Plastic pollution in the aquatic ecosystem: an emerging threat need to be tackled

The accumulation of plastic substances in the aquatic ecosystem is a threat that should not be underestimated. Smaller plastic pieces, such as microplastics and nanoplastics, are of particular concern since their presence in the food web is persistent. Microplastics enter in the food chain and its very bottom, when aquatic organisms eat or ingest contaminated food materials, and keep being transferred in the next food web such as predators including humans. It is evident that aquatic organisms frequently ingest microplastics across a variety of feeding guilds. Marine organisms may cause shock, inner or outer injuries, ulcerating sores, blocking digestive tracts, fake feelings, degraded feeding capabilities, fatigue, weakness, limited predator prevention, or death due to the ingestion of large plastic material and/or particles. However, effects of microplastic particles on marine organisms and the toxicity mechanisms are largely unknown. There is much more limited evidence of the impacts of microplastics intake on freshwater species, both in the limited number of studies performed and the number of species examined. However, a few recent freshwater investigations imply that the physical impacts are similar to those observed in the sea. As a result, we conducted a brief evaluation of the state of the science in order to identify knowledge gaps and research requirements to examine the impacts of microplastics on aquatic ecosystem. To yet, just a few researches have looked at the biological consequences of plastics on aquatic organisms, and the important transport pathways of plastics from freshwater to marine environments and vice-versa.


Introduction
Plastic pollution has been a global environmental problem with the spread and impacts of plastic debris on the aquatic biota, biodiversity and human health in all aquatic environments worldwide (Lares et al., 2018;Li et al., 2018;Beaumont et al., 2019;Blettler and Wantzen, 2019;Du et al., 2020;Haram et al., 2020;Li et al., 2020;Mataji et al., 2020;Scherer et al., 2020;Zhang et al., 2020a). Plastic products have globally used rapidly because of its exceptional features as light-weight, versatile, durable and low-cost production (GESAMP, 2015;Ballent et al., 2016;Ivleva et al., 2017;Hahladakis et al., 2018;Sha q et al., 2019;Capolupo et al., 2020;Parata et al., 2020). In the 1950s, global plastics production was estimated around 1.5 million tons per year whereas in 2007 it was assessed nearly 250 million tons per year (PlasticEurope, 2008). With the increasing of annual global production of plastics, in 2016 it was also calculated around 322 million tones and that is still increasing by 10% each year (PlasticEurope, 2016;Crew et al., 2020). Consequently, these materials signi cantly contribute to the generation of waste (Stoiev and Turra, 2016;Saeed et al., 2020) and annually, between 5 and 13 million tons are estimated to leak into the world's oceans (World Economic Forum, 2016;Xu et al., 2020).
In the today's society, the plastics are therefore universal and plastic pollution is a de ning human legacy on earth (Vince and Hardesty, 2017;Rochman, 2018;Alves and Figueiredo, 2019;Ma et al., 2020).
Despite extensive research efforts investigating plastic contamination (microplastic only, very few of them reported mega-, macro-, meso-, micro-and nano plastics and all size ranges with its pattern, distribution, source, color, shape & size) round the world in marine water, surface or beach sediment and in terrestrial organisms (Eriksen et al., 2013;Imhof et al., 2013;Ivar do Sul and Costa, 2014;Noik and Tuah, 2015;Pereao et al., 2020), few studies have been focused on freshwater aquatic ecosystems (Morritt et al., 2014;Wagner et al., 2014;Klein et al. 2015;. There is already proved that several microplastic particles and bers can accumulate not only marine water but also freshwaters; though fewer microplastic monitoring concentrations have been done on freshwater systems than in seawaters (Klein et al., 2018;Fu and Wang, 2019;Yu et al., 2020). Thus, the knowledge about the plastic waste in river water and sediment in the world are scare, and there is still very little information on their presence, sources and destiny (Thompson et al., 2009;Eerkes-Medrano et al., 2015) along with the social campaign for proper plastic management in freshwater and marine water is very rare.
In order to better understand the plastic pollution and its potentials, this review chapter seeks at exposing the existing knowledge of plastic pollution in the aquatic ecosystem. Target-oriented plastics particle management options, abundance, distribution and rarity learning are far from su cient and further sound research is needed (Peng et al., 2017;Wang et al., 2017;Martins et al., 2019;Masia et al., 2020). Finally, there are several key challenges and suggestions are discussed for further plastic research, and a plastic pollution reduce campaign throughout the society are highly recommended.

