After World War II, with the skyrocketing rise in the production and consumption of plastic products, the plastic flow into the environment appears to have been unstoppable and accelerating. Plastic products have become ubiquitous in everyday life. With an estimated global production of 368 million tons (PlasticEurope, 2020), 4.8–12.7 million tons of plastic are estimated to be released into the marine environment every year (Lebreton et al., 2019). The Mediterranean Sea is a semi-enclosed basin with limited exchanges with the Atlantic Ocean (Tanhua et al., 2013). With an high density coastal population, influx of freshwater from densely populated river catchments and a contribution of 15–30% to the global shipping activity (Compa et al., 2020), the Mediterranean Sea has been recognized as one of the most impacted regions in the world by plastic pollution (Cózar et al., 2015; Suaria et al., 2016; Zayen et al., 2020).

Once in the aquatic environment, due to a combination of chemical, mechanical, and biological processes, plastic debris tends to break down into smaller micrometric debris, namely microplastics (MPs). Most commonly, MPs have been defined as synthetic organic polymer particles, less than 5 mm in size that may differ in shape, color and chemical composition (Duis and Coors, 2016). Microplastic pollution has been reported worldwide, in different environmental compartments, including water, soil, air, and biota (Cincinelli et al., 2019; Zhou et al., 2020). Owing to their small size, MPs are potentially bioavailable to a wide range of organisms, having the potential to interact across trophic levels. There is evidence of MPs ingestion, accumulation and transfer by organisms along the food chain (Jiang et al., 2020). The ingestion of small plastics is known to cause direct adverse effects (e.g., entanglement, suffocation) and to expose organisms to plastic-associated chemical (e.g., POPs, PAHs) or microbial agents sorbed to surface (Mammo et al., 2020). MPs ingestion has been reported in different aquatic organisms, from small zooplanktonic invertebrates up to large marine mammals (Fossi et al., 2012). MPs has been identified also in species of commercial interests, including fishes, bivalve mollusks and crustaceans (Mercogliano et al., 2020), thus raising concern for potential risks to food safety and human health (Bakir et al., 2020).

Although during the last years there has been a virtual explosion of research on MPs pollution, especially in the Mediterranean Sea, there is a significant data gap for the Southern part of the basin (Anastasopoulou and Fortibuoni, 2019; Missawi et al., 2020; Wakkaf et al., 2020). According to its physical, biogeochemical, and biological characteristics, the Gulf of Gabes in Southeast Tunisia has been identified as one of the Mediterranean Sea 11 consensus eco-regions (Ayata et al., 2017). Strongly impacted by hydrodynamics, with tides and anticyclonic winds playing a major role (Béjaoui et al., 2019; Zayen et al., 2020), the Gulf of Gabes is highly productive (D’Ortenzio and Ribera d’Alcala, 2009; Ben Brahim et al., 2010), being an important nursery for several fish species (Hattour et al., 2010; Derbel et al., 2012; Enajjar et al., 2015). Overall, the area contributes approximately 40% of the national fish production in Tunisia (DGPA, 2015), thus being an anomaly in an area, the SE Mediterranean Sea, that is known to be oligotrophic (Béjaoui et al., 2019).

Despite being an important resource for biological marine resources, the growing urbanization and industrialization of the shoreline, notably in the northern (Sfax city) and central (Gabes city) regions of the Gulf of Gabes, is compromising the marine environment quality (Darmoul et al., 1980; Ayadi et al., 2014; Rabaoui et al., 2015; El Zrelli et al., 2017). Many untreated industrial and domestic wastewaters originating from the Ghannouch-Gabes industrial complex and the local municipal wastewater treatment plant, respectively, are discharged into the open sea on a regular basis, worsening coastal pollution (El Zrelli et al., 2018; Zayen et al., 2020), including plastic contamination.

