The production and use of plastic over many decades has resulted in its accumulation in the world’s oceans. Plastic debris poses a range of potential risks to the marine environment and its biota. Especially, the potential hazards of small plastic debris and chemicals associated with plastic have not been extensively studied. When buoyant plastic is exposed to ultraviolet radiation, it will slowly degrade and leach chemicals into surrounding waters. These leachates can include additives, sorbed organic pollutants, and degradation products of the plastic polymers. While most hazard assessments have focused on studying adverse effects due to the uptake of plastic, toxicity studies of the leachates of plastics are less common. To begin to address this knowledge gap, we studied the acute toxicity of leachates from diverse plastics in the harpacticoid copepod Nitocra spinipes. Our results show that leachates caused a higher toxicity after plastic was exposed to ultraviolet light compared to leaching in darkness. We observed differences in toxicity for different polymer types: polyvinyl chloride and polypropylene resulted in the most toxic leachates, while polystyrene and poly[ethylene terephthalate] were least toxic. Furthermore, we observed increased toxicity of leachates from some plastics that had been weathered in the real marine environment compared to matching new materials. Our results indicate that both weathering condition and polymer type influence the toxicity of plastic leachates.

Since the beginning of the mass production in the 1940s, plastic has been manufactured in quickly increasing amounts. Plastic debris accumulates in the environment and lately much attention has been drawn to the pollution in the world’s oceans. Despite the rapid development and ubiquitous presence of plastic, degradation in the marine environment and potential risks associated with plastic are not fully understood. Thus, these knowledge gaps were addressed in this thesis, which adds information about exposure and hazards of marine plastic debris.

Although sampling studies have been conducted all over the world, the concentrations of plastic debris in the Baltic Sea have hardly been investigated. In Paper I, the level and distribution of plastic debris in the Stockholm Archipelago were assessed. Plastic concentrations were higher close to suspected point sources compared to remote areas. Fibers accounted for the most common form and the predominant polymer types were polypropylene (PP) and polyethylene (PE).

In Paper II, the literature was critically reviewed for the most important degradation pathways for plastic under environmentally relevant conditions. Ultraviolet (UV) radiation, leading to photo-oxidation, is the most important initiating factor for buoyant plastic. Consequently, a UV lamp was used in Paper III for an artificial weathering setup in the laboratory to degrade pristine plastics and analyze the chemical leachates by liquid chromatography-mass spectrometry (LC-MS) for degradation products using a nontarget approach. Carboxylic acids and dicarboxylic acids of polymer fragments were the most commonly identified degradation products of the plastic polymers, confirming predictions made in Paper II.

To evaluate potential hazards posed by leachates from weathering plastic debris to marine organisms, an acute toxicity screening study with Nitocra spinipes was conducted in Paper IV. Field-exposed plastic and the corresponding newly purchased plastic were artificially aged with the same weathering setup as in Paper III. Poly(vinyl chloride) (PVC) and PP leachates were generally most toxic, while leachates from polystyrene (PS) and poly(ethylene terephthalate) (PET) were least toxic among the tested materials. For plastics, which were supposed to contain only few additives, we observed no difference in toxicity between leachates from the field-exposed and the newly purchased plastic. However, the other plastic products exposed to the marine environment were more toxic than their corresponding newly bought products. This indicates that the toxicity of the leachates not only depends on the polymer type, but also on the weathering condition of the plastic.

Buoyant plastic in the marine environment is exposed to sunlight, oxidants, and physical stress, which may lead to degradation of the plastic polymer and the release of compounds that are potentially hazardous. We report the development of a laboratory protocol that simulates the exposure of plastic floating in the marine environment to ultraviolet light (UV) and nontarget analysis to identify degradation products of plastic polymers in water. Plastic pellets [polyethylene, polypropylene, polystyrene, and poly(ethylene terephthalate)] suspended in water were exposed to a UV light source for 5 days. Organic chemicals in the water were concentrated by solid phase extraction and then analyzed by ultra-high-performance liquid chromatography coupled to high-resolution mass spectrometry using a nontarget approach with a C18 LC column coupled to a Q Exactive Orbitrap HF mass spectrometer. We designed a data analysis scheme to identify chemicals that are likely chain scission products from degradation of the plastic polymers. For all four polymers, we found homologous series of low-molecular weight polymer fragments with oxidized end groups. In total, we tentatively identified 22 degradation products, which are mainly dicarboxylic acids.

