3 Executive Summary Polystyrene (PS) foam can be found littering habitats in the Great Lakes and St. Lawrence River basin. More commonly known as Styrofoam®, polystyrene foam is widely available, cheap, and often used in food and beverage containers, building insulation, and floating docks. One way polystyrene foam pollutes the Great Lakes is through the fragmentation of expanded polystyrene (EPS) and extruded polystyrene (XPS) used in docks and floats. Polystyrene contamination - When unencapsulated PS foam docks and floats come into contact with their surroundings – sun, wind, waves, ice, and burrowing animals – the foam can break apart and be released into the environment. Small foam pieces are microplastics (plastic <5mm), which are persistent in the environment and pose a risk to fish and wildlife. Widespread and global contamination has resulted in PS foam being found in the gut contents of wildlife, including Great Lakes – St. Lawrence River species. PS foam has been recorded in the gastrointestinal tracts of several species of fish and birds from the Great Lakes (Brookson et al., 2019; McNeish et al., 2018; Thaysen et al., in review; Wagner et al., 2019). Fish and wildlife in habitats around the world are contaminated with PS. PS foam is one of the top items of debris found on shorelines, beaches, and surface water around the world, including the Great Lakes – St. Lawrence River basin. Over 500,000 foam pieces were collected in shoreline cleanups on the Great Lakes recorded by Great Canadian Shoreline Cleanups (GCSC) and the International Coastal Cleanups (ICC) in 2016-2018. In 2019, shoreline clean-ups on Georgian Bay recorded polystyrene foam as the most common litter item and collected over 5,000 pieces in nine shoreline cleanups. Polystyrene foam is one of the top items collected in Seabins on Lake Ontario. Important volunteer efforts continue to remove some of this litter, but polystyrene foam pollution is still widespread. The problem - PS foam can hurt wildlife by ingestion through physical damage, including blockage and abrasion, and through exposure to chemicals. PS foam can contain two types of chemicals: (1) additives and polymeric raw materials (e.g. monomers) originating from the plastics, and (2) chemicals adsorbed from the surrounding environment. Overtime, these chemicals can leach out of plastics and often these leachates can act as toxic or endocrine disrupting chemicals (Hermabessiere et al., 2017). Large PS foam litter collected, 2019 Fragmented PS foam shoreline litter, 2019 Litter source: Unencapsulated PS dock foam Litter source: Unencapsulated PS foam, fragmenting When ingested, PS microplastics pose adverse effects to wildlife. Laboratory experiments show negative impacts of PS on growth, survival, feeding and swimming behaviour, hepatosomatic index (HSI), and reproduction (Cole et al., 2015; Sussarellu et al., 2016; Qiang and Cheng 2019; Yu et al., 2018). Under certain conditions, PS foam leaches styrene, benzene, and ethylbenzene which have known toxic properties (Thaysen et al., 2018). The leaching of PS monomers is one of the reasons why there is greater concern with polystyrene relative to other types of plastic. PS foam impacts aesthetic enjoyment of the Great Lakes and navigation. In some instances PS docks and floats have been banned due the hazards of PS “icebergs” to boat traffic.