Metals in Sediment and Biota - Marine online assessment tool

In all assessed areas, concentrations of lead in biota are above background levels, and mercury above the environmental threshold. Despite bans, concentrations of mercury and lead in biota are increasing in the Southern North Sea. Concentrations of mercury in sediments are often above the environmental thresholds but decreasing in several subregions. Cadmium in sediment is increasing in the Minches and Western Scotland region.

Background

The most toxic metals to humans and animals are mercury, cadmium, and lead, all of which naturally occur in the environment and are known as trace- or “heavy” metals.

Mercury, cadmium, and lead enter the marine environment from a number of natural, agricultural, and industrial processes, via long-range transport by air, riverine input, or run-off from land (OSPAR Heavy metal inputs indicator assessment). Direct inputs of metals used as antifouling chemicals (mainly copper) and corrosion anodes (mainly zinc) cause hot spots in metal concentrations in and around harbours, marine installations and along shipping routes. Zinc anodes usually also contain traces of lead and cadmium.

All heavy metals have a natural background concentration in water, sediment, and biota. For sediment, background concentrations are related to the grain size of the sediments, with clay minerals containing the highest metal concentrations. Mercury is accumulated in the food chain, resulting in higher concentrations of mercury in fish than in lower trophic levels like shellfish. This effect is modelled, and concentration level of shellfish adjusted to fish levels before the environmental quality standard is used.

Further information

Metals are ubiquitous hazardous substances that occurs naturally in the environment, and they are found in sediment, shellfish, and fish in all UK Regions.

An image showing multiple paint pots in a shipyard

Figure 1a: Shipyard (Reykjavik) ©Jakob Strand

The effects of high concentrations of trace metals on humans can include decreased learning ability (lead and mercury); reduced bone strength (cadmium); and damage to the central nervous system (mercury). This has led to restrictions on most uses of cadmium and lead, and strict bans on mercury use.

Mercury has historically been used in medicine as an antibacterial agent and as a liquid anode in electrolysis in the paper industry. It has also been used in dental fillings, thermometers, and other scientific instruments. The main sources of mercury today are the burning of coal and artisan mining for mercury (Streets and others, 2017).

The Minamata Convention is a global treaty to protect human health and the environment from the adverse effects of mercury (https://www.mercuryconvention.org/en). It was adopted in 2013 and entered into force on 16th August 2017 with 128 parties ratifying the convention by 2021. One main objective of the convention is to stop primary mercury mining by 2032.

Mercury has the potential to evaporate and be transported as a gas through the air; other trace metals are mainly transported as fine particles or bound to other particles. Trace metals can be trapped in deeper levels of sediment until mining, geological or biological processes release them, at which point they may affect biota. Background concentrations in sediments vary from region to region depending on the local lithologies.

The volatility and long-range transport properties of mercury make it particularly sensitive to climate change and effects of climate. Higher temperature increases oxidation rates of mercury in the atmosphere, leading to changes in deposition patterns, more forest fires increase the release of mercury from terrestrial soils, and increased precipitation can lead to higher depositions generally. More oxygen depletion can lead to higher methylmercury production in anoxic sediments, releasing it into the water column. Higher temperatures can also increase the uptake rate in food-webs, and ice-free arctic regions can change food-webs, all leading to higher mercury concentrations in the arctic regions (Krabbenhoft and Sunderland, 2013; AMAP, 2021).

Mercury accumulates in the food chain and is considered the most toxic of the three trace metals. It has been estimated that anthropogenic inputs of mercury emitted to the atmosphere are 7-10 times higher than the natural atmospheric levels (Krabbenhoft and Sunderland, 2013). Cadmium accumulates in the lower trophic levels and possibly in benthic food chains but is bio-diluted in the pelagic food chains (Signa and others, 2017). Lead is usually not accumulated in the marine food chain (EQS substance data sheet for lead, 2005, https://circabc.europa.eu/sd/a/f1779cdb-0583-43d3-9990-7d779917c494/20_Lead_EQSdatasheet_310705.pdf), but in the height of leaded fuels usage in the 1970s, atmospheric levels of lead were a factor of 100 above background level, but even after a ban on leaded fuels, the levels in the atmosphere are still a factor of 5 above the expected background levels.

