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If microplastics are found either in their primary or secondary type, what are the sources that leads to their entry into the aquatic environment? Plastics from terrestrial activities consist the majority (80%) of the observed plastics in the marine environment (Andrady, 2011). Various sectors play a role as microplastics sources and lead to their increase in the aquatic systems. Table 2 indicates the sources of plastics as described by GESAMP (2017) as well the type of microplastics it forms. These sources come from four big categories:
– Producers
– Sectoral consumers
– Individual consumers
– Waste management
Among these sources, tourism and human population play an important source of macroplastics.
Almost one quarter of the world population occupy the coastal zones (Small and Nicholls, 2003).
These zones are highly affected by plastics usage and favor plastics production that are likely mismanaged due to the lack of appropriate infrastructure (Figure 4). Cole et al. (2011) indicated that seasonal tourism activities can lead to high amount of wastes in a short period of time. The abundance of these macro wastes on the beaches would rapidly lead to their degradation and are more susceptible into forming secondary microplastics.
Even if most of the sources are considered terrestrial (Jambeck et al., 2015), many marine sources play a role in plastic marine pollution. Fisheries, aquaculture and shipping industries are major sources for marine macroplastics pollution (Andrady, 2011). Fishermen usually abandon or lose fishing nets and lines in the seawater. Aquaculture also generates plastics wastes (Cole et al., 2011) and are locally significant (wastes resulting from shellfish farms for examples). This maritime pollution is highly contributed by maritime traffic: during the 70’s, about 23 thousand tons of packaging were thrown in the sea (Cole et al., 2011). Later that decade, in 1978, international convention (MARPOL) has been held in order to limit waste disposal in the sea, by prohibiting the uncontrolled disposal of plastics and other synthetic materials (cords, nets, plastic bags) (Derraik, 2002). The Mediterranean Sea and the Northwestern France are two zones that are highly affected by maritime traffic. Cole et al. (2011) indicated that even though international agreements exist, the maritime traffic is one of the principal sources of marine pollution because of the lack of control, education and reluctances concerning practical modifications, Table 2: Types and sources of plastics and microplastics in the aquatic environment (adapted from GESAMP, 2017).
The knowledge about most of these stated sources above (11 out of 14) is considered low. The route of microplastics entry and their fate are not well-known, for there are several factors affecting their transportation such as wind, sea currents, river systems and runoff.
Some of these sources are also considered as a route for microplastics entry into the aquatic environment such as the role of wastewater treatment plants (Talvitie et al., 2015). In the last couple of years, the number of studies done on wastewater treatment plant role as microplastics entry sources have increased. But a lot of information are still missing concerning the role of WWTP and solid wastes (landfills) in microplastics entry in the aquatic environment.

