Microplastic Pollution in Coastal Area of Black Sea: Comparison
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Plastic materials, degraded by physical and chemical processes, become micro- and nanoparticles. Microplastic pollution is a complex problem, as it is widespread and the exact harmful effects of long and short exposure are not known, although it certainly has considerable consequences for biota, the environment, and public health. The Black Sea is becoming a strategic area as a battle front and shipping route. This certainly influences the quality of the marine environment, and its microplastic pollution could degenerate into a much more unpleasant situation. Therefore, it is important to have a picture of this pollution in the Black Sea area to facilitate future efforts to understand the impact of this crisis.

  • microplastics
  • black sea
  • contamination

1. Introduction

“Plastic” is a general name for a category of non-biodegradable petroleum-based materials. They are composed of two subcategories: thermoplastics, such as Polyethylene terephthalate (PET), Polypropylene (PP), Polymethyl methacrylate (PMMA), Polyvinyl chloride (PVC), and Polyethylene (PE); and thermosets such as epoxy resins, silicone, acrylic resins, and Polyurethane (PUR). These are an everyday presence in our life and environment. Lately, interest in researching these materials has grown considerably, both in production and environmental protection. Aquatic ecosystems, especially the marine ecosystems, are final destinations for various plastic materials. The transport and dispersion of plastic materials could be the result of natural factors, such as waves or wind. Plastic materials, degraded by physical and chemical processes, become micro- and nanoparticles. In the current situation, the Black Sea is becoming a strategic area as a battle front and shipping route. This certainly influences the quality of the marine environment, and its microplastic pollution could degenerate into a much more unpleasant situation. Therefore, it is important to have a picture of this pollution in the Black Sea area before the current political and military crisis to facilitate future efforts to understand the impact of this crisis.
Lately, a large number of scientific reports show the presence of plastic compounds with dimensions ranged between 100 nm and 5 mm (such as fibers, fragments, spherules), named microplastics (MPs) in different types of environments [1][2] worldwide [3]. For example, in terrestrial environments, such as soils in agriculture, a quantity of 0.34 ± 0.36 microplastic particles per kilogram of dry weight of soil was found on an agricultural farm in Germany [4]. Another study showed that microplastic particles in the soil column can easily migrate and enter groundwater along with other pollutants [5]. These can all act both as receivers/sinks and as carriers of microplastics, which eventually reach the marine ecosystem [6]. Microplastics contamination can even penetrate remote regions, such as snow in the Alps and the Arctic, where most of the detected MPs are spherules with the smallest size range (11 µm) and a highly variable polymer composition [7]. Brahney et al. (2020) [8] showed that the average rates of wind and rainfall deposition were 132 particles m−3 day−1 in protected natural areas in the Western U.S. In comparison, over half of the Black Sea area is covered by protected areas Natura 2000: there are 40 Sites of Community Importance (SCIs) under the Habitats Directive and 27 Special Protection Areas (SPAs) under the Birds Directive [9]. These areas host many protected species of animals and plants, many of them found only in areas exposed to the effects of microplastic pollution. Man-made cellulose fibers are a microplastic highly present in the digestive tract of various fish species. It can lead to harmful effects due to its ability to transfer harmful dyes or contaminants to aquatic fauna through direct ingestion or consumption of contaminated plankton [10][11].
Microplastic pollution is a complex problem, as it is widespread and the exact harmful effects of long and short exposure are not known, although it certainly has considerable consequences for biota, the environment, and public health.
Lately, some studies have evaluated the behavior-linked effects (feeding, speed, predator’s avoidance, and reproductive activity) that MPs have on some species [12]. In these scenarios, MPs reduced growth, energy reserves and nutrient quality in Sebastes schlegelii [13], while brief exposure to MPs from the polyethylene induced a deficiency of anti-predatory defensive response, locomotor changes, and anxiety-like behaviors in the tadpoles of Physalaemus cuvieri [14].
Several other studies were conducted on the potential impacts that microplastics can have in the marine environment [15]. Data show alterations in immunological responses [16][17] and antioxidant systems, neurotoxic effects, an onset of genotoxicity, changes in the gene expression profile in mussels [18], and even carcinogenesis [2].
The frequent presence of microplastics as pollutants in different types of environments has made human exposure, largely through ingestion, inhalation, and dermal contact, inevitable. Under different conditions of microplastics exposure, this may cause inflammatory lesions, correlated with the potential of their surfaces to interact with tissues, and an increased incidence of neurodegenerative diseases, immune disorders, and cancers in humans may also be relevant [19][20]. The studied toxicological effects on the four different taxa are shown in Figure 1.
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Figure 1. The studied toxicological effects on the 4 different taxa.

