Observation and modelling of the macro-waste in the Mediterranean Sea, from the large Scale to the coastal and local Scales

L'objectif général de cette thèse est d'étudier les mécanismes de transport des déchets marins flottants ainsi queleur accumulation potentielle en Méditerranée, en s'appuyant sur la modélisation numérique de leur dérive ainsique sur des observations in-situ de leur distribution. La dynamique du transport des déchets marins y estnotamment analysée en terme d'échelles caractéristiques du bassin, en partant de la grande échelle jusqu'auxéchelles côtière et littorale.Dans un premier temps, l'examen d'un ensemble d'expériences Lagrangiennes numériques a permis d'identifierdans le bassin des zones probables d'accumulation non permanentes à grande échelle (Mansui et al., 2015a).L'impact à l'échelle côtière d'un courant de bord (Courant Nord) et des forçages atmosphériques sur ladistribution locale et l'échouage des déchets marins flottants a ensuite pu être estimé en utilisant notammentdes données originales recueillies en mer et à terre (Mansui et al., 2015b, en révision).The main objective of this work is to study the floating marine debris transport mechanisms and their potentialaccumulation in the Mediterranean Sea, using numerical modelling and in-situ observations of debris. Thetransport dynamics is analysed in terms of typical scales in the basin, from the large scale to the coastal andlocal scales.First, the analysis of a Lagrangian experience data set show the potential existence of non-permanent and largeretention areas (Mansui et al., 2015a). In a second stage, the coastal impact of a boundary current (the NorthernCurrent) and atmospheric forcings on the local distribution and stranding of floating marine litter has beenestimated (Mansui et al., 2015b, under revision), using an original set of data combining offshore sightings andbeaching surveys

Observation et modélisation des macro-déchets en mer Méditerranée, de la large échelle aux échelles côtière et littorale

The main objective of this work is to study the floating marine debris transport mechanisms and their potentialaccumulation in the Mediterranean Sea, using numerical modelling and in-situ observations of debris. Thetransport dynamics is analysed in terms of typical scales in the basin, from the large scale to the coastal andlocal scales.First, the analysis of a Lagrangian experience data set show the potential existence of non-permanent and largeretention areas (Mansui et al., 2015a). In a second stage, the coastal impact of a boundary current (the NorthernCurrent) and atmospheric forcings on the local distribution and stranding of floating marine litter has beenestimated (Mansui et al., 2015b, under revision), using an original set of data combining offshore sightings andbeaching surveys.L'objectif général de cette thèse est d'étudier les mécanismes de transport des déchets marins flottants ainsi queleur accumulation potentielle en Méditerranée, en s'appuyant sur la modélisation numérique de leur dérive ainsique sur des observations in-situ de leur distribution. La dynamique du transport des déchets marins y estnotamment analysée en terme d'échelles caractéristiques du bassin, en partant de la grande échelle jusqu'auxéchelles côtière et littorale.Dans un premier temps, l'examen d'un ensemble d'expériences Lagrangiennes numériques a permis d'identifierdans le bassin des zones probables d'accumulation non permanentes à grande échelle (Mansui et al., 2015a).L'impact à l'échelle côtière d'un courant de bord (Courant Nord) et des forçages atmosphériques sur ladistribution locale et l'échouage des déchets marins flottants a ensuite pu être estimé en utilisant notammentdes données originales recueillies en mer et à terre (Mansui et al., 2015b, en révision)

Modeling 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

The boundary current role on the transport and stranding of floating marine litter: The French Riviera case

The aim of the present study is to evidence the role of a boundary current and meteorological conditions in the transport and stranding of floating marine debris. The used data are from a beach survey and an inter-annual unique effort of marine debris sightings along the French Riviera in the North-Western Mediterranean region. Offshore data have been collected during oceanic cruises while beach surveys were performed around Antibes city. Debris were found on 97% of the ocean transects, with a large spatial and temporal variability, showing contrasted areas of low (~ 1 item/km2) and of high (> 10 items/km2) debris densities. Results suggest that the debris spatio-temporal distribution is related to the Northern current (NC) dynamics, the regional boundary current, with accumulation patterns in its core and external edge. By playing a role in the alongshore transport, such a boundary current can form a cross-shore transport barrier. Stranding events can then occur after strong on-shore wind bursts modifying the sea surface dynamics and breaking this transport barrier. It is also shown that episodic enhancement of the stranding rate can be explained by combining the NC dynamics with the wind forcing and the rainfall effect via the local river run-off. Conversely, off-shore wind bursts could also free the marine litter from the boundary current and export them towards the open sea

Predicting marine litter accumulation patterns in the Mediterranean basin: Spatio-temporal variability and comparison with empirical data

International audienc

Impacts of Changing Ocean Circulation on the Distribution of Marine Microplastic Litter

Marine plastic pollution is currently a major scientific focus, with attention paid to its distribution and impacts within ecosystems. With recent estimates indicating that the mass of plastic released to the marine environment may reach 250 million metric tons by 2025, the effects of plastic on our oceans are set to increase. Distribution of microplastics, those plastics measuring less than 5 mm, are of increasing concern because they represent an increasing proportion of marine litter and are known to interact with species in a range of marine habitats. The local abundance of microplastic is dependent on a complex interaction between the scale of local plastic sources and prevailing environmental conditions; as a result, microplastic distribution is highly heterogeneous. Circulation models have been used to predict plastic distribution; however, current models do not consider future variation in circulation patterns and weather systems caused by a changing climate. In this study, we discuss the potential impacts of global climate change on the abundance and distribution of marine plastic pollution