Trends and status in UK seafloor litter
Overall, litter is widespread on the seafloor across the entire UK, with plastic the predominant material. More data is required to make a full assessment of the trend in the amount of seafloor litter in specific areas of UK waters. There were higher amounts of litter and plastic per square kilometre of seafloor in the Greater North Sea compared to the Celtic Seas, and higher amounts of litter in the southern parts of the Greater North Sea and Celtic Seas which could be a result of increasing human pressures.
Background
Marine litter represents a global problem, with increasing quantities documented in recent decades. The abundance of litter on the seabed is influenced by human inputs and rivers and currents, which can transport and redistribute litter over long distances making it a transboundary problem.
Marine animals can ingest or become entangled in litter on or near the seafloor. Plastic items can also damage or smother areas of the seafloor destroying fragile benthic habitats, preventing photosynthesis and the movement of animals and nutrients.
The presence of litter on the seafloor is a well-documented issue affecting both coastal and deep oceanic waters. Where appropriate, data from trawl surveys, typically carried out for fish stock assessments, can be used to monitor the amount of litter on the seafloor.
Further information
Definitions
Marine litter is any persistent, manufactured or processed solid material discarded, disposed of or abandoned in the marine and coastal environment. Marine litter consists of items that have been made or used by people and deliberately discarded or unintentionally lost into the sea and on beaches including such materials transported into the marine environment from land by rivers, drainage or sewage systems, or winds. For example, marine litter consists of plastics, wood, metals, glass, rubber, clothing and paper. This definition does not include semi-solid remains that include mineral and vegetable oils, paraffin and chemicals that sometimes litter sea and shores.
Litter on the seafloor has been studied in both coastal and deep-sea oceanic waters. The presence of large amounts of plastic litter has been reported in European continental shelf seas. On continental shelves, trawl surveys are a practical way to monitor seafloor litter because they are already ongoing for fish stock assessments, cover a wide surface, and collect a suitable quantity of litter for analysis (Figure 1).
Figure 1. Marine litter bycatch on board of the RV Cefas Endeavour
Benthic trawls are designed to capture marine biota on or near the seafloor over a range of different bottom types. As a result, some trawl designs will plough through while others roll over the seabed. This interaction with the seabed together with the mesh size will influence the amounts of litter captured. The sampled quantities are thus not absolute numbers but represent a relative number allowing us to compare regions sampled with similar gear. For this assessment, only fishing surveys using a Grande Ouverture Verticale trawl in the UK Exclusive Economic Zone waters were considered. The number of stations will determine the confidence we can apply to our assessments and define the time we will need to obtain a certain confidence.
“Harm” can be divided into three general categories:
- social - reduction in aesthetic value and public safety
- economic - cost to tourism, damage to vessels, fishing gear and facilities, losses to fishery operations, cleaning costs
- ecological - mortality or sublethal effects on plants and animals through entanglements, captures and entanglement from ghost nets, physical damage, ingestion that includes the uptake of microplastics and the release of associated chemicals, facilitating the invasion of alien species and altering benthic community structure
Definitions of the acceptable levels of harm in these categories and good environmental status must consider impacts as assessed by the amount of litter in different compartments of the marine environment (seabed, sea surface, water column and coastline), ecological effects of the litter (plastics ingested by marine organisms and entanglement rates and problems associated with degradation of litter into microparticles) as well as social and economic aspects. An overriding objective will be a measurable and significant decrease (for example, 10% per year for litter on coastlines) in the total amount of litter in the environment by 2020.
Assessment method
For each survey the following information was recorded: the definition and specification of the survey, the positions of stop and start of each trawl and its technical specification, such as wingspan, mesh size of net, cod end and blinders. After each tow, fish were deposited in the fish pound or hopper before being sorted, then all litter items were manually picked from the entire net, including ground ropes, lines, hopper and cod end, and classified according to the Cefas classification system which was adopted in the guidance document on Monitoring of Marine Litter in European Seas (OSPAR Commission, 2017).