Cadmium is today used in batteries and electronics and has previously been used in some red paints and plastics. It is found in minerals mined for zinc, copper, and lead, and is a minor constituent of all products of these trace metals. Cadmium can be taken up from the soil by plants. Through this process, it can be concentrated in plants, especially tobacco leaves, sunflower, and linseed.

Both mercury and cadmium are suspected carcinogens (Baines and others, 2021; Liu and others, 2009).

In the Roman empire lead was used for water pipes, as sweetener in wine (lead-acetate) and as colouring for skin-cream. In 1921, automotive engineers introduced lead as an engine lubricant to avoid knocking effects, which became the major source of lead pollution in air, water, and sediment during the 1970s until it was banned in different countries during the 1990s (Larsen and others, 2012). African countries were slower to stop using it, and in 2021, Algeria was the last country to ban the sale of leaded fuels, 100 years after its first introduction (UN, 2021). Lead has also been used as a softener in PVC piping, wires, metal alloys, crystal glass and many other industrial uses, and it is still being used in car batteries.

Metals such as zinc, copper and nickel are essential metals for life and as such have natural mechanisms in place to regulate them. Measured concentrations of essential metals in biota may therefore be purely an indication of maintained physiological levels. A review paper suggests that for copper and zinc, levels are tightly regulated in fish (Bury and others, 2003). Similarly, an exercise by Norway to set national EQS values for metals in shellfish proposed EQS values for the whole body of the blue mussel for nickel, but not copper and zinc that are homeostatically regulated (Ruus and others, 2021). It is therefore possible that any observed temporal trends in these elements actually reflect trends in the fitness of organisms to perform this homeostasis.

Generally, sediment concentrations capture long-term changes (years), whereas concentrations in soft tissues of fish and shellfish indicate recent exposure (days to months) to the metals from surrounding water and prey.

Trace metals do not disappear over time and can be trapped in deeper levels of sediment until mining, geological or biological processes release them, at which point they may affect biota. There are natural concentrations of trace metals in all waters, sediments, shellfish, and fish, referred to as background concentrations. To assess if levels are close to this background, background assessment concentrations (BAC) have been developed, taking into account the levels from remote regions considered to be without anthropogenic inputs (except from long-range transport). The Environmental Quality Standards for secondary poisoning (QSsp) for mercury set by the European Commission (Directive 2013/39/EU) is used as an environmental target, for other metals no QSsp have currently been developed. For human health, the maximum permissible concentration in foodstuff for trace metals in fish and shellfish is set by the European Commission (EC, 2006).

Due to the bioaccumulation potential of mercury, the concentration difference between trophic levels is large. A statistical model adjusting the concentrations of shellfish to that of fish was therefore developed based on the monitoring data (i.e., implicitly taking the trophic level into account based on a statistical model). Before this transformation, shellfish and fish concentrations were clearly separated in the regions while less difference was found between fish species.

For sediments, metals are a natural part of different minerals. Higher concentrations are found in finer (mud, silt/clay) sediment than in sand, and the natural background hence depends on the overall grain size distribution of the sediment. This is taken into account by normalising (Loring, 1991; Kersten & Smedes, 2002; OSPAR, 2018) using sieving and/or normalising concentrations to the aluminium concentration of the sediment samples. For the UK MS 2024 assessment, metal concentrations in sediments are recalculated to 5% aluminium contents to reduce the influence of such grain size effects in all areas.

Assessment method

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

‘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 trace metals 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 trace metals. There is only an environmental quality standard (EQS) available for mercury in fish, not for other metals in biota. Previously the European Commission (EC) maximum concentration limits for trace metals in seafood (fish and shellfish) were used in UK MS 2017 as proxy Environmental Assessment Criteria (EACs), but in this assessment, these are only used as thresholds for human health.

For the UK MS 2024, trends in trace metal concentrations in biota are presented. Two assessment criteria are used to assess the status of trace metal concentrations in biota: For background, Background Assessment Concentrations (BACs) were used for all metals, for environmental thresholds only mercury has been assessed based on the Water Framework Directive Regulation Environmental Quality Standard for secondary poisoning (QSsp) of mercury in fish.

