  Field studies have shown that plastic fragments make up the majority of
plastic pollution in the oceans in terms of abundance. How quickly
environmental plastics fragment is not well understood, however. Here, we study
this fragmentation process 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 compare the modelled PSDs to available observations, and use the
results 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.
