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.

Modeling larval dispersal for the gilthead seabream in the northwestern Mediterranean Sea.

To investigate dispersal and connectivity between spawning and lagoon nursery habitats of the gilthead seabream, Sparus aurata, in the Gulf of Lions (northwestern Mediterranean Sea), we modeled the potential transport of the species' larvae between its supposed main spawning site in the region (the Planier Island) and two of its main local nursery areas (the coastal lagoons of Thau and Salses-Leucate). Passive larval drift simulations using a dispersal biophysical model showed a large variability in the possible trajectories from spawning to nursery areas and in the predicted ages for larvae arrival on the two nursery sites. The most common ages at arrival obtained in the simulations (20-60 days) are broadly consistent with previous modeling studies but contrast with the actual ages of the S. aurata post-larvae collected in 2016 and 2017 at time of the lagoon entrances (60-90 days, from otolith readings). The period between 25 and 70 days being critical for gilthead seabream larvae to acquire sufficient swimming, osmoregulatory, and olfactory abilities to enter coastal lagoons, we argue that ontogenic development plays a crucial role in the transport and local retention of S. aurata larvae in the studied region, explaining the discrepancy between simulation results and observed data.