Status and Trends in Polycyclic Aromatic Hydrocarbons (PAHs) in biota and sediment
Mean concentrations of polycyclic aromatic hydrocarbons (PAHs) in shellfish (biota) and sediment, and PAH bile metabolites in fish, in most assessed areas are above background concentrations, but are below levels likely to harm marine species. Concentrations are decreasing or stable in all areas assessed, except for bile metabolites in the West Channel and Celtic Sea.
Background
Polycyclic aromatic hydrocarbons (PAHs) are natural components of coal and oil and are also formed during the combustion of fossil fuels and organic material, for example during activities at an oil refinery. PAHs also occur as a result of natural processes such as forest fires.
PAHs enter the marine environment through atmospheric deposition, road run-off, industrial discharges and as a result of oil spills. PAHs in the marine environment often end up in marine sediment, where they can become trapped in lower layers unless the sediments are disturbed. Associations have been demonstrated between the incidence of some diseases in flatfish and the concentrations of PAHs in the sediment over which they live and feed. PAHs also accumulate in shellfish, either absorbed directly from the marine environment or indirectly through food consumption. In contrast fish metabolise PAHs and therefore concentrations in fish are low. Exposure to PAH in fish can be screened for by the measurement of PAH bile metabolites in their gall bladders. The problems caused by PAHs in the marine environment vary considerably from tainting the taste of fish and shellfish to potential carcinogenic effects on humans and animals.
Due to their persistence in the marine environment, their potential to bioaccumulate and their toxicity, monitoring of PAHs in shellfish and sediment, and PAH bile metabolites in fish, are required for the UK Marine Strategy. This indicator is used to assess progress against the target set out in the UK Marine Strategy Part One (which requires that concentrations of substances identified within relevant legislation and international obligations are below levels at which adverse effects are likely to occur).
Image 1: Shellfish – Polycyclic aromatic hydrocarbons (PAHs) can accumulate in shellfish, either absorbed directly from the marine environment or indirectly through food consumption © Marine Scotland Science
Image 2: Sediment grab – Polycyclic aromatic hydrocarbons (PAHs) can accumulate in marine sediments © Marine Scotland Science
Further information
Polycyclic aromatic hydrocarbons (PAHs) are hydrocarbons composed of two or more fused aromatic rings, encompassing both parent (non-alkylated) compounds and alkylated homologues. Although PAHs can be produced through natural processes, they also arise from anthropogenic sources (Mojiri and others, 2019). Incomplete combustion processes are a major source of PAHs, but they can also be of petrogenic origin (crude oils or refinery products). PAHs of petrogenic origin include mainly alkylated, 2-ring and 3-ring PAHs formed as a result of diagenetic processes, whereas PAHs from pyrolytic sources mainly comprise the heavier, parent (non-alkylated) PAHs. Assessment of the PAH profile, including PAH ratios such as the phenanthrene/anthracene ratio or the fluoranthene/pyrene ratio can give an indication of the source of the PAHs (Tobiszewski and Namieśnik, 2012).
PAH properties will vary considerably depending on the number of rings. Low molecular weight PAHs can cause tainting of fish and shellfish, rendering them unfit for sale (Davis and others, 2002). In addition, metabolites of some of the high molecular PAHs, such as benzo[a]pyrene, are potent animal and human carcinogens (Honda and others, 2020). Less is known about the toxicity of alkylated PAHs, although one study has demonstrated that alkylated PAHs may have increased toxicity compared to the parent compound (Marvanova and others, 2008).
There are marked differences in the behaviour of PAHs in the aquatic environment between the low molecular weight compounds (e.g. naphthalene) and the high molecular weight compounds (e.g. benzo[g,h,i]perylene) as a consequence of their differing physical-chemical properties (Mojiri and others, 2019). The low molecular weight compounds are appreciably more water soluble and can be bioaccumulated from the dissolved phase by transfer across the gill surfaces of aquatic organisms; whereas the high molecular weight compounds are relatively insoluble and hydrophobic and can attach to both organic and inorganic particulates within the water column. PAHs derived from combustion sources may be deposited directly to the marine environment already adsorbed to atmospheric particulates, such as soot.
