Microplastics transport simulations in the southern North Sea

Marine pollution by microplastics has already become a large problem in marine environments. As in-situ measurements show, the North Sea is already affected. The occurrence and sources of microplastic litter are only estimated. Long term monitoring programmes and surveys often are not feasible, which is why particle transport modelling is a promising and economic way to gather relevant information. This study uses Lagrangian transport simulations in order to determine potential source regions of microplastic particles found in southern North Sea surface waters during cruise He430 in summer 2014. The two-way approach shows particle distributions in a time-reversed and a forward in time setting. Backward in time simulations identified residual currents in the North Sea to be of anticyclonic nature during and about four weeks before microplastics sampling was carried out. Furthermore, estuaries and deltas of rivers were not favoured as potential source regions. Forward in time simulations identified rivers discharging into the southern North Sea as a potential source region for microplastics for coastal regions such as the Wadden Sea, harbours and bays. Sampling stations of cruise He430 were rather uninfluenced by riverine inflow. It is further presumed that the riverine particle distribution in the German Bight is influenced by tidal forces during weak northeasterly winds in summer and by residual currents during strong westerly winds in winter. It is assumed that sampled microplastics potentially origin from the English Channel. Last but not least potential accumulation zones of microplastic particles are supposed to be situated along the mixing zones of freshwater and sea water and in local gyres. The strong variability of the North Sea however makes it hard to determine a general answer to potential source regions.