Within this context, the aim of this study was to generate a baseline characterization regarding MPs pollution level in the Gulf of Gabes, an area of particular interest for its high biological and economical value but still poorly considered. More specifically, this study intended to (i) assess the evolution of MPs occurrence in coastal water samples collected during two different sampling periods (Spring/Autumn), including dynamic of MPs dispersion and accumulation in the studied area, and the correlation with water masses circulation; (ii) evaluate MPs ingestion by different marine species of commercial interests, including fishes (El Zrelli et al., 2017), crustaceans and mollusks, collected from the studied area.

The current study fills a data need on microplastic pollution in the Gulf of Gabes (Southern Mediterranean Sea) by characterizing the evolution of microplastic distribution and dynamics in surface water samples and analyzing MPs ingestion in commercially important species. The Gulf of Gabes is a rich coastal, marine, and freshwater ecosystem with major biological resources, but it is currently under severe anthropogenic strain, especially as a potential source of plastic pollution. Along Tunisian coasts, studies on MPs presence have been mainly carried out in sediment samples (Abidli et al., 2017, 2018; Chouchene et al., 2019; Missawi et al., 2020), whereas studies on MPs in waters are limited to few examples, including the waters from an urban lagoon (Bizerte lagoon, northern Tunisia, Wakkaf et al., 2020) and preliminary data from the Gulf of Gabes (Zayen et al., 2020).

In this work, we reported a remarkable MPs presence in all surface water samples, with an average abundance of 3.1 ± 2 × 10⁵ items/km² in Spring and 7.8 ± 8 × 10⁴ items/km² in Autumn. Our findings are in line with the recently published work of Zayen et al. (2020), showing comparable MPs abundance in surface water samples collected in Autumn 2017 from the Gulf of Gabes. Despite differences among studies conducted in the Mediterranean basin (e.g., net, sampling time, measure unit), overall our values are higher than MPs content found in mostly regions of the Mediterranean Sea (Table 5), thus confirming that the Gulf of Gabes is an area particularly sensitive to plastic pollution.

The sampled stations considered in this work were located close to coastline, in the proximity of important entry points for MPs (i.e., ports, rivers, industrial complex). The high level of anthropogenic pressure along the studied area contributes to explain the high concentration of MPs found. In this area, human activities, including industry, fishing, agriculture, and domestic waters, are likely to play a major role as MPs input (Fourati et al., 2018). In that regard, Liubasteva et al. (2018) have demonstrated by an exhaustive numerical modeling study that plastic emissions from Tunisia contribute more than 80% of their own coastline plastic pollution, defined as a “boomerang effect” (Liubasteva et al., 2018).

The approach used in this study to investigate the influence of hydrodynamics in the spatial distribution of MPs has been already applied in several studies and has given satisfactory results in terms of identifying potential areas of floating particles accumulation and their dispersion in the Mediterranean Sea (Macias et al., 2019) and in the Black Sea (Miladinova et al., 2020). The circulation in the Gulf of Gabes is influenced by the meteorological forcing and tides, being the most important tidal range in the Mediterranean Sea (2 m) as a result of a tidal resonance phenomenon (Abdennadher and Boukthir, 2006). Above all, the circulation is strongly forced by the mesoscale ATC with an Atlantic origin (Boukthir et al., 2019). The ATC is divided into two branches: one flows toward the gulf and the second one toward the south-east and forms a strong jet particularly in winter, namely the Atlantic Libyan Current (Sorgente et al., 2003; Ben Ismail et al., 2015). The main characteristics of the surface circulation scheme have been faithfully reproduced in our simulations.

