MPs have been mainly detected at beaches and surface layers of lakes in significant quantities [52,60,61,62]. Statistically significant particle size reduction has been reported from riverine to coastal areas, where the prevailing polymer types are that of polyethylene (PE), polypropylene (PP) [47], as well as polyester and synthetic dye [53].

MPs abundance can be traced to closely populated areas, as well as at municipal and industrial effluent discharges [48]. In this context, MPs in sediments were reported and their MP concentrations were determined, for the first time, in the surface sediment of seven streams around the lagoon of Bizerte (Northern Tunisia), using a saturated NaCl flotation technique. Categorization of MPs was based on type, color, and size using a stereoscopic microscope [48]. Precisely, sampling certainly contains secondary MPs in the descending quantities of fibers, fragments, and films, while the reported colors are black, clear, white, red, blue, green, and yellow for fibers; white, blue, black, and red for fragments; and red, white, clear, green, and blue-black for films. In the same study, it was reported that MP particles in sampling ranged from 0.2 to 5.0 mm in length. Different stream-sites of sampling revealed significant MPs type differences, ranging at Khima stream (min concentration of 2340 ± 227.15 items kg ⁻¹ dry weight) up to Jedara stream (max concentration of 6920 ± 395.98 items kg⁻¹ dry weight) [48].

While in the relevant literature it is not exactly reported which synthetic substances can be classified as MPs, the sources of MPs detection in water resources in the environment can be precisely determined as plastic granules, or pellets, serving as a raw material for manufacturing plastic sheets or ready-to-use items, while the cosmetic industry also uses microgranules (microspheres, nanospheres, microcapsules, nanocapsules) [4]. An indicative classification according to the dimensional range of MPs particle sizing, in alignment with relevant methods that measure their distribution in water resources, is represented in Figure 6.

Based on Figure 6, there is no scientific affirmation about the exact particle size defining the MP particles, whereas there is a widely accepted agreement that MPs are those particles having a range size 0.5–5 mm and attributing them in their larger dimension. Another proposition of MP sizing is proposing a lower limit close to 0.3 mm, taking into consideration the widespread techniques of water sampling using zooplankton nets of 333 μm mesh size [4]. However, it remains unclear in defining the maximum particle size of MPs, being a continuing matter of discussion [4]. It is scientifically correct to propose that particles ranged at 0.5–5 mm should be conventionally considered as a typical range of definition, since substantial technical difficulties have to be confronted while analyzing particles smaller than 0.5 mm [4].

Based on Figure 6 and Table 3, it can be stated that among the MP detection methods, vibrational spectroscopy and thermosanalysis both necessitate diversified preparation of sampling with reference to specified methodology. A typical application of such a sampling can include the protocol of sampling, the filtration, the digestion of acid, and the density separation. This procedural analysis is case-specific in alignment with the aquatic environment: marine, wastewater effluent, surface water, bottled water [20]. Moreover, the fact that interfering materials are different in each matrix cannot be undermined, thus, a variety of different samples should follow preparation protocols for each case. The scope of such a formalized analysis is the running of accurate and reliable information, based on the size distribution, the chemical composition, and the concentration by mass and particle numbers. Subsequently, a multiple analytical technique should be executed, compared to the traditional analytical approach of a single methodology. MP analysis showed that polymer identification, the particle size, and the MP abundance can be satisfactorily investigated by FTIR and Raman analyses, while mass concentration can be measured by GC-MS coupled with thermal decomposition [20].

Another criterion of characterizing MPs’ intake and environmental damage caused to fisheries is the maximum number of MPs ingested by each fish, which is strongly correlated to exposure, being measured on the distance counted from the source of the river [53]. The integrated representation of the process followed for MPs’ identification and quantification is visualized at Figure 7.

Based on Figure 7, it has been shown that the naturally abundant polymers of polypropylene and polyethylene can be mainly detected by FTIR analysis [48]. The quantification of MPs by FTIR analysis showed that the predominant MP (in abundance terms) is the polyamides (up to 53.3%), followed by polyethylene and polypropylene (up to 17.1%). Comparably, the planting compounds showed the abundance of natural fragments such as cellulose and wood at proportions of more than 45.8% and 18.8%, respectively [56]. In a similar study, the plastic fragments were identified as being of the most prevalent types of MPs, accounting for 68.5% among all MPs found [29]. Consistent with other studies, as shown in Table 4, it can be argued that MPs’ abundance can be positively correlated with fragments’ abundance at an increased catchment urbanization, being also commonly composed by the polymeric substances of PET, PP, PE, PS, PAM, and PVC at various mass proportions [20,25].

From a microbiology point of view, MP particles of ~40 μm can be related to a low concentration level of 1% of the food particles, being reformulated to an average of ~30 particles in the digestive tract. This correlation resembles a high MP contamination but it is still reflecting on a natural situation. Interestingly, neither increased mortality nor morphological changes in the body and tail spine lengths and width or reproductive abnormalities have been reported for adult water fleas of Daphnia magna (a freshwater zooplankton species) [49]. It is also noteworthy that the channeling of MP particles in marine environments is unavoidably influenced by the characteristics of size, shape, type, and age, while those rather weak effects detected in a laboratory may result in reduced fitness in a natural multi-stressor environment [49].

From a toxicological point of view, it can be stated that MPs are prone to absorbing organic contaminants, metals, and pathogens from marine environments into organisms, thus, exacerbating their toxicology features while interacting and causing greater toxic effects. While it has been well known that in marine environments, MPs’ accumulation commonly occurs, MPs are ingested from marine vertebrates that seabirds use as bioindicators [2]. Therefore, MP debris causes asphyxiation through drowning, but a restriction in feeding intensifies the symptoms of starvation, skin abrasions, and skeletal injuries. Furthermore, MP ingestion can block the guts, causing injuries of the gut lining, morbidity, and mortality. Subsequently, small-sized MPs can accelerate their translocation across gastro-intestinal membranes of organisms via endocytosis-like mechanisms into tissues’ and organs’ distribution. Furthermore, MPs can increase dysregulation of gene expression required in biological systems, while the malfunctions caused are those of MPs’ induction of oxidative stress, immunological responses, genomic instability, disruption of endocrine system, neurotoxicity, reproductive abnormities, embryotoxicity, and trans-generational toxicity [2].

Conclusively, research focus should be based on the recognition of new sources of MPs that possibly represent major environmental inputs, compared to those previously considered. In addition to the already renowned sources, pollutants such as the MPs can be also released by embrittlement of plastic litter, microbeads released from personal care products, washing of synthetic clothes, abrasion of vehicle tires, or weathering of different types of paints [12].