Etat et tendances des teneurs en hydrocarbures aromatiques polycycliques (HAP) dans les mollusques et crustacés

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. 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.

PAHs are of concern due to their persistence, potential to bioaccumulate and toxicity. They are therefore included on the OSPAR List of Chemicals for Priority Action. The analyses of PAHs in both sediment and shellfish are reported in the OSPAR Coordinated Environment Monitoring Programme (CEMP).

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 et al., 2002); second, metabolites of some of the high molecular PAHs, such as benzo[a]pyrene, are potent animal and human carcinogens. 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 et al., 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. The low molecular weight compounds are appreciably 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. PAHs are readily taken up by marine animals both across gill surfaces (lower molecular weight PAHs) and from their diet (Baumard et al., 1999). Filter-feeding organisms such as bivalve molluscs can accumulate high concentrations of PAHs. 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 et al., 1995; Richardson et al.,2001). Other marine vertebrate and marine mammals also metabolise PAHs efficiently.

PAHs are also controlled by other international instruments, for example 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.

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.

OSPAR has clarified that in assessing the Co-ordinated Environmental Monitoring Programme (CEMP) data the primary assessment value used in the assessment of PAH concentrations in sediment and biota “corresponds to the achievement, or failure to achieve, statutory targets or policy objectives for contaminants in these matrices” (OSPAR, 2009). This set of assessment criteria was specifically compiled for the assessment of CEMP monitoring data on hazardous substances contributing to the Quality Status Report (QSR) 2010. The use of this set was considered an interim solution for the purposes of the QSR 2010 until more appropriate approaches to defining assessment criteria could be agreed on and implemented. These criteria have also been used in the annually recurring CEMP assessments since 2010 and will be used until OSPAR adopts improved assessment criteria and subject to the conditions set out in the agreement.

Temporal trends in PAH concentrations in biota are presented. Two assessment criteria are used to assess the status of PAH concentrations in biota: Background Assessment Concentrations (BACs) and Environmental Assessment Criteria (EAC).

OSPAR IA 2017 Indicator Assessment values are not to be considered as equivalent to proposed European Union Marine Strategy Framework Directive (MSFD) criteria threshold values. However, they can be used for the purposes of their MSFD obligations by those Contracting Parties that wish to do so.

Provenance and limitations of BACs

Background assessment concentrations (BACs) were developed by OSPAR for testing whether measured concentrations are near natural background levels for naturally occurring substances and close to zero for man-made substances, the ultimate aim of the OSPAR Hazardous Substances Strategy. Mean concentrations significantly below the BAC are said to be near natural background concentrations. BACs are statistical tools defined in relation to the background concentrations or low concentrations, which enable statistical testing of whether observed concentrations could be considered to be near natural background concentrations.

Background concentrations (BCs) are assessment tools intended to represent the concentrations of hazardous substances that would be expected in the North-East Atlantic if certain industrial developments had not happened. They represent the concentrations of those substances at ’remote’ sites, or in ’pristine’ conditions based on contemporary or historical data respectively, in the absence of significant mineralisation and/or oceanographic influences. In this way, they relate to the background values referred to in the OSPAR Hazardous Substances Strategy. BCs for man-made substances should be regarded as zero. It is recognised that natural processes such as geological variability or upwelling of oceanic waters near the coast may lead to significant variations in background concentrations of contaminants, for example trace metals. The natural variability of background concentrations should be taken into account in the interpretation of CEMP data, and local conditions should be taken into account when assessing the significance of any exceedance.

Low concentrations (LCs) are values used to assist the derivation of BACs where there have been difficulties in assembling a dataset on concentrations in remote or pristine areas from which to derive BCs. LCs have been prepared by the International Council for the Exploration of the Sea on the basis of datasets from areas that could generally be considered remote but which could not be guaranteed to be free from influence from long-range atmospheric transport of contaminants.

BACs are calculated according to the method set out in Section 4 of the CEMP Assessment Manual (OSPAR, 2008). The outcome of this method is that, on the basis of what is known about variability in observations, there is a 90% probability that the observed mean concentration will be below the BAC when the true mean concentration is at the BC. Where this is the case the true concentrations can be regarded as ‘near background’ (for naturally occurring substances) or ‘close to zero’ (for man-made substances).