Plastics In The Environment
Page 4/26 2.1 What is plastic?
Plastic production was started in 1950s for a large-scale basis. It was rapidly increased in response to a growing demand for produced commodities and packaging for food or the products to protect from contaminants (Koelmans et al., 2015;Chatterjee and Sharma, 2019;Li et al., 2020;Qi et al., 2020). The term "plastic" refers to a category of synthetic polymers that come in a variety of sizes depending on the purpose. It refers to many ways in various studies as mega-, macro-, meso-, micro-nano plastics (Fig. 1).

Classi cation of plastics
The classi cation of sizes in plastics have emerged alongside their recognition as a type of aquatic litter. Fiber processing from pre-production or melting of resin pellets produces mega-and macro-plastic goods, which have a wide range of applications ranging from garments to industrial constructions. Microplastics are used to make facewash scrubbers, cosmetic microbeads, powders, and macroplastic objects, among other things (pre-production resin pellets). Nanoscale plastics include biomedicines, pharmacological drug delivery systems and medicinal diagnostics (Koelmans et al., 2015;Uddin et al., 2020;Liu et al., 2020a). Table 1 Description and types of different plastics in the environmental

Category Description
Classi cation The plastics of the environment are a very heterogeneous waste group which may be characterized by different descriptors. Until now, there is no speci c universal method for classi cation. It can be laminated by size, origin, form, type of polymer and color [Wagner et al., 2014] Size The size groups for plastic litter as follows: megaplastics (> 1 m), macroplastics (< 1 m), mesoplastics (< 2.5 cm), microplastics (< 5 mm), and nanoplastics (< 0.1 µm) [Lusher et al., 2017a;Chatterjee and Sharma, 2019].
Origin Microplastics can also be categorized based on their source: primary microplastics, for example, are formed from resin pellets (plastic raw materials). Secondary microplastics are the result of UV radiation and physical abrasion breaking down bigger polymers into smaller particles [Lusher et al., 2017a].

Environmental fate of plastics and its application
Plastics' environmental fate is largely determined by their density (Table 2), which determines resistance, location in the water column, and subsequent potential contact with biota (Wright et al., 2013a;Yu et al., 2020). Polymers with a lower density than seawater (e.g., PP, PE) will oat in the water column, but highdensity polymers (> 1.027 g / cm3, PVC) will sink. Invasion of organisms and biofouling on the plastic surface increase the weight of the artifacts, causing them to sink to the bottom sediments; fragmentation, leaching, and oxidation of additives can further change the density and distribution of artifacts throughout the water column (Lobelle and Cunliffe, 2011;Du et al., 2020). Table 2 Types of plastics, their most typical applications, their gravity that are found in the aquatic environment (Andrady, 2011, Nerland et al. 2014, PlasticsEurope 2015, GESAMP 2015 Plastic-type

Sectors generating plastics
Leisure activities, food supply, energy consumption, transportation, and housing supply are all factors that in uence plastic usage, and they vary depending on the social and economic climate. This, in turn, has an impact on technological innovation, product design, market demand, waste generation, and trash disposal. Unfortunately, environmental externalities such as the social, economic, and ecological repercussions of aquatic trash have been overlooked by the business sector. (Wagner and Lambert, 2018). Due to the industrialization the plastic particles can also be accumulated in the environment through speci c applications, processes or products by primary and secondary sources (Fig. 2).
The new 'plastic economy' has been de ned by a linear output and use pattern, producing enormous amounts of waste, which is essentially quite economically ine cient (Defra, 2015;UNEP, 2016;Capolupo et al., 2020). Figure 3 illustrates land and water-based plastics sources, and the routes through which plastics enter the ocean.
Tracks in the land-based sector may go across rivers, through the environment, or directly into the ocean (e.g., through dropping litter on the shoreline). Maritime activities employ a variety of plastics styles for both long-term (e.g., shing gear) and short-term (e.g., ropes) applications (e.g., packaging). Table 3 shows the potential industries involved, as well as garbage or different types of plastic products and the ocean's regular entry points.