We have used only UK data within the UK Exclusive Economic Zone. The assessment has been divided into three regions, due to a different use of trawl data:
- Greater North Sea – using Grande Ouverture Verticale Trawl data
- Northern Celtic Sea – using Grande Ouverture Verticale data
- Southern Celtic Sea – using the Clean Seas Environment Monitoring Programme data with Granton trawls (inshore and offshore)
The Greater North Sea and the Northern Celtic Sea data is from 2012 to 2015; the Southern Celtic Sea is from 2012 and 2014.
The selected surveys used two similar types of otter trawls. Otter trawls derive their name from the large rectangular otter boards which are used to keep the mouth of the trawl net open. These boards act like a plough, digging up to 15cm into the seabed.
We have standardised the number of items found in the area of the haul. This is effectively the distance of the haul multiplied by the cross-sectional area of the net. It is preferable to use trawl wingspan rather than door span, however, there was more complete data for doorspan for the Northern Celtic Sea and the Greater North Sea data. Wingspan was used whenever possible, but when unavailable the doorspan was divided by the mean of the wingspan to doorspan ratio (4.35) for all data available simultaneous observations of the two measurements. For the Southern Celtic Sea data there was complete information on wingspan, and so this method was required to calculate trawl area.
Figures represent spatially smoothed predictions of the litter type at a grid of points. The plots have been smoothed using a Generalised Linear Model with longitude and latitude as the explanatory variables (Wood, 2006). The ‘probability that a haul contains plastic’ map (Figure 4) has been smoothed assuming that the Y-variable has a binomial distribution (1 if haul contains plastic, 0 if not). The litter per square kilometre values (Figure 5) have had ay = ln(x+1) transformation to get them approximately Gaussian distributed. For this plot, the smoothed values have been back-transformed using the transformation x(smooth) = exp(y(smooth)) - 1. The plots are thus approximately median levels on the original scale. For the power studies, we have used the “trend.power” function in the R library emon (Barry and others, 2015). We used the sample sizes that were taken in 2015 (Northern Celtic Sea =166, Greater North Sea=128). The starting value for the linear increase percentage in the mean was the mean litter items per trawl from 2012 (Northern Celtic Sea =0.84, Greater North Sea =0.98). We considered percentage increases over 10 years of 10, 20, 30, 40, 50 and 60. Thus, for example, a 10% increase over 10 years is just under 1% per year. For total litter and for plastic, 95% bootstrap confidence intervals are also given using the percentile method (Manly, 2006). In future, it would be useful for methods be developed to calculate power for litter per square kilometre or for the probability that a haul contains plastic. However, because there are some technical difficulties in doing this, for the moment, we have used total litter counts. Analysis of the data showed that it is reasonable to assume that these follow a Negative Binomial distribution. Power calculations for the Southern Celtic Sea data were not carried out as the current sample sizes are small.
Results
Litter levels were mapped over the area of the surveys (this is using data combined over the years, although similar patterns are shown for the individual years) and are shown in Figures 2 and 3.
Figure 2. Map of smoothed probability that a haul contains plastic (over 2012-2015).
Figure 3. Map of smoothed median total litter per square kilometre.
The distribution and abundance of marine litter on the UK seafloor was investigated using data collected by UK Exclusive Economic Zone trawl surveys and assessed by Marine Strategy Framework Directive sub-regions (Greater North Sea and Celtic Seas). Sampled litter quantities are not absolute numbers but represent a relative number. Widespread distribution of litter items, especially plastics, was discovered on the seabed of the Greater North Sea and the Celtic Seas (Figure 3). The Celtic Seas area contained, on average, significantly fewer items per square kilometre compared to the Greater North Sea area. Different types of litter items were detected in the trawls, especially pieces of plastic sheeting, bags and bottles, metallic objects, glass, and diverse materials including fishing gear. Items of natural origin, like driftwood and branches were less prominent. Data shows that plastic is the most prominent litter type. With only limited years’ data for Greater North Sea and Celtic Seas, we have not attempted to look at trends in litter. While differences are seen between years, this may just be short-term fluctuations and at least 5 years of data would be needed to make confident statements about trends.