2. Microplastic Abundance in the Black Sea

One of the main sources of pollution in the Black Sea is the discharge of rivers, and the largest contribution is expected from the Danube River. Lechner’s study estimated that 4.2 t of plastic per day and a quantity of 1533 t per year reach the Black Sea via the Danube River [21]. There are differences in microplastics abundance between beach and depth sediment samples, even between water column and water surface samples, which probably reflect variations in the original source of microplastics, salinity, and local hydrodynamic conditions. The highest microplastic concentrations in the sediment samples were found on the Black Sea coast and proximal shelf, indicating the fate of low-density particles in coastal sediments. A recent study showed an average abundance of 106.7 MPs kg−1 in all samples. The highest pollution occurred on the northwestern shelf, where the abundance of MPs in the depth sediments was 10 times higher [22]. Studies have also shown a large difference in the abundance of MPs between the beach sediment samples from Romania and those from Bulgaria; in Bulgaria the abundance was four times higher than in Romania and twice as high as in-depth samples (Figure 2B) [22][23]. According to two different studies regarding the sediments from the shore and from shallow depths in Turkey, a difference between the levels can be clearly seen. Thus, an average of 66.06 MPs kg−1 is present at the level of the upper shoreline, and an average of 211 MPs kg−1 is present at 10 m depth [24], while at a depth of 30 m this amount decreases to an average of 180 MPs kg−1 [25]. These data suggest that the spatial distribution of microplastics depends on the depth of the sampling area; however, a lower depth can become a place of accumulation for microplastics.
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Figure 2. The percentage of the different polymers found in different sites and sample types (A) (SD = Sediment Sample, WS = Water Sample); The average of MPs items in different sites and sample types (B) [SD = Sediment Sample (MPs kg−1), WS = Surface Water Sample (MPs m−3), WC = Water Column Sample (MPs m−3)], with p < 0.05 (Anova test), and different shapes (C) of MPs items in different sites and sample types (SD = Sediment Sample, WS = Water Sample), with p < 0.05 (Anova test).
Significant statistical differences were also found in the water sample results; in this case, the highest concentrations of MPs were found in the water column in Turkey with an average of 24.475 ± 26.153 MPs m−3 [26]. The values for surface water were lower than for the water column and ranged from 0.62 MPs m−3 to 17.5 MPs m−3. The lowest quantity was found in Bulgaria with 0.62 MPs m−3 [27], followed by Romania with an average of 7 MPs m−3 [27], and then Turkey with an average of 7.21 MPs m−3 [24][26][28]. (Figure 2B). After summarizing the results of the specialized studies, two different maps were generated regarding the abundance of microplastic particles in different sampling points to obtain an overview of these pollutants in the Black Sea (Figure 3). By analyzing the abundance of microplastics in sediments (Figure 3A), it can be seen that they are present in higher quantities in the northern Black Sea, in the area of Ukraine, but also at the mouths of the Danube River in Romania. A higher abundance was found at Midia Cape and at Kaliakra Cape, and this may be related to the hydrodynamics of vertical currents that force the sedimentation of microplastic particles in these regions. It is worrying that in some areas of the deep sea, especially near Romania, large amounts of microplastics were found; a possible reason would be the sedimentation of particles in the Danube and their transport by deep currents into the open sea to create a “hotspot” for such pollutants. Referring to the abundance of MP in the water column (Figure 3B), higher concentrations were observed at the mouths of the Danube, along the Romanian coast, and especially near Midia Cape and Kaliakra Cape, Bulgaria. Worrying amounts of microplastics are also present along the Turkish shore, in addition to on the Romanian coast and at the two points of high accumulation already mentioned. This may be related to the discharge of the large nearby rivers (Danube River, Sakarya River, Kızılırmak River, Yeşilırmak River), commercial activities in the Black Sea such as navigation and fishing, and the hydrodynamics of the surface currents.
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Figure 3. Sampling sites and the microplastic abundance in sediment samples (MPs kg1) (A) and in water samples (MPs m−3) (B).
The presence of microplastics in marine organisms in the Black Sea has been investigated by various studies in recent years. A study conducted between 2014 and 2015 examined MPs from zooplankton samples collected during two cruises along the southeast coast of the Black Sea. The average concentrations found were 1.2 ± 1.1 MPs m−3 in November 2014 and 0.6 ± 0.55 MPs m−3 in February 2015, and the microplastic morphotypes were mostly fibers (49.4%), films (30.6%), and fragments (20%) [29]. Recently, microplastic particles were detected in copepods from the southeastern Black Sea; the results for 2136 individuals of Acartia clausi and 2123 of Calanus euxinu showed that the microplastics’ frequency of occurrence was 0.002 MPs/Acartia (one MP for every 356 Acartia), respectively, and 0.004 MPs/Calanus (one MP for every 265 Calanus) [30]. Another study showed the presence of microplastics in five bivalve species (Donax trunculus, Chamelea gallina, Abra alba, Anadara inaequivalvis and Pitar rudis) in the southern Black Sea. Microplastics were found in all bivalve species except Abra alba. The average number of microplastics ranged between 1.69 and 4 MPs ind−1. Fibers were the most common type of microplastic in each bivalve species, followed by fragments and films in nine different colors, mostly black and blue [31]. More recently, Aytan et al. [32] conducted a 2022 study on seven commercial species of fish in the Black Sea; the results indicate that microplastics were found in all analyzed specimens, most of them being fibers with lengths between 0.2 and 1 mm. The largest quantities have been found in the species Sarda sarda, Engraulis encrasicolus, and Mullus barbatus; due to their high abundance and distribution in the Black Sea basin, these species could be suitable as bioindicators for the microplastic pollution in the Black Sea. In 2018, a quantity of 1510 tons of mussels was produced in Turkey alone [33]. If a person consumes 22.8 g of mussels per day, according to the average of MP abundance of 0.69 MPs mussel−1, the annual exposure for an adult through consumption can be estimated as 1918 MP per year [34]. Another bivalve affected by microplastic accumulation was recently studied by Gedik [25] and had an average value of between 0.22 and 2.17 MPs ind.−1. In this way, Chamelea gallina is not far from Mediterranean mussels, as both species are being commercialized for human consumption.

