The global modeled sinking characteristics of biofouled microplastic

, Lobelle Delphine, Kooi Merel, Koelmans Albert A., Van Sebille Erik.

Microplastic debris ending up at the sea surface has become a ubiquitous issue in the modern day and the characterisation of the sinking of floating microplastic on a global scale still remains largely unknown. Here, we map the modeled effects of biofouling (algal growth on the substrate) on the removal of microplastic from the sea surface to deeper depths. Using NEMO-MEDUSA output to simulate the hydrodynamic, biological and physical properties of the seawater combined with a particle-tracking framework, we estimate the sinking onset timescale of each virtual particle and the time required to reach its first sinking depth (i.e. the depth it reaches when its vertical velocity becomes zero for the first time). Different sizes and types of plastic (with different densities) are simulated, where our results show that sinking characteristics are largely size-dependent, whereas all plastic densities produce a very similar global distribution of the sinking onset time. Generally, oligotrophic regions with low algal concentrations result in particles not sinking within the 90-day simulation time for particles between 10 mm and 10 µm. On a global scale, the smallest particles simulated here (0.1 µm) sink almost immediately and their trajectories produce the longest time to reach their first sinking depth. The impact of advection and seasonality on the sinking characteristics of the virtual microplastic is small.

View online