Ecological effects of litter in the marine environment

Marine Litter indicators have been dealt with as pressure indicators. However, in some cases, they could also be seen as state indicators, like the common indicators "Plastics in the stomachs of fulmars” and "Ingestion of plastic particles in turtles”.

The bow-tie analysis for marine litter shows the relationships between the DAPSIR components which need to be considered in a cumulative effects assessment. Human activities have been identified which contribute to the marine litter pressures and have the potential to contribute both individually and cumulatively to biodiversity state changes in the thematic assessments for a number of species and habitats. (See bow-tie section on marine litter).

Although there are some indications of reductions in pressure from marine litter that may, over time, result in a relative improvement in environmental status, quantitative data is lacking. Levels of marine litter are still high and objectives and threshold values have not (yet) been reached, and ecological impacts and impacts on ecosystem services are occurring.
A regional breakdown of the state changes is currently not appropriate, other than a breakdown of the relevant (pressure) common indicators.

Harm to biota

Marine litter (including microplastics) has become a major stressor of marine wildlife and ecosystems, adding another severe impact to the existing human pressures on the marine environment. Marine litter further weakens the ecological resilience and adaptive capacity of marine species and ecosystems and, as a result, their ability to withstand adverse impacts from climate change. Known adverse effects on marine animals worldwide, including in the OSPAR Maritime Area, comprise: ingestion of plastic particles via filter feeding, suspension feeding and consumption of prey exposed to microplastics, or direct ingestion in mistake for food, causing blockages and damage to the digestive tract; reduced nutritional value of food and increased exposure to plastic-associated chemical pollutants through the release of additives incorporated during manufacture as well as absorption of persistent organic pollutants by plastics from seawater; entanglement, especially with filamentous litter items (such as loops, packaging bands or net-like structures, e.g., from derelict fishing gear) leading to immobility or direct injury and death; and the smothering of benthic habitats and generation of artificial hard substrate altering the structure of benthic communities and even impacting whole populations of marine species. Furthermore, floating litter acts as a vector for the transport of biota, including microbes which change or modify assemblages of species.

During the last couple of years, the number of species shown to be affected by marine litter has significantly increased. According to a review in 2020, a total of 914 marine species were documented to be affected by marine litter through entanglement and / or ingestion. Ingestion was documented for 701 species, and entanglement was recorded for 354 species. According to the online database “Litterbase”, some 2 788 marine species have encountered plastics to date. This increase correlates with increasing numbers of studies on harm from marine litter.

As elsewhere in the OSPAR Maritime Area, the number of standardised monitoring procedures for impacts of marine litter on biota is still rather small. Until now, the monitoring protocols for ingestion of plastic particles by northern fulmars and by loggerhead turtles are the only agreed methods for quantifying effects. In contrast to the small number of agreed indicators, there are numerous case studies on biota encounters with marine litter and many direct and indirect consequences have been recorded with potentially lethal and sub-lethal effects. In the recent past, the percentage of biota encounters with marine litter has increased for all taxonomic groups. For further information see the full review giving an overview of the available knowledge on harm to various species in the area of interest (the North-East Atlantic) for different taxonomic classes, including endangered species.

The leatherback turtle (all OSPAR Regions) and the loggerhead turtle (Regions IV and V) are listed among OSPAR’s Threatened and Declining Species. Status assessments (see:  leatherback turtle and loggerhead turtle ) show, although data are still limited, that both species are still in decline and are significantly impacted by by-catch and marine litter.

Evidence for ecosystem scale is available but limited, which is probably due to the fact that such effects are difficult to quantify, especially in combination with other anthropogenic pressures.

An underlying ethical aspect of the above-mentioned biological impacts of marine litter is the issue of animal welfare. According to the European Union´s Treaty (as amended in the Treaty of Lisbon), animals are recognised as sentient beings, meaning that they are capable of feeling pleasure and pain. Marine animals which become entangled by, trapped in, or ingest marine litter often experience trauma, damage, infection and compromised ability to feed, move and carry out their normal behaviour. The resulting suffering and pain create a compelling argument that marine litter represents not only a serious environmental and conservation issue, but also a significant global animal welfare issue. To summarise, it can be stated that the numbers of animals affected by negative interactions with marine litter and the associated suffering affecting their welfare, in combination with the extent of such encounters, which in some animals represents a substantial proportion of a population, clearly show that further reductions in the existing inputs and amounts of marine litter are urgently needed.

From the evidence available on the ecological impacts of marine litter it can be inferred that plastic marine pollution is an increasing threat to marine species and habitats in the OSPAR Maritime Area. Many studies recommend potential indicator species for ingestion and entanglement. Direct harm is in general more frequent for entanglement than for ingestion, since negative effects on individuals are more obvious to detect. Regarding ingestion, mussels and lugworms are interesting, because as sessile organisms they are easy to monitor, and as filter feeders, nearly all species of this order contain microplastic particles. The majority of ingested plastics consist of fragments. Mammals reveal a broad size spectrum for ingested plastics, while birds mostly ingest mesoplastics and fish and invertebrates mainly microplastics. The most visible effect of interaction with marine litter is entanglement of wildlife, often in abandoned, discarded or lost fishing gear or rope (so called ghost-fishing). A new protocol for entanglement of northern gannets and other seabirds is already being applied in some breeding colonies in the North Sea. It classifies the amount of plastic per nest based on the number of items and the associated entanglement rates. Seals typically become entangled round the neck for some time, their entanglement is therefore easy to monitor.

For a comprehensive future assessment, further targets and thresholds need to be developed. This calls for additional standardised sampling protocols, necropsies, plastics categorisation and analyses in order to make observations comparable and to generate time series. The latter serves to prepare the data for statistical analyses, which in turn make it possible to define baselines and detect trends. Both baseline and trend analyses serve to implement targets and threshold values for marine litter ingestion and entanglement. As the next step, additional litter type-specific measures need to be implemented, and their results, again, can be controlled by time-series analyses.

Targeted measures to avoid ingestion of plastics are difficult to develop, because fragments are non-identifiable litter types. By contrast, the entanglements of all groups of species most often occur in filamentous litter, which in turn mainly consists of fishing-related litter types, such as the remains of nets, fishing lines, dolly ropes and ropes. Packaging (e.g., strapping bands) has been identified as a further cause of entanglement. The available evidence needs to be assessed carefully on a species basis. As an example, there have been reports of stray finds of northern gannets, often with their beaks entangled in plastic bags. A target to reduce plastic bags, in line with the EU Plastic Bag Directive (Directive (EU) 2015/720), therefore represents a measure which is easy to implement and effective in reducing mortality among northern gannets. Another example is a prohibition on the use of dolly ropes. If applied widely, preferably over the whole of the North-East Atlantic, this would lead to substantial decreases in the use of strings as nesting material and in the associated mortality among seabirds.

To conclude, quantifying the effects of marine litter on biota is a complex task, especially when evaluating multiple species with different ecological requirements. As a next step the available data for the different species could be evaluated with a view to assessing the risk of plastic marine-litter ingestion and entanglement by integrating inter-species factors such as plastic exposure rates and life history traits (e.g., mortality, habitat, and body size). This would require a modelling exercise to identify and estimate their exposure to plastic litter across the OSPAR Maritime Area, using the data in the literature, species distribution maps and plastic dispersion models in order to identify hotspots for the risk of plastic ingestion and entanglement for the chosen species.

In a next step, dose-response relationships would have to be established in order to develop additional threshold values and targeted measures.