PAHs can enter the marine environment through atmospheric deposition, run-off, industrial discharges and as a result of oil spills. Sediment will act as a sink for PAHs in the marine environment. Filter-feeding organisms such as bivalve molluscs can accumulate high concentrations of PAHs (Baumard and others, 1999; Honda and others, 2020). Fish do not generally accumulate high concentrations of PAHs as they possess an effective mixed-function oxygenase (MFO) system which allows them to metabolise PAHs and to excrete them in bile (Stagg and others, 1995; Richardson and others, 2001; Vethaak and others, 2016). Other marine vertebrate and marine mammals also metabolise PAHs efficiently.
Emissions of PAHs are controlled via a number of different regulations. These include the United Nations Economic Commission for Europe Convention on Long-Range Transboundary Air Pollution (UNECE, 2009). This obliges its member countries to reduce their emissions of persistent organic pollutants such as PAHs with the ultimate objective of eliminating discharges and emissions. Persistent Organic Pollutants regulation (EC 850/2004) requires PAH emissions to be captured in an inventory and an action plan put in place to limit releases. The REACH registration process (EC 1906/2007) states conditions for the safe use of substances including PAHs.
Assessment method
In assessing polycyclic aromatic hydrocarbons (PAHs) both ‘relative’ and ‘absolute’ aspects have been analysed:
• ‘Trend assessment’ or spatial distribution assessment to focus on relative differences and changes on spatial and temporal scales – provides information about the rates of change and whether PAH contamination is widespread or confined to specific locations; and
• ‘Status assessment’ of the significance of the (risk of) pollution, defined as the status where PAHs are at a hazardous level, usually requires assessment criteria that take account of the possible severity of the impacts and hence require criteria that take account of the natural conditions (background concentrations) and the ecotoxicology of the contaminants. For example, Environmental Assessment Criteria (EAC) are tools in this type of assessment.
Two assessment criteria are used to assess the status of PAH concentrations in biota (Table a): Background Assessment Concentrations (BACs) and Environmental Assessment Criteria (EAC). Environmental Assessment Criteria (OSPAR, 2009) were derived by OSPAR for the assessment of PAH concentrations in shellfish. Concentrations below the EAC rarely cause any chronic effects in marine organisms. Two assessment criteria are used to assess PAH concentrations in sediment (Table a): Background Assessment Concentrations (BACs) and the United States Environmental Protection Agency (EPA) sediment quality guidelines - Effects Range-Low (ERL). Because EACs were not available for PAHs in sediment, these were assessed against ERL values, developed by the United States Environmental Protection Agency to assess the ecological significance of PAHs in sediment. BACs are an aspirational ultimate objective to achieve the background concentrations which naturally occur in the marine environment and whilst not a UK Marine Strategy target, the status and trend results were used to examine progress towards this objective.
Assessment methods
PAH concentrations are measured in shellfish (Mytilus edulis) and sediment samples and PAH bile metabolites in fish taken routinely (every 1 - 6 years depending on site) from monitoring sites throughout much of the UK regions (Figure 1, Figure 2 and Figure 3). Additional data is available but was not included in the regional status and trend assessment due to there being too few stations or number of years. For an individual time series is assessed for status or trends there must be at least one year with data in the period 2016 to 2021.
Monitoring of PAHs in UK regions began in 1999. The number of time series used in each UK Region is shown in Table b. Only assessment areas with at least three monitoring sites with a reasonable geographic spread were included in the regional assessment of status and temporal trends. The regional assessment only considered coastal and offshore stations and excluded estuarine stations.
For each PAH compound at each monitoring site, the time series of concentration measurements was assessed for temporal trends and status using the methods described in the OSPAR Hazardous Substances Assessment Tool (https://dome.ices.dk/OHAT/?assessmentperiod=2023). The results from these individual time series were then synthesised at the assessment area scale in a series of meta-analyses.
For temporal trends, those monitoring sites that were representative of general conditions were considered and those monitoring sites impacted due to a point source were excluded. Analysis was also restricted to assessment areas where there were at least three monitoring sites with trend information and where those monitoring sites had reasonable geographic spread.
The trend for each PAH compound at each monitoring site was summarised by the estimated annual change in log concentration, with its associated standard error.