The hydrodynamic models used in this study, on the other hand, suggest that the surface water samples taken at various locations came predominantly from outside the Gulf of Gabes and were advected by coastal current. As shown in Figure 5, the first source (the Atlantic Tunisian Current, ATC) entered the Gulf 2–3 months before the sampling date, and then carried to sampling stations by coastal currents (Tunisian Coastal Current). As a result, while the majority of the water analyzed came from near the coasts, some of it is likely to have come from further away. Moreover, the models show that stations 16 and 17 are in a distinct convergence zone than the other stations and are substantially influenced by seasons. At most stations a difference of one order of magnitude was reported between I and II Campaign, that can be partly explained by the high rainfall recorded in November 2019 (during the II Campaign) in addition to the accumulation effects of currents features inversion highlighted by the hydrodynamic models. This evidence a seasonal effect on MPs distribution in the area, as indeed the Gulf of Gabes is characterized by important seasonal variation due to the surface current circulation. In that regard, the Lagrangian model used to track floating debris from two sources located offshore and onshore of the Gulf of Gabes shows that during April month (Campaign I) floating debris accumulates entirely along the Gulf of Gabes coastline and especially in the area where the sampling stations are located (Figure 5). On the other hand, during November (Campaign II), floating debris accumulated not only along the Gulf of Gabes coastline, but there are also transported toward the coast of the Djerba island and Boughrara lagoon (most in the south off the Gulf of Gabes) (Figure 5). This is explained, in part, why the accumulation rates of floating debris at all sampling stations are higher in April than in November.

Microplastic items identified from water samples were mainly fragments, and this trend was confirmed during both the sampling Campaigns (Figure 2), which is in line with data from literature in other Mediterranean regions (Table 5). As suggested by Zayen et al. (2020), the highest presence of fragments could be due to the high rate of fragmentation of large plastic objects rather than MP primary input (i.e., direct release of particles in the form in which they are originally manufactured or from wastewater circuit discharge). In addition, the predominance of smaller MP (<1 or 1–3 mm; Figure 2) can be explained with the rapid breakdown of the particles occurring along the shoreline, considering that all stations sampled were located close to the coast. Considering MPs polymeric composition, a great variability of plastic types during both sampling campaigns were found, with a large predominance of PE, followed by PP, which is in line with other studies in the Mediterranean Sea (Table 5). PE and PP have the highest frequency due to their low density, which allows them to easily travel on surface water, as documented in literature studies (Pedrotti et al., 2016), as well as the fact that they are the most commonly used polymer types in various consumables such as packaging, domestic plastic waste, and various personal care and cosmetics products (Cole et al., 2011; PlasticEurope, 2020). To our knowledge only few studies focused on MPs occurrence in aquatic organisms sampled from the Gulf of Gabes area, specifically in the teleost Serranus scriba (Zitouni et al., 2020) and in the sea worm Hediste diversicolor (Missawi et al., 2020). Such lack of data limits comparisons with our results. To expand the current state of knowledge, this work assessed the presence of MPs in the digestive tracts of several species of commercial interests, including fishes, crustaceans and mollusks sampled in the Gulf of Gabes. Conversely to the high values reported in surface water samples, microplastics were found only in 3% of the total individuals analyzed, specifically in S. scombrus (20% of the analyzed specimen), M. surmuletus (10% of the analyzed specimen), and S. aurita (6% of the analyzed specimen). S. scombrus and S. aurita are pelagic, displaying planktivorous feeding habitat (Neves et al., 2015; Shabaka et al., 2019), while M. surmuletus is demersal, feeding on benthic organisms (Filgueiras et al., 2020; Gündoğdu and Çevik, 2020). The other species considered in this study included both pelagic and demersal organisms. From our results, it is difficult to relate organism feeding behavior to MPs ingestion. Previous studies have deeply investigated such correlation, but the results are often contrasting. For instance, some authors suggested that pelagic species ingest more MPs when compared to demersal species (Rummel et al., 2016; Güven et al., 2017), while on the contrary others reported that pelagic and demersal species did not differ in MP content (Neves et al., 2015; Lusher et al., 2017). Nevertheless, considering that sediments are considered as a major sink for MPs pollution (Woodall et al., 2014), many studies reported that demersal species have more probabilities of being in contact with microplastics than pelagic species (Filgueiras et al., 2020). For instance, Bellas et al. (2016) focused on MPs ingestion in three commercially, relevant demersal fish species (Scyliorhinus canicula, Merluccius merluccius, Mullus barbatus) from the Spanish Atlantic and Mediterranean coast, thus being selected as that are used as indicative species for marine pollution monitoring within the Spanish Marine Pollution Monitoring Programme (SMP). Besides, this study reported a frequency (16%) of MPs ingestion by M. surmuletus, that was comparable to our finding. To provide data on MPs ingestion in species of commercial interests is crucial considering their high economic importance worldwide, and representing the bulk of fish biomass, particularly in upwelling regions (FAO, 2016). Such species play a key role in pelagic food webs, by having an important effect on lower trophic levels (i.e., planktonic organisms) and, at the same time, controlling predatory fish (Cury et al., 2000; FAO, 2016). Among them, S. scombrus represent a widely studied species, and different frequencies of MPs occurrence (Nelms et al., 2018; Lopes et al., 2020) have been reported, both in Atlantic regions (Nelms et al., 2018; Lopes et al., 2020) as well as in Mediterranean Sea (Avio et al., 2020).