Unfortunately, no such value is currently available for other trace metals. The human health assessment is based on thresholds by the European Commission maximum permissible concentrations (MPC) for fish and seafood (EC, 2006), available for all three metals and individual fish and shellfish species. The MPC are treated separately as this is more relevant for the Descriptor 9 (contaminants in seafood assessment).

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 2010-2020. Mean concentrations significantly below the BAC are said to be near background (naturally occurring 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. For sediments, dated sediment cores have also been used to establish pre-industrial levels. In this way, they relate to the background values referred to in the OSPAR Hazardous Substances Strategy 2010-2020. 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 Coordinated Environmental Monitoring Programme (CEMP) data, and local conditions should be taken into account when assessing the significance of any exceedance.

BACs are calculated according to the method set out in Section 4 of the CEMP Assessment Manual (OSPAR, 2009) and updated in OSPAR (MIME 2020, 2021). 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 like trace metals.

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 by the relevant assessment working group, as further CEMP monitoring data are collected.

Provenance and limitations of EC maximum levels in fish and seafood

To assess the effect of metals on human health, the European Commission’s maximum permissible concentrations (MPC) in fish and seafood (EC, 2006) have been used. EC MPC are applied in European Union Member States’ control of foodstuffs with the aim of protecting 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 MPC may be reviewed. If the EC maximum food levels for fish and shellfish are exceeded, the catch should not be put on the market.

The MPCs for lead, mercury and cadmium in fish muscle and shellfish were previously used as alternatives to EACs for trace metals in both fish and shellfish species for the MS 2018 Assessment. 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 common indicator assessment for trace metals in biota should be treated with care. Unfortunately, only one environmentally based threshold (QSsp) is available for mercury in whole fish.

Provenance and limitations QSsp thresholds

The EU Water Framework Directive put forward a method for setting Quality Standards in the EU (EC, 2011), and its interpretation for evaluating monitoring in the EU (EC, 2014). For mercury, the derivation of the QSsp was thoroughly discussed in OSPAR (OSPAR, 2016), and the application of the EU (EC, 2011) approach has large uncertainties when defining the trophic bioconcentration, as it is different for methylmercury and inorganic mercury, and the QSsp is mainly derived from methylmercury but applied to total mercury. The QSsp was also used on mussels using a statistical model to transform the concentration levels found in mussels to fish levels, implicitly taking trophic level differences into account. This is different to how the shellfish data would have been assessed for Water Framework Regulations as here it was used uncorrected and therefore would have been more likely to be below the QSsp.

Provenance and limitations of ERLs

OSPAR has not yet established EACs for trace metals in sediment, and there are no EC equivalent firm EQS values for sediments. For some substances, an interim QSsediment has been suggested, but in most cases, the recalculation from water to sediment is considered too uncertain to make this official. Therefore, a proxy means of assessment has been used. ‘Effects range’ values were developed by the United States National Oceanic and Atmospheric Administration (NOAA, 1999) for assessing the ecological significance of contaminant concentrations in sediment, to protect against the potential for adverse biological effects on organisms. Concentrations below the Effects Range-Low (ERL) level rarely cause adverse effects in marine organisms.

The ERL value is defined as the lower tenth percentile of the data set of concentrations in sediments, which were associated with biological effects. Adverse effects on organisms are rarely observed when concentrations fall below the ERL value, and the ERL therefore has some parallels with the philosophy underlying the OSPAR EACs and the Environmental Quality Standards (EQSs) of the European Union Water Framework Directive. The procedure by which ERL criteria are derived is very different from the methods used to derive EACs and EQSs, and so precise equivalence between the two sets of criteria should not be expected. ERL values are used in sediment assessments of trace metals as an interim solution where recommended EACs are not available.

Assessment method

For each trace metal at each monitoring site, the time series of concentration measurements was assessed for trends and status using the methods described in the contaminant’s online 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.

Trend assessments included those monitoring sites that were representative of general conditions or baseline and excluded those monitoring sites impacted due to a point source. 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:

~ trace metal: UK contaminants assessment areas

and random effects:

~ monitoring site + metal: monitoring site [biota only] + residual variation

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

between monitoring sites common across trace metals (monitoring site).

between metals but common across tissues and species within monitoring sites (metal: 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 trends in trace metal concentrations at the assessment area scale was then assessed by plotting the estimated fixed effects with pointwise 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 QSsp; 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. The analyses using the QSsp also included a term to estimate and adjust for the difference in concentrations between fish and shellfish.