The annual change in log concentration was then modelled by a linear mixed model with a fixed effect:
UK biogeographic region and random effects: compound + compound: UK biogeographic region + monitoring site + compound: monitoring site + within-series variation
The choice of fixed and random effects was motivated by the assumption that the PAH compounds would have broadly similar temporal trends, since they have similar sources. Thus, the fixed effect measures the common trend in PAH compounds in each UK biogeographic region and the random effects measure variation in trends:
-
between compounds common across UK biogeographic region (congener);
-
between compounds within UK biogeographic region (compound: contaminants assessment area);
-
between monitoring sites common across compounds (monitoring site);
-
between compound but common across tissues and species within monitoring sites (compound: monitoring site); and
-
residual variation.
The residual variation is made up of two terms: the variation associated with the estimate of the trend from the individual time series, which is assumed known (and given by the square of the standard error); and a term which accounts for any additional residual variation not explained by the other fixed and random effects.
Evidence of temporal trends in PAH concentration at the assessment area scale was then assessed by plotting the estimated fixed effects with point-wise 95% confidence intervals. Differences between compounds were explored by plotting the predicted temporal trend for each compound and for each compound/assessment area combination with point-wise 95% confidence intervals.
Similar analyses explored status at the assessment area scale. Two summary measures were considered: the log ratio of the fitted concentration in the last monitoring year to the EAC or ERL; and the log ratio of the fitted concentration in the last monitoring year to the BAC. Impacted monitoring sites were also included in these analyses. Finally, concentration profiles across compounds at the assessment area scale were explored using the fitted log concentration in the last monitoring year. BACs and EACs are available for the following PAHs in mussels and oysters and sediment (Table A).
Table A: Background Assessment Concentrations (BACs), Environmental Assessment Criteria (EACs), Effects Range-Low (ERL) and the Quality Standard for human health (QS(hh)) for polycyclic aromatic hydrocarbons (PAHs) in mussels and oysters and sediment. dw – dry weight; ww – wet weight
|
|
|
mussels and oysters |
mussels and oysters |
mussels and oysters |
Sediment |
Sediment |
Sediment |
|
PAH |
Abbreviation |
BAC (μg/kg dw) |
EAC (μg/kg dw) |
QS (hh) (μg/kg ww) |
BAC (μg/kg dw) |
BAC (μg/kg dw) |
ERL (μg/kg dw) |
|
Naphthalene |
|
|
340 |
|
8 |
|
160 |
|
Phenanthrene |
PA |
11.0 |
1700 |
|
32 |
7.3 |
240 |
|
Anthracene |
ANT |
|
290 |
|
5 |
1.8 |
85 |
|
Dibenzothiophene |
|
|
|
|
|
|
190 |
|
Fluoranthene |
FLU |
12.2 |
110 |
30 |
39 |
14.4 |
600 |
|
Pyrene |
PYR |
9.0 |
100 |
|
24 |
11.3 |
665 |
|
Benz[a]anthracene |
BAA |
2.5 |
80 |
|
16 |
7.1 |
261 |
|
Chrysene (including triphenylene) |
CHR |
8.1 |
|
|
20 |
8.0 |
384 |
|
Benzo[a]pyrene |
BAP |
1.4 |
600 |
5 |
30 |
8.2 |
430 |
|
Benzo[g,h,i]perylene |
BGHIP |
2.5 |
110 |
|
80 |
6.9 |
|
|
Indeno[123-c,d]pyrene |
ICDP |
2.4 |
|
|
103 |
8.3 |
|
Table A notes:
-
for sediment BACs are normalised to 2.5% organic carbon
-
PAHs are not routinely monitored in fish, but PAH bile metabolites are. For PAH bile metabolites (1-hydroxypyrene equivalent) in fish, the BACs and EACs are dependent on the species and analytical method used, more details can be found here AC biological effects (ices.dk) (Davies and Vethaak, 2012)
The number of time series used in each Marine Strategy sub-region and UK biogeographic region assessed is shown in Table B.
