Déchets sur les plages - Abondance, composition et tendances

The OSPAR indicator on beach litter is used to identify changes in the input of litter to the marine environment. The assessment is based on a time-series of abundance and type of litter items collected on 100-m stretches of coastline.

What is meant by marine litter?

Marine litter (marine debris) is any persistent, manufactured or processed solid material discarded, disposed of, abandoned or lost in the marine and coastal environment. It includes items entering the marine environment via rivers, sewage outlets, storm water outlets and windblown litter items.

What information do beach litter surveys supply?

Surveys that record all litter items found on a given stretch of beach can be used to assess marine litter pollution. The data collected provide information on amounts, trends in abundance, and sources of marine litter.

The abundance of marine litter at OSPAR survey sites provides information on the magnitude of litter pollution in adjacent waters and coastal areas and can indicate spatial differences in litter pollution. The litter on a given OSPAR beach may be generated locally at sea or on land, but may also arrive via ocean currents, rivers and wind drift from distant sources.

The composition of the litter recorded on the beaches provides information on the frequency of different litter items (e.g. plastic bags, metal tins) or litter categories (e.g. plastic, wood). This in turn provides information on the source of the litter and to some extent its level of threat (wood is not as harmful to the marine environment as plastic).

Spatial differences in litter composition can reflect regional differences in sources or human activities. However, it is important to note that a given survey site or region receives litter from a number of sources. These may be local, regional or distant. Changes in composition can provide information on changes in sources and litter type (i.e. certain litter items becoming more abundant – such as plastic bottles replacing glass bottles – or the introduction of new litter items).

Trends in the abundance of individual litter items or litter types (e.g. fisheries related items) and in total litter abundance provide information on changes in the level of pollution and on the effectiveness of measures introduced to reduce marine litter.

Proportion of marine litter assessed?

The litter encountered on beach litter survey sites can originate from the sea, i.e. flotsam (wreckage of a ship or its cargo) transported to and deposited on the coast by winds and water currents, or can be directly deposited on the coast by humans, for example, tourists, fishermen, or fly-tippers. Litter can also be deposited further inland on riverbanks or be discharged directly into rivers and then transported by rivers into the marine environment.

In the period between any two beach litter surveys, litter items that have been washed ashore by tides or deposited directly on to the beach may be buried, or removed by subsequent tides and winds. However, buried litter can also resurface during strong wave action (Williams and Tudor, 2001) and litter can be blown onto the beach from adjacent land or streets. Therefore, the amount of litter items recorded during any one survey, will constitute a minimum value for litter deposited at the site.

The litter washed ashore is biased towards litter items that float and that do not disintegrate, dissolve or quickly decay in the marine environment. The main category of litter found on beaches in the assessment period was ‘plastic and polystyrene’, which often floats and does not disintegrate rapidly in water. The main components of the other common litter categories all float and / or decay slowly (wood, bottles, jars, lightbulbs, paper cartons, tins and cans). The composition of litter recorded on beaches thus reflects its ability to reach the shore. Metal and glass are therefore under-represented because they are more likely to sink than items made of plastic, polystyrene, rubber and wood. Paper is under-represented because generally disintegrates more rapidly in water than the other items.

Although only a fraction of what is disposed of into the sea reaches the shore, the abundance of the items found can be used as an indication of the level of marine litter pollution in a given area, if it is assumed that the floating element of the litter represents a constant proportion of the total litter input. Surveys of litter items deposited on the seafloor (by recording litter caught in trawls during fish stock surveys) are used to identify trends in items that do not float (see assessment of Composition and Spatial Distribution of Litter on the Seafloor).

