Influence of tidal currents on transport and accumulation of floating microplastics in the ocean

Floating plastic debris is becoming an increasing source of pollution of the world's oceans that can seriously harm marine life. To understand the severity of the problem and find suitable solutions, knowledge of the sources, pathways and fate of plastic in oceans is required. However, observational data about floating plastic in marine environments are hard to obtain, especially for microplastics (smaller than 5 mm in size). Instead, numerical simulations using models of ocean currents are a key tool to gain insight into the transport and distribution of microplastics in oceans. Most models used for such simulations do not account for flow caused by global tides. In this project, we investigate the influence of tidal currents on the transport and accumulation of floating microplastics. We do this by numerically simulating the advection of particles released all over the world's oceans for 13 years. We use data from the GlobCurrent reanalysis project to model geostrophic and Ekman currents, and from the FES global tidal model to model currents caused by the four main tidal constituents (M2, S2, K1 and O1). We analyse the differences between the outcomes of runs with and without these tidal currents included. In each of the runs, we see that microplastic accumulates in so-called garbage patches in the subtropical gyres, which is in agreement with observations. The formation of these garbage patches remains unaffected by the tidal currents. However, there are a number of coastal areas where, on different timescales, clear differences can be seen in particle density, distance travelled by particles, and the separation between particles released at the same location between the runs with and without tides. We show that the differences observed in these regions are caused by the presence of tidal currents. These results suggest that tidal currents have little impact on the transport and accumulation of floating microplastic on a global scale, but might be relevant to include in simulations and research aimed at the behaviour of microplastics in coastal environments.