Table B: Number of monitoring sites within each Marine Strategy sub-region and UK biogeographic region used in the assessment of temporal trends and status
|
|
|
Sediment |
Sediment |
Shellfish |
Shellfish |
Fish |
Fish |
|
Marine Strategy sub-Region |
UK region |
Trends |
Status |
Trends |
Status |
Trends |
Status |
|
Greater North sea |
Northern North Sea |
15 |
21 |
7 |
8 |
9 |
11 |
|
Greater North sea |
Southern North Sea |
6 |
10 |
1 |
1 |
7 |
7 |
|
Greater North sea |
East Channel |
3 |
4 |
|
1 |
3 |
3 |
|
Celtic Sea |
Scottish Continental Shelf |
|
1 |
1 |
1 |
|
|
|
Celtic Sea |
Minches and Western Scotland |
5 |
5 |
1 |
3 |
2 |
1 |
|
Celtic Sea |
Irish sea |
15 |
17 |
8 |
14 |
13 |
10 |
|
Celtic Sea |
West Channel and Celtic Sea |
2 |
2 |
|
|
3 |
3 |
Differences in methodology used for the MS 2018 compared with the UK MS 2024
This assessment includes the assessment of PAHs in sediment and biota, and of bile metabolites. These were assessed separately for the 2018 MS assessment. Although not presented in this assessment, the status assessment against the human health standards (QS(hh)) can be found on the OSPAR Hazardous Substances Assessment Tool https://dome.ices.dk/OHAT/?assessmentperiod=2023
Results
Polycyclic aromatic hydrocarbon (PAH) concentrations were measured in sediment and shellfish, and bile metabolites in fish, collected between 1999 and 2021 from monitoring sites throughout much of the UK biogeographic regions (excluding estuarine sites) (Figure 1, Figure 2 and Figure 3), at frequencies ranging from annually to every six years.
Figure 1 Monitoring sites used to assess PAH concentrations in sediment in each Marine Strategy region (dark lines) and biogeographic subregion (light lines). The filled circles indicate sites where there are sufficient data to assess both status and trends; the open circles indicate sites where only status can be assessed. There are additional sites that are not shown because they were not sampled often enough.
Figure 2 Monitoring sites used to assess PAH concentrations in shellfish in each Marine Strategy region (dark lines) and biogeographic subregion (light lines). The filled circles indicate sites where there are sufficient data to assess both status and trends; the open circles indicate sites where only status can be assessed. There are additional sites that are not shown because they were not sampled often enough.
Figure 3 Monitoring sites used to assess PAH bile metabolite concentrations in fish in each Marine Strategy region (dark lines) and biogeographic subregion (light lines). The filled circles indicate sites where there are sufficient data to assess both status and trends; the open circles indicate sites where only status can be assessed. There are additional sites that are not shown because they were not sampled often enough.
Only assessment areas with at least three monitoring sites and a reasonable geographic spread were included in the assessment of status and temporal trends. The regional assessment only considered coastal and offshore stations and excluded estuarine stations. For shellfish only the Irish Sea had enough sites for a regional assessment.
PAH concentrations in sediment and shellfish and the PAH metabolite (1-hydroxy pyrene equivalents) in fish, were compared to the OSPAR Background Assessment Concentration (BAC). In addition, PAH concentrations in shellfish (and 1-hydroxy pyrene equivalents for fish) were compared to Environmental Assessment Criteria (EACs) and concentrations in sediment compared to the United States Environmental Protection Agency’s Effects Range-Low (ERL). Adverse effects on marine organisms are rarely observed when concentrations are below the EAC or ERL value.
Mean PAH concentrations in sediment and shellfish and 1-hydroxy pyrene equivalents in fish, are statistically significantly below the ERL or EAC in all UK regions (Figure 4, Figure 5 and Figure 6). Therefore, adverse biological effects in marine species are unlikely. No area was at background for sediment, shellfish or fish.
Figure 4 The mean PAH concentration (coloured circles) in sediment in each biogeographic region relative to the Effects Range Low (ERL). A value of 1 occurs when the mean concentration equals the ERL. The horizontal line indicates the upper one-sided 95% confidence limit on the mean. The mean concentration is significantly below the ERL (p < 0.05) if its upper confidence limit is less than 1. The dark blue circle indicates that the mean concentration is significantly below the ERL (p < 0.05) but is not also significantly below the Background Assessment Concentration.
Figure 5 The mean PAH concentration (coloured circle) in shellfish in the Irish Sea relative to the Environmental Assessment Criterion (EAC). A value of 1 occurs when the mean concentration equals the EAC. The horizontal line indicates the upper one-sided 95% confidence limit on the mean. The mean concentration is significantly below the EAC (p < 0.05) if its upper confidence limit is less than 1. The dark blue circle indicates that the mean concentration is significantly below the EAC (p < 0.05) but it is not also significantly below the Background Assessment Concentration.
