Various animals around the globe are frequently reported being entangled to marine litter, such as discharged fishing nets, ropes, plastic bags and six-pack drink holders. When trying to free themselves from litter, animals may get injured or die – especially abandoned or lost fishing gear are responsible of countless deaths of marine mammals, seabirds, fish and invertebrates annually. Even though the effects of entanglement are known to be deleterious for individuals, the knowledge on population-level impacts is limited. Entanglement has a potential to further decline populations of endangered species and threaten the sustainability of fish stocks.
Toxicity of plastics
Many ingredients used in plastic production, such as some monomers and additives, are considered harmful. When plastic enters the environment and weathers, these ingredients are able to leach out from the plastic matrix to the surrounding media. Common leachates found from the environment include for example bisphenol A, phthalates and brominated flame retardants, which have in laboratory settings observed to leach out from plastic. The leaching potential of harmful substances is dependent on the plastic type, properties of the leachate and environmental conditions.
Impacts of hazardous substances
Plastic litter can contain harmful substances derived from the manufacturing processes and adsorbed from the environment; therefore, plastic is assumed to act as a vector for hazardous substances to biota. The intake of harmful compounds may be passive via skin and respiratory surfaces or occur actively through ingestion of plastics. Both plastic leachates and adsorbed chemicals are observed being able to cause acute and chronic toxic effects. However, the relative importance of harmful substances in plastics compared to other exposure pathways is still under research.
Many animals including various marine invertebrates, shellfish, fish, birds and mammals are known to ingest marine litter. Plastic ingestion may cause slow sublethal physical and chemical effects or direct mortality. Rapid death may occur if the gastrointestinal tract is completely blocked or severely damaged by ingested plastics, whereas chronic effects may take place if ingested material leaches harmful substances inside the animal. Although the research on prevalence of especially microplastics in marine fauna has recently arisen around the globe, the knowledge about their effects in natural populations is still limited. It is possible that microplastics and associated substances can be transferred along the food web ending up even to the highest trophic levels.
Smothering is known to cause harm to benthic habitats and communities. Plastic litter on the seafloor inhibits the gas exchange between sediment and overlying water resulting in local hypoxia or anoxia and therefore also affects the faunal composition in the sediment. Litter impairs the photosynthetic activity of plants, algae and other organisms capable to photosynthesize and can have long-lasting effects to the vegetation on the seafloor. For example the recovery of seagrass beds after smothering event may last even more than 17 months. Smothering can also have an impact on the feeding behavior and efficiency of animals and change the community structure by providing a growing platform to organisms that could not otherwise occur in a certain habitat.
Floating litter can host a vast number of organisms, such as barnacles and bryozoans, on its surface. Various hitch-hikers attached to plastic can be transported by the winds and currents wherever the litter is accumulating and end up to areas outside their natural range. These invasions may lead to drastic changes in the ecosystem, if the new species are able to settle in new areas. It is suggested, that the increasing amount of plastic in our oceans would enhance the rafting opportunities of many species from macro-sized invertebrates to micro-organisms including potential pathogens.