For trace metals, BACs have been developed for mussels, oysters, and fish. An EC Environmental Quality Standard of 20 µg/kg ww for mercury has been developed for whole fish, below the established BAC for mercury of 35 µg/kg ww in fish, and very close to the BAC for shellfish (~18 µg/kg ww after conversion of the 90 µg/kg dw value for mussels). Therefore, for fish and shellfish, this BAC is probably not an accurate value.

BACs and EC levels are available for three trace metals only, cadmium, mercury, and lead (Table A).

Table a: Assessment criteria used in for trace metals in sediment, fish, and shellfish: BAC is Background Assessment Concentrations developed by OSPAR, QSsp are Environmental Quality Standards for secondary poisoning developed under the Water Framework Directive, MPC are European Commission maximum levels in fish and seafood; ERL, Effects Range-Low (O’Conner, 2004); dw, dry weight. ww, wet weight.

Sediments

	Symbol	BAC (5% Aluminium)	BAC	ERL
mg/kg dw		All subregions except Iberian Sea and Gulf of Cadiz	Iberian Sea and Gulf of Cadiz	All subregions
Cadmium	Cd	0.31	0.129	1.2
Mercury	Hg	0.07	0.091	0.15
Lead	Pb	38	22.4	47

Biota

	BAC	BAC	BAC	QSsp	MPC	MPC	MPC
	mussels	oysters	fish	Whole fish	Fish muscle	Bivalves	Crustaceans
	μg/kg dw	μg/kg dw	μg/kg ww	μg/kg ww	μg/kg ww	μg/kg ww	μg/kg ww
Mercury	90	180	35	20	500	500	500
Cadmium	960	3 000	26		50	1 000	500
Lead	1 300	1 300	26		300	1 500	500

Table A notes:

BAC for sediment is normalised to 5% aluminium.

ERLs are not normalised but considered valid for 5% aluminium.

BACs for mussels and oysters are expressed as μg/kg dw (dry weight) and BACs for fish, QSsp and MPC are expressed as μg/kg ww (wet weight); Cadmium BAC for fish is only applied when lipid >3% and mercury BAC for fish was above QSsp so not used.

BACs and EC levels are converted to other bases (wet, dry or lipid weight) using species-specific conversion factors.

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

Marine Strategy sub-Region	UK region	Sediment	Sediment	Fish and shellfish	Fish and shellfish
		Trends	Status	Trends	Status
Greater North Sea	Northern North Sea	11	14	33	38
Greater North Sea	Southern North Sea	5	10	19	19
Greater North Sea	East Channel	3	4	4	7
Celtic Sea	Scottish Continental Shelf	0	0	1	1
Celtic Sea	Minches and Western Scotland	4	4	7	5
Celtic Sea	Irish sea	15	17	35	47
Celtic Sea	West Channel and Celtic Sea	2	2	6	6

Differences in methodology used for the MS 2018 compared with the UK MS 2024

This assessment includes the assessment of metals in sediment and biota. These were assessed separately for the MS 2018 assessment. Although not presented in this assessment, the status assessment against the human health standards (MPC) can be found on the OSPAR Hazardous Substances Assessment Tool (with mercury, lead and cadmium all below MPC values in all assessed regions) https://dome.ices.dk/OHAT/?assessmentperiod=2023.

The assessment thresholds in fish and shellfish used in MS 2018 are reported as “human health” thresholds in the UK MS 2024, and environment thresholds for mercury have not been used before in fish and shellfish. There were no environmental thresholds for metals other than mercury, whereas BACs were available for other metals, but not for mercury in fish, as this was above the QSsp for fish, and hence excluded from the assessment criteria.

Results

Metals concentrations were measured in sediment and biota from monitoring sites throughout much of the UK biogeographic regions (excluding estuarine sites) (Figure 1 and Figure 2). The frequency of sampling ranges from annually to every six years.

Figure 1: Monitoring sites used to assess metal concentrations in fish and 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 2: Monitoring sites used to assess metal 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.