Figure 6 The mean PAH bile metabolite concentration (coloured circle) in fish in each biogeographic region relative to the Environmental Assessment Criterion (EAC). A value of 1 occurs when the mean concentration equals the EAC. The horizontal line indicates the upper one-sided 95% confidence limit on the mean. The mean concentration is significantly below the EAC (p < 0.05) if its upper confidence limit is less than 1. The dark blue circle indicates that the mean concentration is significantly below the EAC (p < 0.05) but is not also significantly below the Background Assessment Concentration.
Temporal trends in the PAH concentrations in sediment and shellfish were assessed in areas with at least three stations and five years of data, with four regions being assessed for sediment, one for shellfish and five for PAH bile metabolites in fish (Figure 7, Figure 8 and Figure 9). Concentrations for bile metabolites (1-hydroxy pyrene equivalents) in fish from the West Channel and Celtic Sea were increasing, and stable in all other regions. Decreasing trends were observed in shellfish from the Irish Sea, and for sediment PAH concentrations showed no statistically significant trend in all four regions.
Figure 7 The percentage annual change (circle) in the mean PAH concentration in sediment in each biogeographic region. The horizontal line is the associated 95% confidence interval. There is a significant change in mean concentration (p < 0.05) if the confidence interval does not cut the vertical line at 0. The circle indicates that there is no significant change in mean concentration (p > 0.05).
Note: No statistically significant (p <0.05) change in mean concentration (circle), mean concentration is significantly decreasing (downward triangle), mean concentration is significantly increasing (upward triangle)
Figure 8 The percentage annual change (triangle) in the mean PAH concentration in shellfish in the Irish Sea. The horizontal line is the associated 95% confidence interval. There is a significant change in mean concentration (p < 0.05) if the confidence interval does not cut the vertical line at 0. The downward triangle indicates that the mean concentration is significantly decreasing (p < 0.05).
Figure 9 The percentage annual change (circle, triangle) in the mean PAH bile metabolite concentration in fish in each biogeographic region. The horizontal line is the associated 95% confidence interval. There is a significant change in mean concentration (p < 0.05) if the confidence interval does not cut the vertical line at 0. Circle: no significant change in mean concentration (p > 0.05). Upward triangle: significant increase in mean concentration (p < 0.05).
Further information
Mean PAH concentrations in sediment (normalised to organic carbon) and biota (shellfish) relative to the BAC for each assessment area are shown in Figure a and Figure b. Mean individual PAH concentrations in sediment were mainly above BACs. Concentrations were statistically significantly below BACs for six individual PAHs (phenanthrene, anthracene, fluoranthene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene and benzo[g,h,i]perylene) in the Northern North Sea and for one PAH (indeno[1,2,3-c,d]pyrene) in the Minches and Western Scotland region. No individual PAH compound was below the BAC in the Irish Sea or Southern North Sea. Individual PAH concentrations for shellfish from the Irish Sea all exceeded the BAC.
Figure A The mean concentration of individual PAHs (coloured circles) in sediment in each biogeographic region relative to the Background Assessment Concentration (BAC). A value of 1 occurs when the mean concentration equals the BAC. The horizontal line indicates the upper one-sided 95% confidence limit on the mean. The mean concentration is significantly below the BAC (p < 0.05) if its upper confidence limit is less than 1. Light blue: the mean concentration is significantly below the BAC (p < 0.05). Dark blue: the mean concentration is significantly below the Effects Range Low (ERL) (p < 0.05) but not the BAC. Amber: the mean concentration is not significantly below the BAC (p > 0.05) and no ERL is available. ICDP, indeno[1,2,3-cd]pyrene; BGHIP, benzo[ghi]perylene; BAP, benzo[a]pyrene; CHR, chrysene (including triphenylene); BAA, benzo[a]anthracene; PYR, pyrene; FLU, fluoranthene; ANT, anthracene; PA, phenanthrene; NAP, Naphthalene.
Figure B The mean concentration of individual PAHs (coloured circles) in shellfish in the Irish Sea relative to the Background Assessment Concentration (BAC). A value of 1 occurs when the mean concentration equals the BAC. The horizontal line indicates the upper one-sided 95% confidence limit on the mean. The mean concentration is significantly below the BAC (p < 0.05) if its upper confidence limit is less than 1. Dark blue: the mean concentration is significantly below the Environmental Assessment Criterion (EAC) (p < 0.05) but not the BAC. Amber: the mean concentration is not significantly below the BAC (p > 0.05) and no EAC is available. ICDP, indeno[1,2,3-cd]pyrene; BGHIP, benzo[ghi]perylene; BAP, benzo[a]pyrene; CHR, chrysene (including triphenylene); BAA, benzo[a]anthracene; PYR, pyrene; FLU, fluoranthene; PA, phenanthrene.