BACs are calculated on the basis of variability within the CEMP dataset currently available through databases held by the ICES Data Centre and will be refined at the working level by the relevant assessment group as further CEMP monitoring data are collected.

Provenance and limitations of EACs

Environmental Assessment Criteria (EACs) were developed by OSPAR and ICES for assessing the ecological significance of biota concentrations. Some EAC values were specifically compiled for the assessment of CEMP monitoring data on hazardous substances contributing to the QSR 2010 (OSPAR Agreement 2009-2). EACs do not represent target values or legal standards under the OSPAR Convention and should not be used as such. EACs were set so that hazardous substance concentrations in biota below the EAC should not cause chronic effects in sensitive marine species, including the most sensitive species, nor should concentrations present an unacceptable risk to the environment and its living resources. However, the risk of secondary poisoning is not always considered. EACs continue to be developed for use in data assessments.

Concentrations below EACs are considered to present no significant risk to the environment, and in most cases EACs are considered analogous to the Environmental Quality Standards applied to concentrations of contaminants in water or biota, for example under the European Union Water Framework Directive.

Caution should be exercised in using these generic environmental assessment criteria in specific situations. Their use does not preclude the use of common sense and expert judgement when assessing environmental effects and/or the potential for them. Furthermore, the EACs neither take into account specific long-term biological effects such as carcinogenicity, genotoxicity and reproductive disruption due to hormone imbalances, nor do they include combination toxicology.

Assessment methods

PAH concentrations are measured in shellfish samples taken annually (or every few years) from monitoring sites throughout much of the Greater North Sea, Celtic Seas, and Bay of Biscay and Iberian Coast (Figure 1).

Monitoring of PAH in the OSPAR Maritime Area began between 1995 and 1999. Measurements of PAH concentrations in blue mussel (Mytilus edulis) made up the majority of time series inallOSPARareas, although data were also available for Mediterranean mussels (Mytilus galloprovincialis) (Bay of Biscay and Iberian Coast) and Pacific oyster (Crassostrea gigas) (Greater North Sea, Celtic Seas, Bay of Biscay and Iberian Coast). The number of time series used in eachOSPARregion and contaminants assessment area is shown in Table c. Only assessment areas with at least threemonitoring sites with a reasonable geographic spread were included in the regional assessment of status andtemporaltrends.

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 contaminants online assessment tool (http://dome.ices.dk/osparmime2016/main.html). 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 and baseline monitoring sites where temporal trends would not be expected 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 of 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:

                ~ OSPAR contaminants assessment area 

and random effects:

                ~ compound + compound: OSPAR contaminants assessment area + 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 assessment area and the random effects measure variation in trends:

•    between compounds common across contaminants assessment areas (compound);

•    between compounds within contaminants assessment areas (compound: contaminants assessment area);

•    between monitoring sites common across compounds (monitoring site); and

•    residual variation (compound: monitoring site + within-series variation).

There are two residual terms. Within-series variation is the variation associated with the estimate of the temporal trend from the individual time series and is assumed known (and given by the square of the standard error). Compound: monitoring site allows for any additional residual variation.

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; and the log ratio of the fitted concentration in the last monitoring year to the BAC. Baseline 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.

Human health Environmental Quality Standards (EQS) are also available for fluoranthene and benzo[a]pyrene, these are on a wet weight basis but can be converted to a dry weight basis by multiplying by 5.

BACs and EACs are available for the following PAHs in mussels and oysters (Table a).

The number of time series used in each OSPAR region and assessment area assessed is shown in Table c.

Differences in methodology used for the IA 2017 compared with the QSR 2010

For the IA 2017, a meta-analysis is used to synthesise the individual time series results and provide an assessment of status and temporal trend at the assessment area level. Meta-analyses take into account both the estimate of status or temporal trend in each time series and the uncertainty in that estimate. They provide a more objective regional assessment than was possible in the QSR 2010, where a simple tabulation of the trend and status at each monitoring site was presented.