Effects of Plastics and Microplastics on Freshwater Ecosystems
In aquatic ecosystems, the mobility and degradation of plastics potentially generate a mixture of parent materials, fragmented particles of different sizes, and other nonpolymer degradation products. Such complex mixture of plastics and associated chemicals that change in time and space influence the biota significantly .
The first interaction between the biota and microplastics is biofouling, which involves colonization of the particle surface by a biofilm . It may lead to transport of biota and/ or invasive species [113,114]. Another early interaction is ingestion, for example, oysters and mussels. [115,116]. Other possible interactions could be stress responses such as inflammation or oxidative stress at tissue and cellular levels [117,118]. Microplastics may also serve as a medium to expose biota to environmental contaminants adsorbed to their surfaces .
Ingestion of MPs and its Biological Impact
MPs may be taken up from the water column and sediment by a range of organisms directly through ingestion or dermal uptake most importantly through respiratory surfaces (gills) .
One example is the zebrafish (Danio rerio) in which PS particles accumulate in the gills (5 and 20 pm), gut (5 and 20 pm) and liver (5 pm) .
Among the clearest evidence to date that freshwater species are exposed directly to microplastic pollutants is provided by the work of Windsor et al. , who recently documented the presence of microplastics in the guts or other tissues of several species of invertebrates (including insect larvae) (Figure 11.3) collected at locations along the rivers Usk, Taff and Wye in South Wales .
Although plastic is largely excreted following ingestion, evidences suggest that microplastics can be retained in the gut over timescales beyond those expected for other ingested matter . Particles may even cross the gut wall and translocate to other body tissues, with unknown consequences. [11,74]. Some species are capable of rapid excretion while others accumulate and/or mobilize microplastics into their circulation. Sanchez et al.  investigated gudgeon (Gobio gobio) caught in 11 French streams and found MP in the digestive tract of 12% of the fish. However, the rate of MP ingestion in different species of fishes certainly depends on their feeding strategy. Rosenkranz et al.  demonstrated that the water flea Dapbnia niagna rapidly ingested MP under laboratory conditions. MP (0.02 and 1 mm) appear to cross the gut epithelium and accumulate in lipid storage droplets inducing severe effects. Imhof et al.  reported the uptake of MP by annelids (Lumbriculus variegatus), crustaceans (D. magna and Gammarus pulex), ostracods (Notodromas monacba) and gastropods (Potamopyrgus antipodarum). Karami et al. observed histological alterations in the gills and blood chemistry parameters (such as plasma cholesterol levels, blood HDL levels) of African catfish (Clarias gariepinus) at low
FIGURE 11.3 Caddisflv larva with incorporated plastic.
concentrations of HDPE fragments (50 pg/L) and severe changes (like epithelial sloughing, hyperplasia, extensive cell sloughing) at a higher particle concentration (500 pg/L) .
Surprisingly, many species are able to identify particles with nutritional value. Studies with labeled bacteria have shown that some ciliates (estuarine oligotrichs) and flagellates prefer bacteria over MPs, while other species (estuarine scuticociliates; e.g., Uronema narina) cannot discriminate between bacteria and MPs. Therefore, we hypothesize a similar pattern regarding species-specific size and taste discrimination: some species have the tendency of directly feeding on available MPs in the size range of their food, while more selective feeders avoid MP ingestion .
The ingestion of MPs may lead to the development of physical stress in organisms. The extent of physical stress depends on:
- • Particle size: Larger particles are harder to digest.
- • Particle shape: Needle-like particles attach more readily to internal and external surfaces.
- • Physical irritation: Smaller and angular particles are more difficult to dislodge than smooth spherical particles and therefore may cause blockage of gills and the digestive tract.
- • Exposure to secondary MPs: MPS of mean particle size 2.6 pm may cause elevated mortality, increased interbrood period and decreased reproduction at very high MP levels .
Research to date has revealed the ingestion of microplastics in a wide range of species at many organizational levels with different feeding strategies, including filter feeders and predators. Apart from accumulation of particles in organisms at lower trophic levels , there is also evidence for the trophic transfer of microplastic particles, such as from mussels to crabs . The potential of MP transfer from meso- to macro-zooplankton using PS microspheres (10 pm) at much lower concentrations of 1000, 2000 and 10,000 particles/mL was demonstrated .
Because excretion rates are unknown and MP uptake is often defined as particles present in the digestive tract (i.e., the outside and not the tissues of an organism), it is
FIGURE 11.4 Bioavailability.
not yet established whether the trophic transfer of MP also results in a bioaccumulation or biomagnification. However, MPs are transferred from the prey to the apex predator, where, in certain situations, they can be retained for longer periods in the body of the latter . The functioning of the ecosystem is directly influenced by MP physical and chemical characteristics, biological aspects like molecular targets, the bioavailability of the MPs and the penetration of submicron MPs into the cells (Figure 11.4).