Inferring microplastics origins in the Mediterranean Sea by coupling modelling and in-situ measurements

This work focuses on the dynamics and retention of microplastics (MP) in the Mediterranean. MP manta-net trawls were performed in autumn 2019 north of the Balearic Islands and along the Balearic Front (BF). Lagrangian modelling was used to find the MP collected origin during the campaign. These combined results show that North of Mallorca is a temporary retention zone of 3 months variability, with MP origin being the Northern Current (NC) and the Gulf of Lion (GOL). Retention processes were less clear along the BF, due to frontal dynamics together with the strong northerly winds. However, it appears that the origin can differ between the North (i.e. the large North-Westerly basin, including the GOL and the NC path) and the South (short distances around the zone) of this front. In both areas, the wind and the current variability are strongly conditioning the existence and position of the MP concentration zones.

3D hotspots of marine litter in the Mediterranean: A modeling study

The 3D dispersion of marine litter (ML) over the Mediterranean basin has been simulated using the velocity fields from a high resolution circulation model as base to run a 3D Lagrangian model. Three simulations have been performed to mimic the evolution of ML with density lower, similar, or higher than seawater. In all cases a realistic distribution of ML sources was used. Our results show that the accumulation/dispersion areas of the floating and buoyancy neutral particles are practically the same, although the latter are distributed in the water column, 80% of them found in the photic layer (average depth of 35m). Regarding to the densest particles, they rapidly sink and reach the seafloor close to their source. The regions of higher temporal variability mostly coincide with the ML accumulation regions. Weak seasonal variability occurs at a sub-basin scale as a result of the particles redistribution induced by the seasonal variability of the current field.

Deep winter convection and phytoplankton dynamics in the NW Mediterranean Sea under present climate and future (horizon 2030) scenarios

Deep water convection (DC) in winter is one of the major processes driving open-ocean primary productivity in the Northwestern Mediterranean Sea. DC is highly variable in time, depending on the specific conditions (stratification, circulation and ocean-atmosphere interactions) of each specific winter. This variability also drives the interannual oscillations of open-ocean primary productivity in this important region for many commercially-important fish species. We use a coupled model system to 1) understand to what extent DC impacts phytoplankton seasonality in the present-day and 2) to explore potential changes in future scenarios (~2030). Our model represents quite accurately the present-day characteristics of DC and its importance for open-ocean phytoplankton blooms. However, for the future scenarios the importance of deep nutrients in fertilizing the euphotic layer of the NW Mediterranean decreases. The model simulates changes in surface density and on the levels of kinetic energy that make mesoscale activity associated with horizontal currents to become a more important fertilization mechanism, inducing subsequently phenological changes in seasonal plankton cycles. Because of our focus on the open-sea, an exact quantification of the impact of those changes on the overall biological production of the NW Mediterranean cannot be made at the moment.

Modelling the transport and accumulation of floating marine debris in the Mediterranean basin

In the era of plastic and global environmental issues, when large garbage patches have been observed in the main oceanic basins, this work is the first attempt to explore the possibility that similar permanent accumulation structures may exist in the Mediterranean Sea. The questions addressed in this work are: can the general circulation, with its sub-basins scale gyres and mesoscale instabilities, foster the concentration of floating items in some regions? Where are the more likely coastal zones impacted from open ocean sources? Multi-annual simulations of advected surface passive debris depict the Tyrrhenian Sea, the north-western Mediterranean sub-basin and the Gulf of Sirte as possible retention areas. The western Mediterranean coasts present very low coastal impact, while the coastal strip from Tunisia to Syria appears as the favourite destination. No permanent structure able to retain floating items in the long-term were found, as the basin circulation variability brings sufficient anomalies.