Development of the OSPAR beach litter monitoring (2001–2014)

The OSPAR beach litter monitoring began as a pilot project in 2001. The method for monitoring marine beach litter was initially developed by experts of the Beachwatch and Coastwatch projects and was agreed upon by the OSPAR Pilot Project Steering Group in 2000. Based on experience from the field surveys, the survey protocol was subsequently amended and updated by the Pilot Project Steering Group in 2001, 2002 and 2009. The OSPAR Guideline for Monitoring Marine Litter on the Beaches in the OSPAR Maritime Area was published in 2010 (OSPAR Agreement 2010-02).

Between 2009 and 2015, OSPAR beach litter surveys were undertaken in Belgium, Denmark, France, Germany, Ireland, the Netherlands, Norway, Portugal, Spain, Sweden and the United Kingdom.

The number of survey sites and the number of surveys have both increased since the start of the monitoring programme in 2001. The increase was particularly noticeable after 2011, in response to the implementation of the EU Marine Strategy Framework Directive (MSFD) (Figure a). The present network of survey sites is more comprehensive (Figure b). The number of survey sites and surveys peaked in 2013 (77 survey sites and 268 surveys) before falling due to reduced surveying in the Celtic Seas area.

Despite an overall increase in the number of survey sites, the specific sites used and the number of surveys undertaken per year at those sites have varied. As a result, the beach litter data have been assessed in two ways:

• Trends in the abundance of litter items have been calculated using data for the period 2009–2014, collected from the 19 most consistently monitored survey sites.
• Abundance and composition have been calculated using data for 2014 and 2015, collected from 76 survey sites (because these data provide greater spatial coverage).

OSPAR Litter Monitoring Programme

In 2010, OSPAR published a Guideline for Monitoring Marine Litter on the Beaches in the OSPAR Maritime Area (OSPAR Agreement 2010-02), as a tool for standardising the collection of litter data in the marine environment.

Within the OSPAR Litter Monitoring Programme, litter is counted on standard stretches of coastline (survey sites) situated in eleven countries (Belgium, Denmark, France, Germany, Ireland, the Netherlands, Norway, Portugal, Spain, Sweden and the United Kingdom) on the North-East Atlantic coast. The data used for the assessment were collected during surveys on 100-m stretches of coastline. The distribution of the 100-m beach litter survey sites is presented in Figure b.

The reference beaches mainly comprise sand or gravel and are exposed to the open sea. They are in most cases accessible to surveyors all year round for surveying and litter removal. However some sites, such as in the north of the OSPAR Maritime Area, are not accessible or not possible to survey during the winter survey period. Beaches selected have a minimum length of 100 m, are generally free of buildings all year round, and are (in most cases) not subject to litter collection activities (beach cleaning). National coordinators of the surveys have used expert judgement and local knowledge of coastal areas when selecting reference beaches. For example, in some countries local conditions do not allow for selection of beaches mainly comprising sand, and in some locations it is not possible to select beaches of 100 m in length.

The start and end points of the survey sites are marked clearly, to ensure that exactly the same site is monitored for all surveys. Reference beaches are (normally) surveyed four times a year.

The survey periods are as follows: winter (between mid-December and mid-January), spring (April), summer (between mid-June and mid-July), and autumn (between mid-September and mid-October).

The surveys on all reference beaches within a given region should be carried out within the shortest timeframe possible within a given survey period. Coordinators within the regions arrange the survey dates bilaterally. Ideally, a given beach would be surveyed on the same days each year.

Survey sites are a 100-m long section of beach. The entire area from the waterline to the back of the beach is surveyed, with the back of the beach determined by the presence of a dune or a sea wall, for example.

Data on the amount of litter on a given stretch of coastline is recorded at item level. Items to be recorded are predefined by the Guideline for Monitoring Marine Litter on the Beaches in the OSPAR Maritime Area (OSPAR Agreement 2010-02). Items comprise identifiable pieces of litter, including items such as plastic bottles, tetra-pak containers, metal drinks cans, and rubber gloves. The items recorded are assigned to specific categories according to the material from which they are made (e.g. plastic, wood, metal) or their use (e.g. medical, sanitary). In total, there are 112 predefined litter item types: Plastic / polystyrene (54 items), Metal (15 items), Paper and cardboard (9 items), Wood (9 items), Sanitary waste (6 items), Cloth (5 items), Rubber (4 items), Glass (3 items), Pottery/ceramics (3 items), Medical waste (3 items) and Faeces (1 item).