Further information
The locations of the hauls in the regions are shown in Figure 4.
Figure 4. Grande Ouverture Verticale haul locations in Greater North Sea (red) and Northern Celtic Sea (green), and Clean Seas Environment Monitoring Programme locations in Southern Celtic Sea (blue).
Tables 1 and 2 give summaries of how litter items changed over the four years of the study, for each region. Table 1 uses the number of litter items per square kilometre statistic described above. The results of total litter and plastic litter are illustrated in Figures 2 and 3 respectively. The figures also contain 95% bootstrap confidence intervals using the percentile method (Manly, 2006). Table 2 gives the percentage of hauls in which each of the litter types were found at least once. Essentially, these entries are estimates of the probability that a haul contains a litter item.
Table 1. Mean litter per square kilometre for each litter type. For total and plastic, 95% bootstrap confidence intervals are also given, in brackets. Misc = miscellaneous.
|
|
Northern Celtic Sea |
Greater North Sea |
Southern Celtic Sea |
|||||||
|
|
2012 |
2013 |
2014 |
2015 |
2012 |
2013 |
2014 |
2015 |
2012 |
2014 |
|
Total |
14.2 (10.9,17.9) |
10.3 (7.9,12.6) |
10.8 (8.2,13.6) |
10.6 (8.4,13.2) |
16.3 (12.9,20.2) |
25.7 (21.5,30.4) |
23.7 (17.9,30.2) |
20.4 (16.4,24.5) |
166.5 (93.4,251.2) |
10.0 (5.8,15.0) |
|
Plastic |
11.8 (8.9,15.2) |
8.1 (6.3,10.0) |
7.8 (5.7,9.9) |
7.5 (5.7,9.5) |
11.0 (8.2,14.0) |
17.4 (14.0,20.5) |
17.5 (12.8,22.4) |
14.6 (11.2,17.8) |
158.5 (87.3,244.8) |
7.5 (3.3,12.5) |
|
Metal |
0.4 |
0.5 |
1.0 |
0.7 |
1.1 |
1.0 |
1.0 |
0.6 |
0.9 |
1.7 |
|
Rubber |
0.2 |
0.4 |
0.4 |
0.9 |
1.2 |
2.4 |
1.2 |
2.1 |
2.2 |
0.4 |
|
Glass |
0.0 |
0.0 |
0.0 |
0.0 |
0.1 |
0.6 |
0.4 |
0.9 |
0.9 |
0.0 |
|
Natural |
0.9 |
1.0 |
0.8 |
1.3 |
1.8 |
3.4 |
2.1 |
1.4 |
3.6 |
0.0 |
|
Misc |
0.9 |
0.3 |
0.9 |
0.3 |
1.0 |
1.0 |
1.5 |
1.0 |
0.4 |
0.0 |
Table 2. Percentage of hauls containing each litter type. For total and plastic, 95% bootstrap confidence intervals are also given, in brackets. Misc = miscellaneous.
|
|
Northern Celtic Sea |
Greater North Sea |
Southern Celtic Sea |
|||||||
|
|
2012 |
2013 |
2014 |
2015 |
2012 |
2013 |
2014 |
2015 |
2012 |
2014 |
|
Total |
48 (41,56) |
45 (37,53) |
45 (35,54) |
40 (33,48) |
50 (42,59) |
69 (60,76) |
58 (49,66) |
55 (48,64) |
100 (100,100) |
48 (28,68) |
|
Plastic |
45 (38,52) |
40 (31,49) |
36 (27,45) |
33 (25,40) |
41 (33,49) |
64 (55,72) |
52 (43,61) |
47 (38,55) |
100 (100,100) |
40 (20,56) |
|
Metal |
2 |
3 |
5 |
4 |
7 |
6 |
7 |
4 |
8 |
12 |
|
Rubber |
1 |
3 |
3 |
5 |
7 |
11 |
7 |
11 |
17 |
4 |
|
Glass |
0 |
0 |
0 |
0 |
7 |
11 |
7 |
11 |
8 |
0 |
|
Natural |
5 |
7 |
5 |
8 |
11 |
15 |
11 |
5 |
25 |
4 |
|
Misc |
5 |
1 |
4 |
1 |
5 |
6 |
7 |
5 |
8 |
0 |
The results of the power analysis indicate that with current sample sizes, the surveys can detect, with 80% probability, a change of around 35% over 10 years – or roughly 3% per year. This is a reasonable power, suggesting that the current sample sizes are adequate to detect the effectiveness of measures.