Wastewater Treatment Plants

Wastewater treatment plants are able to treat both industrial and domestic water before discharging it into the aquatic environment (Zbyszewski et al., 2014). Whether it is a secondary or a tertiary plant, WWTP is not made for eliminating microplastics and particles will be released via the discharging water (Browne et al., 2011; Mathalon and Hill, 2014; Murphy et al., 2016; Roex et al., 2013). Yet, normal treatment steps are capable of removing microplastics. These different processes differ from a WWTP to another. Primary treatment processes can remove 63% to 98% (Sun et al., 2019); and an additional 7 to 20% are captured during secondary treatment (Gies et al., 2018; Ziajahromi et al., 2017). This retention during early stages suggest that MPs are removed during grit and grease screening and sludge formation (Lares et al., 2018; Leslie et al., 2017; Murphy et al., 2016). Other factors might increase the MPs in WWTP such as the plant’s equipment of advanced treatment technologies (Lares et al., 2018; Magni et al., 2019; Michielssen et al., 2016; Ziajahromi et al., 2017). Among these techniques, membrane bioreactor sludge (MBR) (Lares et al., 2018), dissolved air flotation (Talvitie et al., 2017a) and reverse osmosis and decarbonation (Ziajahromi et al., 2017) showed high microplastics removal efficacy. Other studies showed that advanced technologies do not enhance microplastics retention (Carr et al., 2016; Leslie et al., 2017). This retention percentage generally ranges between 72 and 99.4% (Table 3) (Reviewed by Gatidou et al., 2019; Sun et al., 2019) but remains limited by current detection techniques. It is speculated that during grease and grit removal steps, most of the microplastics are retained (Magni et al., 2019; Murphy et al., 2016). Only half of the published articles analyzed the sludge inside WWTPs and each study analyzed their sludge samples using different protocols. Fragments, fibers and microbeads have been observed during grit and grease removal as well as sludge samples representing various types of polymers. Even though important amounts of microplastics are retained, but the microplastics particles still reach the effluent water. The amount of discharged particles differs from one study to another (Table 3) and several shapes are observed: microbeads, fibers, fragments and films (Blair et al., 2019; Carr et al., 2016; Dris et al., 2015; Mason et al., 2016). Released volumes of treated water vary from one plant to another, and from one day to another. Studies indicated that up to 6.5 x 107 and 5 x 104 MPs/day are released in Scotland (UK) (Murphy et al., 2016) and San Francisco (USA) (Mason et al., 2016), respectively. These differences are influenced by the lack of a homogenous protocol: different sampling and analyses methodologies, diverse mesh sizes and various observation and identifying techniques. Whether using steel buckets, filtration pump, sieves and automatic sampler, or by samples filtration or density separation; this disparity between studies lead to an unreliable data comparison. Also, low identification percentage of polymers cam be observed in WWTP: most of the studies identified polymers using FTIR spectroscopy whereas a couple studies used Raman microscopy. A small percentage of the total observed items were identified and were ranged from 1.3% (Lares et al., 2018) to 8% (Simon et al., 2018) and 16% (Murphy et al., 2016). Thus, there is a need of a standardized sampling and analyses protocol to have a more reliable comparison between studies.
Landfills are sites for a large variety of wastes: municipal, commercial, industrial, agricultural and constructional. Plastics are among these deposited dry wastes. Plastics are durable and, once buried, persist in the environment. Several types of plastics can be observed: plastic bottles, carrier bags, packaging sheets, single films, polyvinyl chloride (PVC) pipes and large plastics. Inappropriate management would lead to major plastics release into the surrounding environment. Whereas in a properly managed landfill, wastes are daily covered with soil or synthetic material and fences, with an appropriate recycling program that would decrease 20 to 40% of the wastes composition (Barnes et al., 2009). From 2006 to 2016, landfilling has decreased by 43% in Europe while recycling increased by 79% (PlasticsEurope, 2018). Landfills are a potential source of microplastics, several factors affecting landfills (high temperature, pH and physical compacting) as well as anaerobic circumstances increase plastics degradation (Mahon et al., 2017; Sundt et al., 2014). These degraded microplastics enter the surrounding environments via air-born pathway (Rillig, 2012) and leachates (He et al., 2019; Praagh et al., 2018; Su et al., 2019). Leachates are “the fluid percolating through the landfills and are generated from liquids present in the waste and from outside water, including rainwater, percolating though the waste” (Jayawardhana et al., 2016). Not only do untreated leachates contain higher microplastics than treated leachates (Praagh et al., 2018), but they are also vectors of heavy metals and organic contaminants (Sui et al., 2017). The role as a microplastics entry source will increase if the landfill is poorly managed or abandoned, or if its leachates are not treated. What about abandoned coastal landfills for example? As Figure 5 shows, abandoned coastal landfills with their waste directly along the coast would increase their role in macro and microplastics entry to the marine environment. With no proper management, wastes (including plastics) are in direct contact with the surrounding seawater and enter the aquatic environment via waves, high tides and winds. If landfills were considered as a primary disposal for wastes, they should be regarded as important sources of microplastics (Su et al., 2019).


Once microplastics find their way in the aquatic environment, they are either observed in the surface water, sediments or biota. Several factors influence their distribution and abundance: items size, shape, density and biofouling (Andrady, 2011). Density plays an important role on the presence and distribution of plastics: low density (<1.02 g/cm3) polymers (PUR, PE and PS) tend to float on the seawater, whereas high density polymers (PVC, PET) tend to sink to the bottom. Yet, this density is influenced by biofouling, as well as by currents and resuspension. In the latter, the biofouling phenomenon affects low density polymers in the surface water: the items are covered by a colony of biofilms that will increase the density and lead to their sinkage to the bottom (Andrady, 2011). Resuspension phenomena have been already observed when high-density polymers were detected on the surface water but not fully understood (Enders et al., 2015; Suaria et al., 2016).