Overall, an average of 1 particle/individual was found in the digestive tracts, which is in line with data from literature (Table 2). All the identified MPs were fragments according to other works (Digka et al., 2018; Savoca et al., 2019; Avio et al., 2020), as well as being reflected in the high percentage of fragments found in seawater surface samples (Cole et al., 2011; Zitouni et al., 2020). In that regard, several investigations on MPs ingestion by aquatic vertebrates and invertebrates reported fibers as the most ingested particles (Avio et al., 2020). Also in our study, many fibers were identified during biota analysis, but these were not considered to avoid false results since we cannot exclude that contamination occurred during laboratorial analysis, although several precautional methodologies have been applied as recommended by literature (Rummel et al., 2016; Wesch et al., 2017; GESAMP, 2019). This conservative approach might in turn lead to underestimation of the plastic pollution level, thus contributing to explain, for example, differences found in this study with data from literature on MPs ingestion by aquatic organisms.

Ingested particles were constituted by PE and EVA, and two particles were identified as dye additives, thus confirming their anthropogenic origin. This data is in agreement with Zitouni et al. (2020) that have reported in Serranus scriba ingested particles made by a wide range of additives such as lubricants, stabilizers and plasticizers. These are known as “plasticizers,” not properly plastic. The presence of these substances in the digestive tract of aquatic organisms is, however, an indication of plastic ingestion by organisms, probably attracted by their color Koelmans (2015), and for this reason these particles were included in the estimation of ingested items.

The low frequencies of MPs ingestion found in this study appear to be in contrast with the high level of plastic debris found in water samples. In that regard, the low number of analyzed individuals per species (10 on average) may explain the low frequencies found, as indeed previous works highlighted that a high number of individuals to be analyzed is essential to find a robust indicator regarding trends of ingested litter in a specific area (Neves et al., 2015; Avio et al., 2020). In addition, it should be also highlighted that these investigations should be considered a snapshot of MPs currently trapped as well as those yet to be egested or translocated (Parker et al., 2021), thus being representative of MPs contamination level at a given point in time and in space.

This research contributes to expanding our state of knowledge on microplastic pollution levels in a region, the Gulf of Gabes, that still poorly considered, but of great importance for its high ecological value. Our results confirmed the ubiquitous nature of MPs in the marine environment, by polluting the southern region of the Mediterranean basin.

This study reported a high level of MPs in surface water samples, with a seasonal variability in MPs distribution observed between the sampling campaigns. Despite high presence in waters, low frequencies of plastic ingestion by investigated species were reported, thus claiming for further and long-term investigations.

Considering the severe anthropogenic pressure insisting on the studied area, future work is recommended in order to define plastic pollution levels in the area and its reliable threat to marine ecosystems, that are essential to set effective management measures to face this emerging global threat.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

Ethical review and approval was not required for the animal study because the research was done on captured commercial fishes.

SB, EC, and SM wrote the manuscript initial draft. SB, HJ, MB, MAB, EC, SM, and RM collected and processed the samples. AM and RM processed the sample by chemical characterization. SB, EC, SM, MB, RM, FG, CS, and MF reviewed the manuscript. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.