All PAH compounds in sediment and biota were significantly below the ERL or EAC in all assessment areas. Therefore, adverse biological effects in marine species are unlikely in the assessed areas. The PAH profile in all assessment areas was typical of pyrolytic sources, being dominated by the heavier 4-ring to 6-ring PAH compounds.
PAH concentrations in sediment from the Minches and Western Scotland region were classed as being at background in the 2018 UK Marine Strategy Part 1 (UKMS1) assessment, with 7 of the 9 PAHs assessed being below the BACs. This suggests there has been a deterioration in the status of this region, however no upward trend was detected in the 2018 or 2024 assessments. In 2018 PAH concentrations in sediment from the West Channel and Celtic Sea, were unacceptable (> ERL), with all 7 individual PAHs exceeding the ERL. However, there were insufficient sites for an assessment of this region in 2024. The only region to be assessed for PAHs in shellfish in 2024 (Irish Sea) had the same status as in 2018, with PAH concentrations in both 2018 and 2024 assessments being above the BAC but below the EAC. Four regions were assessed for PAHs in shellfish in total in 2018, all had acceptable concentrations (<EAC). For 1-hydroxy pyrene equivalents in fish there was no change in the status assessment with mean regional concentrations in all regions being below the EAC in 2018 as well as 2024.
Temporal trends in PAH concentrations in sediment and biota were assessed in areas where there were at least three stations and five years of data. The percentage yearly change for each PAH in each assessment area is shown in Figure c and Figure d. In all regions trends were stable or downwards. Shellfish from the Irish Sea showed decreasing trends for 6 out of 10 PAHs (benzo[ghi]perylene, benzo[a]pyrene, benz[a]anthracene, fluoranthene, anthracene, naphthalene), for sediment only perylene in the Irish Sea showed a downwards trend.
Figure C The percentage annual change (circle, triangle) in the mean concentration of individual PAHs in sediment in each biogeographic region. The horizontal line is the associated 95% confidence interval. There is a significant change in mean concentration (p < 0.05) if the confidence interval does not cut the vertical line at 0. Circle: no significant change in mean concentration (p > 0.05). Downward triangle: significant decrease in mean concentration (p < 0.05). ICDP, indeno[1,2,3-cd]pyrene; BGHIP, benzo[ghi]perylene; PER, perylene; DBAHA, dibenzo[a,h]anthracene; BBKF, benzo[b,k,f]fluoranthene; BEP, benzo[e]pyrene; BAP, benzo[a]pyrene; CHR, chrysene (including triphenylene); BAA, benzo[a]anthracene; PYR, pyrene; FLU, fluoranthene; DBT, dibenzothiopene; ANT, anthracene; PA, phenanthrene; FLE, fluorene; ACNE, acenaphthene, ACNLE, acenaphthylene; NAP, naphthalene.
Figure D The percentage annual change (circle, triangle) in the mean concentration of individual PAHs in shellfish in the Irish Sea. The horizontal line is the associated 95% confidence interval. There is a significant change in mean concentration (p < 0.05) if the confidence interval does not cut the vertical line at 0. Circle: no significant change in mean concentration (p > 0.05). Downward triangle: significant decrease in mean concentration (p < 0.05). ICDP, indeno[1,2,3-cd]pyrene; BGHIP, benzo[ghi]perylene; BAP, benzo[a]pyrene; CHR, chrysene (including triphenylene); BAA, benzo[a]anthracene; PYR, pyrene; FLU, fluoranthene; ANT, anthracene; PA, phenanthrene; NAP, naphthalene.
Individual Time Series Results per Monitoring Site
A summary of individual time series results per monitoring site for PAH concentrations in sediment and biota is presented here https://dome.ices.dk/ohat/?assessmentperiod=2023. In summary, at 605 out of 651 time series (93%) across UK regions, mean concentrations of PAH in sediment are below the ERL. At 743 out of 763 time series (97%), mean concentrations have stayed stable or decreased over the assessment period. For shellfish, 235 out of 252 time series (93%) were below the EAC and 163 of 164 mean concentrations (99%) have decreased or stayed stable over the assessment period. For 1-hydroxy pyrene equivalents, in fish all the 35-time series were below the EAC and mean concentrations have stayed stable or decreased for 33 of 37 (89%) sites. It should be noted that not all individual time series results are included in the assessments (see number of time series used in each assessment area in Table b), due to the criteria set out in the assessment methods.
