20 3.7 Effects Overall, thousands of studies have tested the effects of microplastics, and PS is one of the most widely tested polymers (Bucci et al 2019). The available data on microplastics in freshwater and marine species is dominated by investigations at the organismal and sub-organismal levels (Bucci et al 2019; Rochman et al. 2016). Increasing data is also available on the toxicity of nanoplastics (plastic particles <1um), although methods to detect particles smaller than 300um in the environment are still lacking (Covernton 2019). Laboratory studies suggest that fish as well as benthic and invertebrate taxa will ingest polystyrene microplastics if they are introduced under experimental conditions. These include fish (Jabeen et al. 2018; Qiao et al 2019; Qiang and Cheng 2019), polychaetes (Leung and Chan 2018), benthic macro invertebrates (Redondo-Hasselerharm et al. 2018), copepods (K.-W. Lee et al., 2013), zooplankton (Cole et al., 2015; Schür et al., 2020), lugworms (Besseling et al., 2013), nematodes (Lei et al. 2018b), frogs (De Felice et al. 2018 ), crabs (Yu et al 2018), oysters (Sussarellu et al., 2016), and algae (Mao et al. 2018). As the size of microplastics decreases, the potential for particles to transfer outside of the gut and into other tissues is expected to increase. This transfer, also called translocation, may facilitate bioaccumulation or even biomagnification in food webs, although the size of particles that are able to translocate between tissues remains unclear. Some studies have demonstrated translocation of PS 0.5 – 9.6 um, however, a study by Sussarellu et al. (2016) using Crassostrea gigas (Pacific oysters) showed no evidence of PS sphere (2 and 6 μm) translocation. The highest concentrations of microplastics are often detected in the gastrointestinal tracts. Lower concentrations have been detected in other tissues such as the liver or fish tissue. There is significant data available on the impacts of PS microplastics exposure on reproductive, developmental, and feeding processes in animals, showing mainly effects on the liver, gastrointestinal tract and on growth. The studies that have investigated effects in freshwater and marine species are outlined below. 3.7.1 Freshwater species Due to the prevalence of PS foam in the Great Lakes St. Lawrence River Basin, a literature search was done to investigate the known toxicity of PS microplastics on freshwater species. Examples of effects demonstrated include reduced feeding behaviour and growth, mortality, changes to lipid composition, oxidative stress, changes to swimming behaviour, and reduced reproductive output (Table 3). In these studies, exposure to PS microplastics is most likely via ingestion. Limited data are available on other paths of exposure, such as PS foam leachates, such as work by Thaysen et al., (2018). The significance of these pathways in the environment is unclear, as leachates may only occur under certain conditions and PS foam can contain different chemicals used as ingredients or adsorbed from the environment. However, due to the known toxicity of additives and the potential to leach, leachates are additionally a source of exposure for potential effects. Acute (4-hour) and chronic (21-day) toxicity studies Aljaibachi and Callaghan 2018 Study in Daphnia magna (Water flea) comparing short- and long-term toxicity of PS (2 μm) Findings: • Impacts on feeding behaviour, mortality • No impact on reproduction Supporting files: • Peer reviewed study Acute (developmental stages 36-46) toxicity study De Felice et al. 2018 An evaluation of blue PS microplastic (2.75 μm) in Xenopus laevis (African clawed frog) tadpoles Findings: • PS observed in digestive tracts • No impact on mortality, body growth, or swimming activity Supporting files: • Peer reviewed study STUDY DESCRIPTION CITATION FINDINGS & SUPPORTING FILES Table 3: Summary findings in studies that investigate the effects of PS microplastics to freshwater species.