Modelling size distributions of marine plastics under the influence of continuous cascading fragmentation

Field studies in the global ocean have shown that plastic fragments make up the majority of plastic pollution in terms of abundance. It is not well understood how quickly plastics in the marine environmental fragment, however. Here, we study the fragmentation process in the oceanic environment by considering a model which captures continuous fragmentation of particles over time in a cascading fashion. With this cascading fragmentation model we simulate particle size distributions (PSDs), specifying the abundance or mass of particles for different size classes. The fragmentation model is coupled to an environmental box model, simulating the distributions of plastic particles in the ocean, coastal waters, and on the beach. We demonstrate the capabilities of the model by calibrating it to estimated plastic transport in the Mediterranean Sea, and compare the modelled PSDs to available observations in this region. Results are used to illustrate the effect of size-selective processes such as vertical mixing in the water column and resuspension of particles from the beach into coastal waters. The model quantifies the role of fragmentation on the marine plastic mass budget: while fragmentation is a major source of secondary plastic particles in terms of abundance, it seems to have a minor effect on the total mass of particles larger than 0.1 mm. Future comparison to observed PSD data allow us to understand size-selective plastic transport in the environment, and potentially inform us on plastic longevity.