Numerical analysis of transport and accumulation of floating marine litter in the Black Sea.

Floating marine litter (FML) is a global problem because of the risk it poses to marine life and human health. In a semi-enclosed basin such as the Black Sea, the slow replenishment of water and the strong input from European rivers potentially favoured the increasing accumulation of FML. In this sense, it is absolutely necessary to generate strategies in the Black Sea to mitigate the impacts on the marine ecosystem and human populations. This is one focus of the DOORS European Union Project (Developing Optimal and Open Research Support for the Black Sea) within which this work is framed.

In recent years, scientific studies on marine litter in the Black Sea have increased at regional and coastal scales. Such works include counting, analysis of distribution, estimation of riverine input and the use of numerical models to identify circulation and accumulation patterns (Bouzaiene et al., 2021; González-Fernández et al., 2022). Using Lagrangian models has opened the door to the discussion of how such models should be configured and the importance of whether to include phenomena such as stokes drift. In addition, some areas have been suggested as high accumulation areas but these results diverge between authors and available data. 

LOCATE is a tool built with the Lagrangian solver OceanParcels and developed for the prediction of areas of high FML accumulation, which has been adapted and validated for the Black Sea. The experiments were performed using surface current velocity and Stokes drift data taken from the Copernicus Marine Service with items of FML represented by Lagrangian particles in the model. Two simulations were run with a homogeneous particle release over the whole basin, every month during one year. The first one with only the surface currents and the second one adding Stokes drift, in order to evaluate the contribution of including the Stokes drift taken from the wave data. A third simulation was carried out with both drivers and releasing particles daily during one year according to the estimated amount of waste transported at the mouths of the nine main contributing rivers, to identify the trends of particle movement from these discharge points.

The results indicate the south-western area as an area of high coastal accumulation in all three simulated cases. The mainly cyclonic circulation, the large input of FML from the Danube River and other northern rivers including a relevant fraction of the outflow from the Kerch Strait probably explained this. In addition, the percentage of particles beached on shore and the residence time in offshore waters were strongly influenced by including Stokes drift, moving from a percentage of 45.5% to 75.5% and from an average residence time of 99 to 63 days. These values are in agreement with recent literature supporting an overestimation of residence times by omitting Stokes drift. Finally, this is only the beginning of a forecasting tool for FML in the Black Sea that is expected to be further improved by using coupled hydrodynamic models, extending the resolution with nested areas and incorporating higher accuracy in coastal processes including beaching.