Plastics As A Pollutant
The environmental impact of plastics can differ based on the chemical composition of plastics. Till now, the most widely used plastics have been polyethylene (PE) and polypropylene (PP), which have been used to produce packaging materials and malleable lms, as well as automobile components, shing gear, pipelines, and kitchenware. PET is a type of polyethylene terephthalate (PET) that is extensively used in apparel and drinking cups. PVC is employed in the automotive and construction industries, and polystyrene (PS) is used in a variety of applications, including building insulation and packaging .
Most plastic polymers are low in toxicity due to their unique properties, such as being biochemically inactive due to their high molecular weight and insolubility in water. Monomers are used to make plastic polymers, which are subsequently combined to form synthetic polymers. The majority of monomers, such as vinyl chloride or styrene, are poisonous and carcinogenic, and monomer residues in plastic goods may be dangerous (Lithner et al., 2011;Haram et al., 2020). Four plastics (PVC, PU, PS, and PC) that make up about 30 percent of global production are considered especially di cult because they often contain toxic monomers or additives (Rochman et al., 2013b). Plasticizers and llers that modulate texture or coloring agents, ame retardants, antimicrobials, and other compounds that alter the material's qualities in bene cial ways are examples of these additions (Deanin 1975). Such compounds may endanger the health of people and other animals (Rochman, 2015;Koch and Calafat, 2009;Parata et al., 2020) and may reduce the potential for recycling and reuse. Bisphenol A (BPA), which is extensively used in the making of polycarbonate (PC) plastic water bottles and other resins used in food containers but has been subject to regulation due to its hormone-like characteristics, estrogen mimicking, and human accumulation, is one well-known example (Koch and Calafat, 2009;Egessa et al., 2020). Likewise, certain common plasticizers (adipates and phthalates) have hormone-like effects and are commonly used to smooth brittle plastics like PVC so that they can be used in food packaging, toys, and a variety of other everyday products (Koch and Calafat, 2009;Da Costa et al., 2016;Qi et al., 2020).

Commercially important ora and fauna
Plastic particle ingestion by ecologically vulnerable species has now been documented in a variety of environments, including beaches, aquaculture, estuaries, sea surface, water column, and benthos (Lusher 2015;GESAMP 2016;Amoatey and Baawain, 2019;Pereao et al., 2020), and deep sea (Taylor et al., 2016). The diversity of the species investigated and the habitats from which they are taken necessitates a variety of collecting procedures (Lusher et al., 2017b). Through several studies it has been found that more than 220 different animals eat microplastic debris across nature (GESAMP, 2016;UNEP, 2016). Beyond the individual stage, assemblages, and populations, interactions with environmental variables, including pollutants, and contamination of a long-lived organism at various levels and times during ontogeny, it is also critical to understand the ecology of microplastics ( ora and fauna interactions (Ferreira et al., 2016;Wang et al., 2020b).

Marine mammals and seabirds
Microplastics have been discovered in aquatic birds and marine mammals, and they may be of relevance to humans and the environment. Humans eat a variety of bird species, and each seabird species tested has at least one person with digestive tract microplastics ( Marine mammals can ingest microplastics but it is hard to determine the source of microplastic. It was detected in baleen whales, Mesoplodon mirus (Lusher et al., 2015a), beaked whales, Megaptera nevaeangliae (Besseling et al., 2015a) and seal stomachs Phoca vitulina (Bravo Rebolledo et al. 2013).
For marine animals, microplastics may be produced from feeder on the aquaculture or by ingesting microplastic prey. Indeed, marine mammals can be regarded as an ocean safety sentinel for microplastic ingestion and plastic contaminants consumption in different baleen whales (Fossi et al., 2012(Fossi et al., , 2014(Fossi et al., , 2016Baini et al., 2017;Lavers et al., 2019;Kuhn et al., 2020).

Shell sh (bivalves and crustaceans)
Bivalves seem to be the most commonly employed species in microplastic exposure investigations. Around the Minch and Orkney Islands, the lobster revealed an increased quantity of plastics and a poor uptake of plastics in the heavily damaged Clyde Sea area (Murray and Cowie, 2011;Welden and Cowie, 2016a). Additionally, the common shrimp (Crangon crangon), a decapod crustacean was also sampled from the North Sea (Devriese et al., 2015), Blue Mussels from wild and farm Mathalon and Hill, 2014;Van Cauwenberghe & Janssen, 2014), paci c cup oyster from the coastal waters of Atlantic Ocean (van Cauwenberghe & Janssen, 2014), Chinese mitten crab (Eriocheir sinensis) from coastal waters of the Baltic Sea (Wjcik-Fudalewska et al., 2016), brown mussel (Perna perna) from the Santos Estuary of Brazil (Santana et al., 2016), Manila clams (Venerupis philippinarum) from wild and farm (Davidson and Dudas, 2016) have contained by microplastics. Small bivalves with high microplastic pollution have shown similar results in Asian markets (Li et al., 2015). While this microplastic is commonly de ned in the aquatic setting, it is di cult to identify or monitor the size and variety of potential sources.
Subsequently, Fish purchased from sh markets comprised microplastics in Indonesian waters  and the sh markets in Shanghai (Jabeen et al., 2016). Microplastics were also identi ed in the digestive systems of market-purchased freshwater sh, including Nile Tilapia (Oreochromis niloticus) and Nile Perch (Lates niloticus) from Lake Victoria (Tanzania). Microplastics have recently been discovered in the digestive tracts of commercially valuable species of wild sh larvae (2.9 percent of tested individuals) from the English Canal (Steer et al., 2017). While it is obvious that many commercial sh consume microplastics, nothing is known about the human consequences of doing so.

