The Mediterranean is considered a hot-spot for plastic pollution, due to its semi-enclosed nature and heavily populated coastal areas. In the present study, a basin-scale coupled hydrodynamic/particle drift model was used to track the pathways and fate of plastics from major land-based sources (coastal cities and rivers), taking into account of the most important processes (advection, stokes drift, vertical and horizontal mixing, sinking, wind drag, and beaching). A hybrid ensemble Kalman filter algorithm was implemented to correct the near- surface circulation, assimilating satellite data (sea surface height, temperature) in the hydrodynamic model. Different size classes and/or types of both micro- and macroplastics were considered in the model. Biofouling induced sinking was explicitly described, as a possible mechanism of microplastics removal from the surface. A simplified parameterization of size-dependent biofilm growth has been adopted, as a function of bacterial biomass (obtained from a biogeochemical model simulation), being considered a proxy for the biofouling community. The simulated distributions for micro- and macroplastics were validated against available observations, showing reasonable agreement, both in terms of magnitude and horizontal variability. An 8-year simulation was used to identify micro- and macroplastics accumulation patterns in the surface layer, water column, seafloor and beaches. The impact of different processes (vertical mixing, biofouling, and wind/wave drift) was identified through a series of sensitivity experiments. For both micro- and macroplastics, distributions at sea surface were closely related to the adopted sources. The microplastics concentration was drastically reduced away from source areas, due to biofouling induced sinking, with their size distribution dominated by larger (>1 mm) size classes in open sea areas, in agreement with observations. High concentration patches of floating plastics were simulated in convergence areas, characterized by anticyclonic circulation. The distribution of macroplastics on beaches followed the predominant southeastward wind/wave direction. In the water column, a sub-surface maximum in microplastics abundance was simulated, with increasing contribution of smaller particles in deeper layers. Accumulation of microplastics on the seafloor was limited in relatively shallow areas (<500 m), with bottom depth below their relaxation depth due to defouling. The simulated total amount of floating plastics (∼3,760 tonnes) is comparable with estimates from observations.

), suggesting that X k should be treated as a site-specific parameter. Food density (X) was adapted to include not only Phyto-C but also POC in the diet of M. galloprovincialis and only when Phyto-C density was low compared to POC density. Results showed a small contribution of POC during spring in the Maliakos Gulf and almost none at Thermaikos Gulf. The simulated mussel growth showed good agreement with field data. Sensitivity tests on the calibrated parameters (E, R and X k ) were performed to investigate model uncertainty. The standard deviation of simulations with perturbed parameter/initial values remained relatively small and appeared to increase as the modeled mussel grew, in agreement with observations.

Abstract. Microplastics (MPs) are a contaminant of growing concern due to their widespread distribution and interactions with marine species, such as filter feeders. To investigate the MPs accumulation in wild and cultured mussels, a dynamic energy budget (DEB) model was developed and validated with the available field data of Mytilus edulis (M. edulis, wild) from the North Sea and Mytilus galloprovincialis (M. galloprovincialis, cultured) from the northern Ionian Sea. Towards a generic DEB model, the site-specific model parameter, half-saturation coefficient (Xk), was applied as a power function of food density for the cultured mussel, while for the wild mussel it was calibrated to a constant value. The DEB-accumulation model simulated the uptake and excretion rate of MPs, taking into account environmental characteristics (temperature and chlorophyll a). An accumulation of MPs equal to 0.53 particles per individual (fresh tissue mass 1.9 g) and 0.91 particles per individual (fresh tissue mass 3.3 g) was simulated for the wild and cultured mussel after 4 and 1 years respectively, in agreement with the field data. The inverse experiments investigating the depuration time of the wild and cultured mussel in a clean-from-MPs environment showed a 90 % removal of MPs load after 2.5 and 12 d respectively. Furthermore, sensitivity tests on model parameters and forcing functions highlighted that besides MPs concentration, the accumulation is highly dependent on temperature and chlorophyll a of the surrounding environment. For this reason, an empirical equation was found, directly relating the environmental concentration of MPs, with the seawater temperature, chlorophyll a, and the mussel's soft tissue MPs load.