<p>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.</p> <p>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. </p> <p>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.</p> <p>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.</p>

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<p>Transport mechanisms of floating marine litter (FML) in coastal zones are poorly understood. Tracking FML dispersion with numerical models is complex due to the geometry, hydrodynamic processes and the influence of coastal processes, the latter being especially challenging to incorporate. Within the TRACE (Tools for a better management in coastal environments to accelerate tRAansition to Circular plastic Economy) project, however, the LOCATE tool was developed to simulate the motion and accumulation of plastic particles in coastal areas, using nested grids of varying spatial scales and resolutions (2.5 km, 350 m and 70 m) to account for coastal processes. LOCATE couples Eulerian hydrodynamic data with a Lagrangian particle solver, thus requiring configuration and optimization. Regional Eulerian hydrodynamic data are obtained from Copernicus Marine Environment Monitoring Service (CMEMS) products whereas coastal hydrodynamic simulations use the open-source Coupled Regional Modeling System (COAWST) system. The Lagrangian solver uses the open-source OceanParcels (Probably a Really Efficient Lagrangian Simulator) model (van Sebille, et al., 2020). As proof of concept, the model was applied to the Barcelona coastline where breakwaters can behave as marine litter traps and concentrations are comparable to some other heavily polluted areas such as the Atlantic and Pacific gyres (Sánchez-Vidal, et al, 2021).</p> <p>Observational data from 2017 from the Llobregat and Besòs rivers, two known sources of FML around Barcelona, were used to run simulations to determine how LOCATE can predict litter accumulation zones when compared to beach cleanup data. Both rivers are the major rivers around the Barcelona coastline and have been hypothesised to be an important source of plastic in the region. A beaching module that detects, quantifies, stores location and time, and removes particles that have crossed the land-water boundary was developed that uses a vector-resolution coastline, and as such is independent from the limiting spatial resolution of the nested grids at coastal scales. The coastline was divided into 16 zones, as well as five water boundary zones on the perimeter of the study area (40.88°N to 41.81°N, 1.38°E to 3°E). </p> <p>Simulations of particles released at hourly intervals were carried out between February and October 2017, with 552,400 released in total. Results show overall beaching rates of 91.5%, higher than other larger scale studies. The most impacted areas were around the release sites and adjacent beaches to the south, showing a NE to SW transport; the Llobregat river mouth being the most affected with over 200 particles km<sup>-1 </sup>day<sup>-1</sup>. A non-dimensional beaching bias index was used to identify areas that have a greater partiality to receive particles from a release point, such as areas south of the Llobregat river. Residence time at sea was highly variable with very short times recorded for the Besòs river mouth and adjacent areas (< 1 hour). Preliminary results will be presented to determine possible factors contributing towards high beaching rates and litter accumulation in zones. The ultimate aim of the present system and model is to demonstrate its exportability and adaptability to other coastal regions.</p>