Status and Trend for Heavy Metals (Mercury, Cadmium, and Lead) in Fish and Shellfish

In assessing heavy metals both ‘relative’ and ‘absolute’ aspects have been

• ‘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 contamination is widespread or confined to specific locations; and
• ‘Status’ assessment of the significance of the (risk of) pollution, defined as the status where heavy metals are at a hazardous level, usually requires assessment criteria that take account of the possible severity of the impacts and hence requires criteria that take account of the natural conditions (background concentrations) and the ecotoxicology of the heavy metals. For example, the European Commission (EC) maximum concentration limits for heavy metals in seafood (fish and mussels) are used by OSPAR as proxy Environmental Assessment Criteria (EACs).

OSPAR has clarified that in assessing the Co-ordinated Environmental Monitoring Programme (CEMP) data the primary assessment value used in the assessment of heavy metal 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 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 agrees on the adoption of improved assessment criteria and subject to the conditions set out in the agreement.

Trends in heavy metal concentrations in biota are presented. Two assessment criteria are used to assess the status of heavy metal concentrations in biota: Background Assessment Concentrations (BACs) and European Commission maximum levels for fish and seafood (EC, 2006).

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 background 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 background (naturally occurring 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 background concentrations.

Background concentrations (BCs) 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 synthetic, 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 heavy 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 on the basis of datasets from areas that could generally be considered remote but which could not be guaranteed to be free from the influence of 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 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 International Council for the Exploration of the Sea (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 EC maximum levels in fish and seafood

OSPAR has not yet established EACs for heavy metals in biota. Therefore the European Commission maximum levels in fish and seafood (EC, 2006) have been used as a proxy means for assessing the ecological significance of biota concentrations of heavy metals. EC maximum levels are applied in European Union Member States’ control of foodstuffs with the aim to protect public health by excluding the most contaminated food from the market. The limits are set on the basis of the diet of the ‘average’ European consumer, and should according to Regulation 1881/2006 be set at “a strict level which is reasonably achievable by following good agricultural, fishery and manufacturing practices and taking into account the risk related to the consumption of the food”. So if there are improvements in the practices (i.e. a decrease in the levels of environmental contaminants) the EC maximum levels may be reviewed. If the EC maximum food levels for fish and mussels are exceeded, the catch should not be put on the market.

The maximum EC levels for lead, mercury and cadmium in fish muscle and bivalve molluscs are used as alternatives to EACs for heavy metals in both fish and shellfish species. These values are firmly established by EC regulation, but have the disadvantage that the standards for cadmium and lead have not been directly designed for all the matrix/contaminant combinations required for the assessment. In general, it is recognised that the use of dietary standards is not fully satisfactory for assessing environmental risk, and that these values and their use to draw conclusions concerning the heavy metals in biota common indicator assessment should be treated with care. They are used as an interim solution to address the need for criteria until a more appropriate approach can be defined and agreed.

Assessment method

Concentrations of heavy metals in biota are monitored on a yearly basis in the CEMP monitoring programme at 304 monitoring sites (Figure 1).

For each heavy metal at each monitoring site, the time series of concentration measurements was assessed for 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.

Trend assessments included those monitoring sites that were representative of general conditions, and excluded those monitoring sites impacted due to a point source and baseline monitoring sites where trends would not be expected. The 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 in each metal 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 fixed effects:

    ~ heavy metal: OSPAR contaminants assessment areas

and random effects:

    ~ monitoring site + metal: monitoring site + within-series variation

The choice of fixed and random effects was motivated by the assumption that the heavy metals could have very different trends as they have different sources and are metabolised differently. Thus, the fixed effects measure the trend in each heavy metal in each OSPAR contaminants assessment area and the random effects measure variation in trends:

• between monitoring sites common across heavy metals (monitoring site); and
• residual variation (metal: monitoring site + within-series variation)

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

Evidence of trends in heavy metal concentrations at the assessment area scale was then assessed by plotting the estimated fixed effects 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 EC maximum levels; 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.

For heavy metals, BACs have been developed for mussels and fish. The upper limit for good status is set as the EC maximum levels in fish and seafood for the heavy metals, which might not be precautionary for the most sensitive species. An EC Environmental Quality Standard of 20 µg/kg ww for mercury has been developed for whole fish, but is not used as it requires re-calculation of the results to a top predator level in the food chain, resulting in all measurements failing to comply (OSPAR, 2016). It is also below what is considered a natural background concentration of mercury.

BACs and EC levels are available for three heavy metals only, cadmium, mercury and lead (Table a)

EC (2006). European Union: Commission Regulation (EC) No. 1881/2006 of 19 December 2006 setting maximum levels for certain contaminants in foodstuffs

Larsen M.M., Blusztajn J.S., Andersen O., Dahllöf I. (2012). Lead isotopes in marine surface sediments reveal historical use of leaded fuel. Journal of Environmental Monitoring, 2012:14, 2893-2901. DOI: 10.1039/c2em30579h

OSPAR (2008). OSPAR Publication 2008-379 CEMP Assessment Manual: Co-ordinated Environmental Monitoring Programme Assessment Manual for contaminants in sediment and biota

OSPAR (2009). OSPAR Publication 2009-461 Background Document on CEMP Assessment Criteria for the QSR 2010

OSPAR (2016). OSPAR Publication 2016-679 Assessment criteria comparison (EAC/EQS) for mercury