For the assessment of status and temporal trends, only areas with at least three monitoring sites and a reasonable geographic spread were included, reducing the number of subregions available to 6 out of 7 for fish and shellfish and 4 out of 7 for sediments. Fish and shellfish data were combined to a fish-normalised concentration level for mercury comparison with the QS_sp.

An environmental threshold has only been defined for mercury in fish and shellfish. All regions were significantly above this threshold (Figure 3). Mercury, cadmium and lead concentrations in fish and shellfish were above the background concentration in all regions (Figure 4).

Figure showing the mean mercury concentration in fish and shellfish relative to the Quality Standard secondary poisoning in six biogeographic regions. The regions are the Northern North Sea, the Southern North Sea, the Eastern Channel, the Minches and Western Scotland, the Irish Sea, and the Western Channel and Celtic Sea. The mean concentration exceeds the Quality Standard in all six regions.

Figure 3: The mean mercury concentration (coloured circles) in fish and shellfish in each biogeographic region relative to the Quality Standard secondary poisoning (QSsp). A value of 1 occurs when the mean concentration equals the QSsp. The horizontal line indicates the upper one-sided 95% confidence limit on the mean. The mean concentration is significantly below the QSsp (p<0.05) if its upper confidence limit is less than 1. The red circle indicates that the mean concentration is not significantly below the QSsp (p>0.05).

Figure 4: The mean concentration of cadmium, lead and mercury (coloured circles) in fish and shellfish in each biogeographic region relative to the Background Assessment Concentration (BAC). A value of1 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. Amber: the mean concentration is not significantly below the BAC (p>0.05) and no Quality Standard secondary poisoning (QSsp) is available. Red: the mean concentration is not significantly below the QSsp (p>0.05).

For concentrations in sediment, all subregions were statistically significantly below the ERL for cadmium, whereas for mercury and lead, only the Northern North Sea and Minches and Western Scotland were below the ERL, and both Southern North Sea and Irish Sea were above (Figure 5).

Figure 5: The mean concentration of cadmium, lead and mercury (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. Light blue: the mean concentration is significantly below the Background Assessment Concentration (BAC) (p<0.05). Dark blue: the mean concentration is significantly below the ERL (p<0.05) but not the BAC. Red: the mean concentration is not significantly below the ERL (p>0.05).

Temporal trends for the metal concentrations in fish, shellfish and sediments were modelled between the earliest monitoring dates (1999) and 2021, and the change in concentration calculated as the yearly change, if more than 5 years of data were available.

For mercury, cadmium and lead in fish and shellfish, no decreasing trends were observed for any UK biogeographic region, and for the Southern North Sea, increasing trends of 2.3 % and 4.2 % yearly change (Figure 6) were observed for mercury and cadmium, respectively.

Figure 6: The percentage annual change (circle, triangle) in the mean concentration of cadmium, lead and mercury in fish and shellfish 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).

In sediments, mercury has significantly decreased trends in Northern North Sea (-2.3%), Minches and Western Scotland (-3.8%) and Irish Sea (-2.8%). Cadmium mostly showed no significant change except for Minches and Western Scotland where there was a +4.9% increase per year on average. Lead also mostly showed no significant change except for the Irish Sea which had a -1.8% decrease per year on average (Figure 7).

Figure 7: The percentage annual change (circle, triangle) in the mean concentration of cadmium, lead and mercury 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). Upward triangle: significant increase in mean concentration (p<0.05).

Of the 222 trend assessments in the Celtic Seas and Greater North Sea for cadmium, lead and mercury, 33 (15%) showed a significant upward trend and 26 (12%) showed a significant downward trend. Overall, levels of these three metals can be considered to be stable as 73% show no significant trend.

Further information

A wider range of metals are monitored under the Coordinated Environmental Monitoring Programme (CEMP) than just mercury, lead and cadmium. Data is also available for zinc, copper, nickel, silver, selenium, arsenic, chromium and cobalt, though not all of these have BAC and/or ERL values available. Mean metal concentrations in fish and shellfish (Figure A) and sediment normalised to aluminium content (Figure B) are shown relative to BAC for the assessed areas for all available metals.

Similar to mercury, cadmium and lead, status for biota samples of zinc and copper are both above background levels in two UK biogeographic regions (Figure C). As status could only be assessed in Northern North Sea and Irish Sea regions, it is difficult to compare patterns of these with other metals.