Figure 5. Power plot for Northern Celtic Sea (NCS) and Greater North Sea (GNS).
Conclusions
There is evidence of seafloor litter across all areas of the UK. Data shows that the amount of litter on the seafloor is higher in southern waters than in northern waters and southern seafloors have a greater proportion of plastic litter than those in northern regions. While it is noted that further monitoring is required before a trend in litter levels can be fully made, this assessment has shown that the current sampling regime is sufficient to detect change and the effectiveness of measures over a longer timescale.
Further information
The OSPAR Marine Litter Regional Action Plan 2014-2021 (OSPAR Commission, 2014) is presently being implemented, therefore reductions in seafloor litter quantities may be seen in the future. However, it must be noted that there will probably be a time lag in seeing the effects of certain measures. Although removal of litter might be useful at certain hotspot locations, prevention should still be seen as the key attribute to address marine litter inputs.
Knowledge gaps
Longer time series and better coverage of the southern Celtic Sea are required to improve the assessment.
Further information
There are some areas where further knowledge could improve the assessment. Currently, seasonal influences, weather patterns, and changes in currents which could affect the distribution of litter, are not taken into account. Although we only used surveys using similar gear, there could also be an influence of the sampling gear and programmes (fixed and random stratified). Further, there is a need to compare catchability and differences across gear types. Several data issues slowed down the assessment and these could be improved in the future.
References
Barry J, Maxwell D (2015) ‘R library emon: Tools for environmental and ecological survey design and analysis’ (viewed 21 September 2018)
Manly BFJ (2006) ‘Randomization, Bootstrap and Monte Carlo Methods in Biology, 3rd edition’ Chapman and Hall.
OSPAR Commission (2014) ‘Marine Litter Regional Action Plan’ ISBN: 978-1-906840-86-0 (viewed on 21 September 2018)
OSPAR Commission (2017) ‘Coordinated Environmental Monitoring Programme (CEMP) Guidelines on Litter on the Seafloor’ Agreement 2017-06; source: EIHA 17/9/1 Annex 12 (viewed on 21 September 2018)
Acknowledgements
Assessment metadata
| Assessment Type | UK MSFD Indicator Assessment |
|---|---|
D10 Marine Litter Seafloor Litter | |
| Point of contact email | marinestrategy@defra.gov.uk |
| Metadata date | Thursday, August 1, 2019 |
| Title | Trends and status in UK seafloor litter |
| Resource abstract | The data contained counts of each type of litter category found in material collected from fishing surveys between 2012 and 2015. |
| Linkage | |
| Conditions applying to access and use | © Crown copyright, licenced under the Open Government Licence (OGL). |
| Assessment Lineage | |
| Dataset metadata | |
| Links to datasets identifiers | Please, see below |
| Dataset DOI | Barry and Maes (2020). UK EEZ zone Seafloor Marine Litter MSFD 2017. Cefas, UK. V1. doi: https://doi.org/10.14466/CefasDataHub.92 |
The Metadata are “data about the content, quality, condition, and other characteristics of data” (FGDC Content Standard for Digital Geospatial Metadata Workbook, Ver 2.0, May 1, 2000).
Metadata definitions
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MEDIN discovery metadata - a list of standardized information that accompanies a marine dataset and allows other people to find out what the dataset contains, where it was collected and how they can get hold of it.
Recommended reference for this indicator assessment
Thomas Maes1 and Jon Barry1 2018. Composition and Spatial Distribution of Litter on the Seafloor. UK Marine Online Assessment Tool, available at: https://moat.cefas.co.uk/pressures-from-human-activities/marine-litter/seafloor-litter/
1Centre for Environment, Fisheries and Aquaculture Science