Confidence Assessment
There is high confidence in the quality of the data used for this assessment. The data have been collected over many years using established sampling methodologies. There is sufficient temporal and spatial coverage and no significant data gaps in the areas assessed over the relevant time periods. The synthesis of monitoring site data for the assessment area scale are based on established and internationally recognised protocols for monitoring and assessment per monitoring site, therefore there is also high confidence in the methodology.
Conclusions
Mean polycyclic aromatic hydrocarbon (PAH) concentrations in sediment and shellfish, and bile metabolites in fish were below the ERLs or EACs in all assessment areas and therefore are unlikely to cause adverse effects in marine organisms.
However, PAH concentrations need to be kept under surveillance, because concentrations are above background in sediment and shellfish in most regions with individual PAH compounds also exceeding the EAC / ERL at some stations. Concentrations are mainly stable or decreasing however there are also some increasing trends, particularly for 1-hydroxy pyrene in fish bile from the West Channel and Celtic Sea.
PAHs originate from natural sources and so they will always be present in the marine environment. However, better use of emission control technology in combustion processes could improve the situation further and reduce concentrations to natural levels.
Further information
PAH concentrations in sediment and biota have in general been stable or decreasing with only 2.6% of sediment and 0.6% of shellfish time series showing increasing trends. However, for 1-hydroxy pyrene equivalents in fish, 10% of time series showed increasing trends, and at the regional scale the West Channel and Celtic Seas showed an increasing trend.
The majority of time series were below concentrations that could cause adverse effects in marine organism with only 7.1% time series for sediment exceeding the ERL, and for shellfish 6.7% of time series exceeded the EAC. For 1-hydroxy pyrene equivalents in fish, no time series exceeded the EAC. However, few stations had concentrations below the BAC (close to background).
Knowledge gaps
There is a lack of monitoring data, particularly for shellfish, as many sites were in estuarine waters and therefore not included in the assessment, and in some regions, there are insufficient monitoring sites with a good geographic spread for a status and trends assessment. In 2018 PAH concentrations in sediment from the West Channel and Celtic Sea, exceeded the ERL and therefore were classed as unacceptable. However, this region could not be assessed in 2024 as there were insufficient sites for a regional assessment.
Effects Range-Low (ERL) values were used in the sediment assessment because there are no Environmental Assessment Criteria (EACs) available. There is a need for EACs to be developed for PAHs in sediment.
Further information
Environmental Assessment Criteria (EACs) were used in the assessment of parent PAHs in shellfish only; there are no assessment criteria for alkylated PAHs. There is a need for EACs to be developed for alkylated PAHs in shellfish. The limitations in using EACs and Background Assessment Concentrations (BACs) should be addressed with further research.
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Authors
Lynda Webster (Marine Directorate of the Scottish Government)
Assessment Metadata
Please contact marinestrategy@defra.gov.uk for metadata information
Assessment metadata
| Assessment Type | UK Marine Strategy |
|---|---|
| Point of contact email | marinestrategy@defra.gov.uk |
| Metadata date | Sunday, June 1, 2025 |
| Title | |
| Resource abstract | |
| Linkage | |
| Conditions applying to access and use | |
| Assessment Lineage | |
| Dataset metadata | |
| Dataset DOI | The Scottish Government, Marine Directorate. 2025. https://doi.org/10.7489/12541-1 |
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
Assessment Lineage - description of data sets and method used to obtain the results of the assessment
Dataset – The datasets included in the assessment should be accessible, and reflect the exact copies or versions of the data used in the assessment. This means that if extracts from existing data were modified, filtered, or otherwise altered, then the modified data should be separately accessible, and described by metadata (acknowledging the originators of the raw data).
Dataset metadata – information on the data sources and characteristics of data sets used in the assessment (MEDIN and INSPIRE compliance).
Digital Object Identifier (DOI) – a persistent identifier to provide a link to a dataset (or other resource) on digital networks. Please note that persistent identifiers can be created/minted, even if a dataset is not directly available online.
Indicator assessment metadata – data and information about the content, quality, condition, and other characteristics of an indicator assessment.
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.