We collected plastic debris in the Stockholm Archipelago using a manta trawl, and additionally along a transect in the Baltic Sea from the island of Gotland to Stockholm in a citizen science study. The samples were concentrated by filtration and organic material was digested using hydrogen peroxide. Suspected plastic material was isolated by visual sorting and 59 of these were selected to be characterized with Fourier transform infrared spectroscopy. Polypropylene and polyethylene were the most abundant plastics identified among the samples (53% and 24% respectively). We found nearly ten times higher abundance of plastics near central Stockholm than in offshore areas (4.2 x 10(5) plastics km(-2) compared to 4.7 x 10(4) plastics km(-2)). The abundance of plastic debris near Stockholm was similar to urban areas in California, USA, and the overall abundance in the Stockholm Archipelago was similar to plastic abundance reported in the northwestern Mediterranean Sea.

Plastic in the global oceans fulfills two of the three conditions for pollution to pose a planetary boundary threat because it is causing planetary-scale exposure that is not readily reversible. Plastic is a planetary boundary threat if it is having a currently unrecognized disruptive effect on a vital Earth system process. Discovering possible unknown effects is likely to be aided by achieving a fuller understanding of the environmental fate of plastic. Weathering of plastic generates microplastic, releases chemical additives, and likely also produces nanoplastic and chemical fragments cleaved from the polymer backbone. However, weathering of plastic in the marine environment is not well understood in terms of time scales for fragmentation and degradation, the evolution of particle morphology and properties, and hazards of the chemical mixture liberated by weathering. Biofilms that form and grow on plastic affect weathering, vertical transport, toxicity, and uptake of plastic by marine organisms and have been underinvestigated. Laboratory studies, monitoring, and models weathering on plastic debris are needed to reduce uncertainty in hazard and risk assessments for known and field of the impact of suspected adverse effects. However, scientists and decision makers must also recognize that plastic in the oceans may have unanticipated effects about which we are currently ignorant. Possible impacts that are currently unknown can be confronted by vigilant monitoring of plastic in the oceans and discovery-oriented research related to the possible effects of weathering plastic.

Each year vast amounts of plastic are produced worldwide. When released to the environment, plastics accumulate, and plastic debris in the world's oceans is of particular environmental concern. More than 60% of all floating debris in the oceans is plastic and amounts are increasing each year. Plastic polymers in the marine environment are exposed to sunlight, oxidants and physical stress, and over time they weather and degrade. The degradation processes and products must be understood to detect and evaluate potential environmental hazards. Some attention has been drawn to additives and persistent organic pollutants that sorb to the plastic surface, but so far the chemicals generated by degradation of the plastic polymers themselves have not been well studied from an environmental perspective. In this paper we review available information about the degradation pathways and chemicals that are formed by degradation of the six plastic types that are most widely used in Europe. We extrapolate that information to likely pathways and possible degradation products under environmental conditions found on the oceans' surface. The potential degradation pathways and products depend on the polymer type. UV-radiation and oxygen are the most important factors that initiate degradation of polymers with a carbon-carbon backbone, leading to chain scission. Smaller polymer fragments formed by chain scission are more susceptible to biodegradation and therefore abiotic degradation is expected to precede biodegradation. When heteroatoms are present in the main chain of a polymer, degradation proceeds by photo-oxidation, hydrolysis, and biodegradation. Degradation of plastic polymers can lead to low molecular weight polymer fragments, like monomers and oligomers, and formation of new end groups, especially carboxylic acids.