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Coastal marine systems

Scientific research has mainly focused on the abundance of microplastics in the marine environment with oceans considered as the final sink for these pollutants (Horton and Dixon, 2018). Microplastics distribution in marine environment is affected by both temporal and spatial variability due to seasonal patterns in oceanic currents (Cole et al., 2011; Ryan et al., 2009). The majority of studies on microplastics occurrence were reported in the surface and subsurface water of several areas worldwide (reviewed by Akdogan and Guven, 2019). Most of them have taken into consideration the spatio-temporal variation of MPs abundance, including their shape, size, color and polymer type. Microplastics have been reported in the water of Pacific Ocean (Desforges et al., 2014; Díaz-Torres et al., 2017; Lebreton et al., 2018; Rios Mendoza and Jones, 2015), the North Sea (Liebezeit and Dubaish, 2012; Lorenz et al., 2019), the Atlantic ocean (Kanhai et al., 2017; Lusher et al., 2014; Reisser et al., 2015), Bohai and South China Seas (Cai et al., 2018; Zhang et al., 2017), Artic polar waters (Lusher et al., 2015), and the Mediterranean Sea (Baini et al., 2018; Cózar et al., 2015; Gündoğdu and Çevik, 2017; Lefebvre et al., 2019) (see Annex 1). Microplastics in coastal waters are affected by strong hydrodynamic factors (tides, wind, waves and salinity gradients), and undergo beaching, drifting and settling, with only a small faction being carried into the open ocean (Zhang, 2017). Various sampling methods were used along with several mesh sizes (Table 4). Different analyses protocols were used in various studies: for clean water samples, no digestion method was used and they were directly filtered. If this was not the case, organic material destruction was used (enzymatic or mechanical) and samples underwent density separation (various solution were used between studies) (Reviewed by Stock et al. (2019)). Recovered items in the coastal water samples, were made of all kind of shapes: fibers, fragments, films and microbeads. The dominant shape varied from one study to another and from one region to another (Lusher et al., 2015, 2014; Pedrotti et al., 2016; Suaria et al., 2016): Fibers were abundant in the Western Mediterranean surface water (Lefebvre et al., 2019), whereas fragments were dominant in the Eastern Mediterranean (Gündoğdu, 2017; van der Hal et al., 2017). The dominant polymer type differed with the dominant shape: PE, PP and PS were dominant in the areas with high abundance of fragments (Pedrotti et al., 2016; Zhang et al., 2017); Polyester (PET) and PA were highly dominant in samples with important fibers occurrence (Lefebvre et al., 2019; Lusher et al., 2015).
Microplastics in the surface water can be either washed up on the coast or sunk at the bottom. Deep sea sediments have been recognized to potentially accumulate microplastics and are considered as major sink of MPs (Martellini et al., 2018). Microplastics on the shore are affected by important anthropogenic pressure (tourism) whereas the offshore sediments are influenced by environmental conditions such as tides, wind speed and direction, resuspension, current and biofouling (Wang et al., 2018; Zhang, 2017). The abundance of microplastics in sediments vary worldwide. There is no standard sampling and extraction protocol: some digested their samples before using a density separation method, whereas others skipped digestion and went directly to density separation, and some even visualized their samples without treatment. Even different solutions were used as a density separation medium with sodium chloride (NaCl) the most used solution (see Figure 28). Microplastics observed after extraction from sediments are expressed in different units (items/kg; items/m2) complicating the comparison between various studies. Extreme high values were observed in sediments of Venice Lagoon reaching 2175 MPs/Kg d.w (Vianello et al., 2013) and in the Xiangshan Bay (China) 1739 ± 2153 items/Kg (Chen et al., 2018). Whereas other regions had lower number: 67 ± 76 items/Kg d.w in the French Atlantic Ocean (Phuong et al., 2018a); 85 ± 141 items/kg d.w in the North of Crete (Piperagkas et al., 2019), 45.9 ± 23.9 items/Kg in the Spanish Mediterranean coast (Filgueiras et al., 2019) and 60– 240 items/Kg in the Yellow Sea and East China Sea (Zhang et al., 2019). All kinds of shapes can be observed: fragments, films, fibers and pellets with abundant shapes and polymers different between studies (Reviewed by Yao et al., 2019). Several studies indicated the high significance of pellets in the sediment samples (Fanini and Bozzeda, 2018; Karkanorachaki et al., 2018; Turner and Holmes, 2011; Turra et al., 2014). A wide range of polymers were detected in sediments: PE, PP, PS, PES, PA, PVC and rayon, and their distribution varied with different studies.