3. Polymer Nature and the Shapes of Microplastics in the Black Sea

Various polymeric compositions were found in the Black Sea, both in the water and sediments samples (Figure 2A). The most present polymer type in the sediment samples was Polyethylene terephthalate (PET) followed by Polyethylene (PE) and styrene acrylonitrile copolymer (SAC). In the water samples, the most present polymer type was Polypropylene (PP), followed by Polyethylene (PE) and Polyethylene terephthalate (PET). The reason for this reversal is very likely to be density, as polypropylene (PP) has a lower density (0.905 g cm−3) than polyethylene (PE) (0.965 g cm−3) [35], which favors floating in the water column or at the surface. These materials are also the most common types found as dominant floating polymers in marine environments, both globally and regionally [36][37][38]. Statistically speaking, in the Black Sea, there is some correlation between different types of polymers. The presence of Polyethylene terephthalate (PET) is strongly correlated with the presence of Polystyrene (PS), Polypropylene (PP) and Polyacrylonitrile (PAN), an organic polymer resin. Various shapes of MPs were found (Figure 2C) in different sites and different types of samples from the Black Sea. Fibers were the most common shape found, followed by fragments and films. Statistical analysis showed a higher correlation between the presence of fragments and the presence of spherules in the samples. A positive correlation was also observed between fragments and films, as well as between fibers and films, and foams and pellets, respectively.

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