It is important to note that some litter items categorised as sanitary and medical are made of materials listed individually. For example, the litter item cotton bud sticks, which are a major component of beach litter in some regions, is included in the sanitary category despite being mainly made of plastic. In this case, the litter item has been captured in a separate category to link the item type to its sources, with the ultimate aim of enabling the development of suitable litter reduction measures.

Each litter item has a unique OSPAR identification number. In each survey for a given beach the number of individual pieces of litter is recorded for each litter item. The survey forms allow for the registration of identifiable items, unknown items and litter fragments in different size categories. Litter items which do not fit into a definite category, are registered under the litter item ‘other’ for the given material or use category. A short description of these items is included on the survey form. Multilingual photo guides are available to assist surveyors with the identification and categorisation of litter items. All litter items are removed from the beach during the survey.

Although the beaches surveyed are generally sandy, several other beach types are included in the OSPAR Litter Monitoring Programme (OSPAR Agreement 2011-01). These include rocky, boulder and shingle beaches as well as beaches with different levels of pebble, rock, shingle and vegetation coverage. Physical characteristics of the survey site influence deposition and retention levels of litter, for example rocky coasts with sandy bays have the potential to accumulate and retain more litter than long open sandy beaches. Furthermore, water currents, weather conditions and prevailing wind conditions, have a significant influence on deposition and retention of litter and thus litter abundance. This is also the case for beach use and proximity to point and diffuse sources such as towns.

For these reasons the location and physical characteristics of each reference beach are recorded in detail. This information includes the proximity of possible sources of marine litter to the beach, as well as other factors that could help explain the amount, type, and composition of marine litter found. This information is stored in the OSPAR Beach Litter Database. Such parameters should also be considered in future assessments.

Assessment method

Data from a six-year period (2009–2014) were used to assess trends in abundance of litter items, trends in groups of litter items and trends in material categories. The present distribution of survey sites and the availability of survey data at these sites, allow trend assessments to be undertaken for the southern North Sea, the Swedish west coast and the Spanish north-west coast (Table aandTable b).

An assessment of the composition of litter, using all available data from surveys in the years 2014 and 2015 (Table c), is presented for the Celtic Seas (22 beaches), Bay of Biscay / Iberian Coast (approximately 21 beaches), Arctic Waters (5 beaches), Northern North Sea (11 beaches, six of these in Sweden) and Southern North Sea (17 beaches).

Assessment for the period between 2009 and 2014 (abundance, composition and trends)

Abundance

The total number of litter items recorded per 100-m survey varies by an order of magnitude between survey sites. The average abundance of recorded items are highest on the sites on the Swedish west coast (873 items per 100 m). The abundance of litter items is similar for sites on the Southern North Sea (380 items per 100 m) and the north-western coast of Spain (432 items per 100 m), however, the composition of the litter in the two areas differs considerably.

Most common items

Plastic / polystyrene fragments (<50 cm) (OSPAR ID 301) and plastic nets and ropes (OSPAR ID 300) are the most common items on the survey sites in the Southern North Sea, with each item contributing a third of all litter recorded there. Plastic caps and lids (OSPAR ID 15) made up a further 7% of all items recorded (Table d).

Cotton bud sticks (OSPAR ID 98), plastic nets and ropes, and plastic caps and lids are the most common items on the survey sites in the north-western coast of Spain, each item contributing about a fifth of all litter recorded. Plastic / polystyrene fragments (<50 cm) made up a further 11.2% and cigarette butts (OSPAR ID 64) a further 5.9% of all items recorded.