Trophic Transfer and Bioaccumulation
The possibility of trophic transfer and bioaccumulation is a common concern for many persistent contaminants. Bioaccumulation and trophic transfer of plastics with accompanying chemical pollutants in the aquatic ecosystem through the food web is increasing daily (Fig. 4). The initial evidence indicates the potential for trophic movement of wild-caught species from microplastics (Romeo et al., 2015;Welden and Cowie, 2016;Du et al., 2020). Benthic lter feeders, such as oysters and mussels, are well-studied for collecting plastic micro bers and other particles from the water column and transmitting them to benthic predators (Farrell and Nelson, 2013;Walkinshaw et al., 2020) and farm or wild shell sh consumers (Mathalon and Hill, 2014). Microplastic particles were also found in larger pinniped and cetacean gastrointestinal tracts, indicating a trophic transition from prey sh to top predators (Lusher et al., 2015b;Eriksson and Burton, 2003;Scherer et al., 2020). Toxins like PBDE (polybrominated diphenyl ether) have been found to be transferred from plastics to insects, amphipods, lungworms, and sh in laboratory trials (Rochman, 2016;Goswami et al., 2020).
The relative pollutant load in the environment, the plastics involved and the receiving individual depending on the situation are a main variable. Correlative data from sh (Rochman, 2016), mussels (Jang et al., 2016) and seabirds (Tanaka et al., 2013) con rms plastics' potential for causing environmental contaminants to bioaccumulate (Worm et al., 2017). As a result, the issue and question are how plastic affects sh and shell sh, as well as how it will affect seafood consumers and human health. The widespread occurrence of marine plastic debris in seafood, as well as the toxicity of chemical compounds connected with it, has raised concerns, and the weight of evidence suggests that toxins can be passed from plastic to animals (Rochman, 2015;.

Conclusions And Future Research Perspectives
Plastics' presence and concentration in the aquatic environment is now an undeniable fact. Microplastics have sparked considerable scienti c interest and a growing body of information during the previous decade. Nonetheless, the issues surrounding fundamental questions remained unanswered. Microplastics are a pervasive and widespread marine pollution found throughout the water column, according to a growing number of monitoring methodologies. Moreover, differences in microplastic size classi cations and the lack of comparability of microplastic sampling methodologies limit our ability to compare quantitative research in order to better understand the geographical and temporal patterns of this contaminant.
Typically, the biggest concentrations of microplastics are found along coastlines and in mid-ocean gyres, but their fates are unknown. Laboratory eld trials have shown the use of microplastics in a range of marine biota, but it's unknown if ingestion of microplastics alone would be harmful to one's health (for example, mortality, morbidity, breeding success) or if such a contaminant would be regularly transmitted to the food chain. It is a signi cant concern that harmful chemicals are passed by microplastic intake into biota. But, the effects of MPs on freshwater organisms are much less well recognized.
We highlight several important aspects to debate based on the ndings of this study in order to better understand the issue of plastic fragments or microplastics in aquatic systems: (a) Perform further studies of plastic effects at concentrations appropriate for the environment; (b) Carry out further laboratory studies targeting a number of most commonly developed species ( bers and fragments) and the size number (800-1600 µm) of microplastics present in eld biological samples; (c) Carry out further eld and laboratory experiments with freshwater species such as marine ones; (d) Investigate the processes by which microplastics in uence certain classes of species (e.g. Echinodermata, cnidaria and porifera) to help assess the importance of small crustacean studies in general to invertebrates; (e) Consider the mechanisms by which PE affects certain classes of species other than sh and PSMPs, namely sheries and small crustaceans; (f) Measure the ecotoxicity of microplastics under more environmentally acceptable conditions, such as experience of multispecies and mesocosms.
(g) Public awareness campaign and strict regulation can disclose the adverse plastic effects on people and society.
Over the coming years further investigations are expected. They will be crucial to understanding the processes or processes by which plastic fragments as microplastics affect the aquatic species in order to answer realistically the speci c effects of these micro and nano level contaminants on the aquatic ecosystem.

Con icts of Interest:
There are no con icts of interest declared by the author. The founding sponsors had no involvement in the study's design, data collection, analysis, or interpretation, manuscript preparation, or the decision to publish the ndings.