the water column and floating on the sea surfaces are a lighter density than the sea surface, e.g. polyethylene, polypropylene, polystyrene (Hidalgo‐Ruz et al. 2012). Although plastic is treated as non‐toxic because of its less reactive nature (Hwang et al. 2020).
3.1.1 Microplastic in the Marine Food Web
Due to wide distribution and unmanaged dumping (some through rivers) in the ocean, MP is a common ailment. However, European countries are more prone to this problem because they have busy sea routes as well as industries near coastal areas. The MPs are added to the ocean from terrestrial sources along with the secondary MPs from larger submerged plastics. Ballast water from ships release huge amounts of MPs. Those MPs are hotspots of toxic chemicals, pathogens, harmful algal bloom, etc. (Naik et al. 2019). Microplastic is of serious concern due to its wide distribution from pelagic to benthic marine biota (Thompson et al. 2009). These are a great matter of concern as they are affecting every segment of the ecosystem. They become attached to the planktons and cause disturbances in performing photosynthesis, and make a film over the water surface and provide the breeding ground for bacterial pathogens. The transfer of MPs from one trophic level to the next is a big concern. Another serious threat is the biomagnification of MPs along with the associated chemicals to the successive trophic level (Walkinshaw et al. 2020). The associated chemicals of MPs have a large area‐to‐volume ratio, which absorbs hydrophobic pollutants from the surrounding marine environment (Figure 3.1) (Smith et al. 2018). The most serious threat is of bioaccumulation of heavy metal in the presence of micro MPs at every trophic level of food chain, and which may lead to biomagnification of toxic heavy metals among the higher‐level organism in food chain, which have a high probability of being eaten by the human population.
3.1.2 Toxic Impacts on Primary Producers
Phytoplankton are considered the main contributor to the primary productivity in the oceans, which fixes almost half of the carbon dioxide of the earth during photosynthesis process by using photosynthetic active radiation (PAR) from Sun and carbon dioxide (Uitz et al. 2010). The MP deposition over phytoplankton decreases the chlorophyll concentration, photosynthesis, cell growth, and morphology of phytoplankton. Microplastics deposited upon the phytoplankton penetrate the cell walls and interfere with the chlorophyll mechanisms in green algae (Nerland Bråte et al. 2014). Phytoplankton absorb persistent organic pollutants released during the degradation of MPs which is further transferred along the marine food web (Chandra et al. 2020). This hazardous chemical has the property of bioaccumulation in successive trophic levels, which causes toxicity to them. When MPs are deposited over harmful alga, they release phycotoxin, which is transferred to phytoplankton, bivalves, and crustaceans (Sharma & Chatterjee 2017). The toxins are then bioaccumulated in their bodies and move to the next trophic level. These are then consumed by humans, which may result in many health issues. The coral reef, which has the highest biodiversity in the tropical shallow parts of marine realm, is also badly impacted by MPs. These coral reefs have the mutual collaboration of algae and fungi. However, most of the time, algal partners depend on phytoplankton, zooplankton, etc., for their food requirements, but they are confused by the colorful MPs and consume them. The digestive tracts of coral reefs (coral polyps) cannot deal with MPs and they have very harmful impacts on their health.
Figure 3.1 Flow diagram of the fate of plastic entering the environment.
These phytoplankton are trapped as part of the marine snow and an important constituent of marine organic matter, this organic matter is taken as food by benthos and nektons. Marine algae aggregates over the floating MPs and settle down to the sediment water interface. This reduces the residence time of floating organic matter in the water column, which in turns lowers the food availability to those organisms residing in the water column. Furthermore, hetero‐aggregates of MP and phytoplankton are consumed by zooplankton and have harmful impacts to them. These MP contaminated zooplankton are bioavailable to the predator and transferred to successive trophic levels. In these ways MP potentially disturbs the food transfer and, most importantly, reduces the energy flow from primary producers level.
3.1.3 Toxic Impacts on Consumers
Free‐floating zooplankton are key members of the marine food web, as they are the connecting link in transferring energy from producer to consumer level in the food chain, and then to the higher trophic levels. They feed upon phytoplankton and the MPs present over the surface of the phytoplankton are accumulated into their bodies (Cózar et al. 2014). Zooplankton consist of many species and have different life cycle stages with a wide range of feeding mechanisms (Wirtz 2012). Studies from lab experiments revealed that these zooplankton are capable of absorbing tiny plastic latex beads (Cole et al. 2013) as MPs of <5 mm have been found in 15 different taxa of zooplankton, from copepods to jellyfish. Ciliated heterotrophic planktons engulf MPs by phagocytosis (Laist 1987). The zooplankton ingests MPs particles, which may either pass through their digestive tract or get stuck in the gut, causing disturbed digestive health, such as lack of hunger due to feeling of filled stomach by MPs, and behavioral changes, which ultimately leads to their death. Sometimes these MPs are successfully excreted from the zooplankton body in the form of pellets (and become part of marine snow), and are distributed to the water column and ultimately settle to the bottom. These MP‐contaminated pellets are then available for benthic organisms. The benthic invertebrates comprise 98% of overall marine biota, which includes oysters, blue mussels, barnacles, lobsters, etc., and these have all been reported to have MPs in them (Nerland Bråte et al. 2014).
According to Possatto et al. (2011) and Lusher et al. (2013), 30% of the fish species that humans extensively consume, including sea bass, are contaminated with MPs. The main exposure route of the MPs in fishes is ingestion during feeding upon MP‐contaminated phytoplankton and zooplankton, or direct ingestion due to confusion with prey. Gills are another exposure root to MPs for marine creatures. This organ serves the purpose of osmoregulation, acid–base regulation, gaseous exchange, and nitrogenous exchange. Interaction of MPs to the gills may cause partial or total blockage, which disrupts these functions and causes fatal harm to the organism (Watts et al. 2016). These MPs often accumulate in their bodies, causing starvation, hormonal imbalance, behavioral changes, and malnourishment, ultimately leading to fatality (Welden & Cowie 2016). Sometimes MPs stuck in the gills of organisms, cause suffocation, and even death of organism. Microplastics of larger size (5 mm) are more harmful as they remain for a longer time in the fish body compared with smaller size (2 mm). Smaller sized MPs are easily excreted with feces. Many of the organisms extensively consumed by humans are reported with MP contamination during field studies, some of them are mentioned in Table 3.1.
Sea birds feed upon fishes. Sometimes they take contaminated fishes with MPs accumulated in them. Sea birds like Albatross and Shearwater feed on the ocean surface and in that way, they take in a huge amount of floating MPs inside their gastrointestinal tract. Ryan (2008) found the presence of MPs in south Atlantic birds. These MPs have many negative impacts on their bodies, including starvation due to lack of hunger caused by MPs accumulated in their gastro‐intestinal tract, and blockage of respiratory organs like gills leading to suffocation and ultimately death. The sediment water interface is the main hub of these artificial polymers due to sinking and sedimentation. MPs have lower density than the oceanic water; however due to biofouling (deposition and colonization of microorganisms on any surface exposed to them in water) by microorganism they settle down. Biofouling increases the density of MP and make it more dense than the oceanic water, although this may take up to a week, a month, or more than year (Fazey & Ryan 2016). These phenomena are highly surface area‐to‐volume ratio dependent, as smaller fragments have more surface area‐to‐volume ratio than the larger debris
