Comparing models and observations of the surface accumulation zone of floating plastic in the North Atlantic subtropical gyre

Plastics have been invented as a durable, reusable alternative to other organic materials. Their mass production started only in the 20th century, but plastic pollution is already now one of humankind’s most enduring legacies, penetrating virtually every ecosystem on this planet. Marine plastics are especially persistent and hard to clean up, as they often sink down to the seafloor or fragment into smaller pieces that are called microplastics if they are smaller than 5 mm in size, accounting for over 90% of all floating plastic particles. In order to protect the marine environment, it is therefore essential to understand the sources, transport and sinks of floating debris. Only this way, a comprehensive evaluation of the risk caused by plastic pollution to the marine ecosystem can be achieved. To do so, observations of plastic samples are interpolated with predictions from ocean general circulation models to create maps of the regions that are most vulnerable to plastic pollution, for instance the surface accumulation zones in the center of the subtropical gyres. This thesis focusses on the surface accumulation zone of microplastics in the North Atlantic subtropical gyre, where previous modelling efforts have failed to agree with the observed accu- mulation pattern derived from microplastic samples. With the Lagrangian particle tracking tool Parcels, virtual particle distributions are generated in the North Atlantic, based on the hydrody- namic fields from four different ocean general circulation models: GlobCurrent, NEMO, SMOC (a CMEMS product) and HYCOM. The outcomes of these models are compared against each other, predicting the maximum of the surface accumulation in the North Atlantic subtropical gyre to lie between 25 and 55àW. The models’ prediction powers are tested against the most complete data set of microplastic samples in the North Atlantic yet, building up on the data set used in the global inventory of small floating plastic debris aggregated by van Sebille et al. (2015). Still, observations in the eastern part of the North Atlantic remain too sparse to show the true zonal extent of the surface accumulation zone. A linear regression between the observed concentrations and the kernel density estimate of the virtual particle distributions generated by a model measures the correlation between a model and the observations. Generally, the most realistic release scenario of virtual particles in the Lagrangian simulations, which includes a repeated coastal release and a sinking timescale, results in a surface accumulation zone that matches best to the observations in the North Atlantic subtropical gyre. Because observations are still lacking around the predicted maxima of the surface accumulation zone, it is too early to claim that one model corresponds best to the observations.