both the Lagrangian and Eulerian ones.

MP biofouling has been modeled by Kooi et al. (2017), who develop a 1D Lagrangian model to describe size- and density specific vertical motion of MP. They have found that

• Denser particles settle sooner than the less dense particles when they have the same size.
• The settling velocity decreases with decreasing particle size.
• MP particle can oscillate due its buoyancy controlled by biofouling. MP density is balanced by the source-and-sink terms in a biofouling equation for algae attached to the MP surface.
• Oscillation periods increase with decreasing particle size.

 

Lobelle et al. (2021) improve the Kooi et al. (2017) model developing a 3D Lagrangian model with horizontal and vertical advection. They consider a global distribution of MP of different size and density and note that the timescale is largely size-dependent as opposed to density dependent.

Finally, Fischer et al. (2022) modify the physics part of the model by Lobelle et al. (2021), focusing on the vertical movement, both vertical advection and vertical turbulence diffusion. Additionally, they add two loss terms in the biofouling equation. They conclude that the vertical movement of particles is mainly affected by wind induced mixing within the upper mixed layer and by biofilm dynamics in the deep ocean.

Biofouling as a possible mechanism of the MP removal from the surface has been incorporated into a 3D Lagrangian model for 6 size classes of MP (Tsiaras et al., 2021). In the water column, a sub-surface maximum in MP abundance is obtained, with increasing contribution of smaller particles in deep layers.

MP has been embedded in a biogeochemical 3D Euler model by Kvale et al. (2020), who consider the processes of MP aggregation in sinking marine snow and faecal pellets in the global ocean. In the subsequent work (Kvale, 2022), a two-way coupling developed in the model allows finding a way of the MP influence on global marine carbon cycling and climate.

Biofouling, the MP transport by marine snow and fecal pellets have been simulated by Berezina et al. (2021), who incorporate MP into a biogeochemical 2D Euler model with translational symmetry. They reveal that the so-called “biological pump” (or vertical transport of MP by marine snow and fecal pellets) can be one of the important drivers controlling the distribution of MP in the water column and bottom sediments in the Oslo Fjord.

A new Lagrangian model that we plan to implement in the future will help to advance our understanding of biota-mediated processes in MP transport and fate in the Mediterranean Sea.