Due to the size of the MP items and characteristics, they can be mistaken as food for various animals and may, therefore, be ingested and integrated within the trophic chain. Once microplastics have reached the different environmental aquatic environment (water and sediments), they are more or less available to the aquatic biota depending on the items shape and size (Cole et al., 2011; Thompson et al., 2009). The ingestion of microplastics has been observed in invertebrates, mollusks, fish, seabirds and big predatory mammals. The MPs items characteristics lead to their uptake by aquatic species. Evidence of microplastics ingestion has been observed in several marine species: crustaceans (Desforges et al., 2014), pelagic fish (Lefebvre et al., 2019), demersal fish (Neves et al., 2015), sea turtles (Duncan et al., 2018; Tomás et al., 2002), seabirds (Rodríguez et al., 2012; Tanaka et al., 2013) and mammals (De Stephanis et al., 2013). Also, these MPs were observed in invertebrates, mollusks and fish existing in the freshwater systems (reviewed by O’Connor et al., 2019). The same as in the other matrices, no standard protocol exist and the comparison between studies would not be accurate even if the MPs concentration were expressed in the same units.

Table of contents :

1. General overview: Plastics pollution
2. The new emerging pollutant ‘Microplastics’
2.1 History and definition
2.2 Origin of microplastics
2.3 Sources of microplastics in the aquatic environment
3. Occurrence of microplastics in the aquatic environment
3.1 Microplastics abundance in the water
3.2 Microplastics in sediments
3.3 Microplastics in biota
4. Microplastics biomonitoring
5. Objectives of the thesis
1. Study sites
1.1 Estuaries: the Seine, the Canche and the Liane
1.2 Coastal systems: Sainte-Adresse and the Lebanese coast
1.3 Wastewater treatment plant: Edelweiss
2. Sampling techniques of all three matrices (water, sediments and native biota)
2.1 Water surface
2.2 Sediment samples
2.3 Native biota
3. Caging experiment
3.1 Juvenile flounder caging experiment
3.2 Farmed blue mussels caging experiments
3.2 Caging model comparaison
4. Laboratory analyses and samples preparation
4.1 Contamination prevention
4.2 Water samples
4.3 Sediment samples
4.4 Biota samples
5. Microplastics analyses
5.1 Visual observation
5.2 Micro-Raman spectroscopy analysis and polymer identification
1. Introduction
2. Material and methods
3. Results
4. Discussion
5. Conclusion
Supplementary files
B. Synthesis of Article 2
II. Article 2: Microplastics pollution along the Lebanese coast (Eastern Mediterranean Basin): Occurrence in surface water, sediments a Graphical abstract
1. Introduction
2. Materials and methods
3. Results and discussion
4. Conclusion
Supplementary files
A. Synthesis of Article 3
III. Article 3: Juvenile fish caging as a tool for assessing microplastics contamination in estuarine fish nursery grounds
1. Introduction
2. Material and methods
3. Results
4. Discussion
5. Conclusion
Supplementary files
B. Synthesis of Article 4:
IV. Article 4: Effect of exposure period on caged blue mussels (Mytilus edulis) microplastics bioaccumulation
1. Introduction
2. Material and Methods
3. Results
4. Discussion
5. Conclusion
Supplementary files
C. Synthesis of Article 5
V. Article 5: Is blue mussel caging an efficient method for monitoring environmental microplastics pollution?
Graphical abstract
1. Introduction
2. Material and methods
3. Results
4. Discussion
5. Conclusion
Supplementary files


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