Plastic industrial packaging (OSPAR ID 40), nets and ropes, and plastic caps and lids are the most common items on the two survey sites on the Swedish west coast, with each item contributing 12–15% of all litter recorded. Plastic / polystyrene fragments (<50 cm), cotton bud sticks, and plastic tangled nets and string (OSPAR ID 33) each made up approximately a further 8% of all items recorded.

All of the most common items are considered especially harmful to the marine environment, mainly owing to their potential for entanglement, ingestion or injury for marine animals. However, all litter items can also cause socio-economic impacts, through decreases in revenue and by incurring additional costs, such as beach cleaning.

Trends

On survey sites on the north-west coast of Spain, the abundance of cotton bud sticks (OSPAR ID 98) decreased significantly (by 17.8 pieces per year), whereas the abundance of plastic polystyrene pieces (<50 cm) (OSPAR ID 301) increased significantly (by five pieces per year). Three other items decreased significantly in the assessment period: plastic crisp / sweet packets and lolly sticks (OSPAR ID 19), plastic drinks bottles and containers (OSPAR ID 4), and plastic lobster pots (OSPAR ID 26). A small but significant decrease was recorded in the abundance of industrial scrap metal (OSPAR ID 83) (Table d).

The category sanitary items (mainly cotton bud sticks) also showed a significant decrease over the assessment period (2009–2014). Furthermore, recordings of litter items included in the category rubber showed a significant downward trend but the decrease was small. However, rubber items are not among the 15 most recorded items at any of the survey sites.

For the Southern North Sea only the category paper / cardboard (including cigarette butts) showed a significant decrease but was only in the top 15 most recorded items at one survey site in the region.

No significant trends in the abundance of litter items were measured on the west coast of Sweden.

Assessment for the period 2014–2015 (abundance and composition)

Abundance

The average total number of litter items recorded per 100-m survey in the period 2014–2015 was about the same for the Southern North Sea, Celtic Seas and Bay of Biscay / Iberian Coast (Table g). Whereas, it is generally higher at the Northern North Sea survey sites, which are mainly located in the Skagerrak area (Table h). However, the average total number of litter items recorded per 100-m survey on the 11 Northern North Sea sites is extremely variable. The difference between sites ranges between 14 and over 23 000 items per 100 m and the difference between surveys at the same site ranges between 220 and over 23 000 items at its most extreme.

Distribution and number of survey sites

There is a lack of information on beach litter for large parts of the North-East Atlantic coastline and the distribution, as well as the number of survey sites, needs to be optimised for future assessments. The OSPAR Litter Monitoring Programme (OSPAR Agreement 2011-01) surveyed 7.7 km of coastline in the year with most surveys and survey sites (2013). For some sub-regions, more survey sites are needed in data-poor coastal areas in order to improve the reliability of the results. The Bay of Biscay is lacking survey sites and the number of survey sites in Arctic Waters is also low. Coverage on the west and north-east coasts of the North Sea should be improved; large gaps in the distribution of sites are widespread on the coasts of England and Scotland.

Improved identification of sources

lthough some major sources of litter are apparent form the analysis of the data for the periods 2009–2014 and 2014–2015, a detailed identification of sources using the OSPAR beach litter survey data will require the allocation of particular OSPAR litter items to given sources at sub-regional level. The present allocation of sources in Litter Analyst is not precise enough to be used to identify sources for the OSPAR assessment units. Use of a ‘Matrix Scoring Technique’ to determine litter sources (e.g. Tudor and Williams 2004) should be considered on the basis of the report produced by the EU MSFD Task Group on Marine Litter (Veiga et al., 2016).

Improved knowledge of harm caused

The simple characterisation of harmful litter items used in this assessment (some items are considered especially harmful due to the danger of entanglement, ingestion or injury) will need to be reassessed on the basis of a report by the MSFD Task Group on Marine Litter (Werner et al., in prep).

Reference levels and baselines

Reference levels and baselines have not been defined for beach litter.