Figure A: Mean metal concentrations (coloured circles) in fish and shellfish 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. Amber: the mean concentration is not significantly below the BAC (p>0.05) and no Quality Standard secondary poisoning (QSsp) is available. Red: the mean concentration is not significantly below the QSsp (p>0.05).

In sediments, zinc has a similar pattern to mercury and lead, with Southern North Sea and Irish Sea above ERLs. Arsenic and copper have a similar pattern to cadmium, with 3 regions (Northern North Sea, Minches and Western Scotland and Irish sea at background concentrations), although only copper out of these is above ERL at Southern North Sea. Generally Southern North Sea has the worst status, with four metals (zinc, copper, lead and mercury) above ERLs and no metals at or below BAC. Irish Sea is next worse with three metals (zinc, lead and mercury) above ERLs, though three metals (copper, arsenic and cadmium) are at background levels. Northern North Sea has the best status, with five metals (nickel, zinc, copper, arsenic and cadmium) at background levels and no metals above ERL, followed by Minches and Western Scotland with four metals (copper, mercury, arsenic and cadmium) at background levels and also no metals above ERL.

Figure B: Mean metal concentrations (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. Red: the mean concentration is not significantly below the ERL (p>0.05).

There is a wider suite of metals available for assessing temporal trends although, for some of these, regional coverage is poor because sufficient data is only available for assessment from Scottish sites. Temporal trends in metal 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 metal in each assessment area is shown in Figure c for fish and shellfish and Figure d for sediment.

In fish and biota stations, the majority of metals and regions had no statistically significant upwards or downwards trends. The exceptions were: the Southern North Sea which had significant upwards trends for mercury (+2.3% per year), copper (+3.1% per year) and cadmium (+4.2% per year), the Northern North Sea which had a significant upward trend for chromium (7.8% per year), and the Irish Sea which had both a significant upward trend for copper +(2.5% per year) and significant downwards trends for silver (-17% per year) and chromium (-3.0% per year). Ranking the metals in fish and shellfish according to the number of subregions with increasing concentrations, copper is the worst – increasing in two of the five subregions, and no subregions show decreasing trends. Cadmium, chromium and mercury are all increasing in one region, whilst silver and chromium are both decreasing in one region.

In sediments, mercury has significant decreasing trends in Northern North Sea (-2.3%), Minches and Western Scotland (-3.8% per year) and Irish Sea (-2.8% per year). Several other metals also had decreasing trends in the Irish Sea, including zinc (-1.4% per year), copper (-2.7% per year) and lead (-1.8% per year). Significant upward trends were found for arsenic in the Northern North Sea (+2.0% per year) and cadmium in Minches and Western Scotland (+4.9% per year). Most other metals and regions had no significant trends.

Figure D: The percentage annual change (circle, triangle) in the mean metal concentrations 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). Upward triangle: significant increase in mean concentration (p<0.05).

As the expectations are that inputs are decreasing from air and rivers, the general sediment downward time trends make sense, whereas the upward time trends for fish and shellfish in the Southern North Sea point to an internal release from the sediment to the water and marine organisms, or a change in the uptake of metals in fish and shellfish due to higher temperatures (Kibria and others, 2021) or increased bioavailability due to ocean acidification (Stockdale and others, 2016).

Conclusions

Mercury was found to be problematic in all areas for shellfish and fish, being above both the BAC for mussels and the QSsp assessment criteria. A new, more protective environmental quality standard has been used for mercury than in the previous assessment which has resulted in a different assessment outcome. Lead and cadmium were above background levels for shellfish and fish in all areas, but there are no environmental quality thresholds to compare these against. In sediments, mercury, lead and zinc were above the ERL assessment criteria in the Southern North Sea and the Irish Sea. However, trends for mercury in all assessed regions and most metals in the Irish Sea showed concentrations were decreasing.

Despite the global convention on mercury, and much legislation on reducing or banning mercury and other metals being in place, time trends are showing increasing levels in the Southern North Sea for biota, though levels were decreasing in sediments. As the input to the marine environment seems to be decreasing, the upward time trends in biota for mercury and other metals could be related to changes in availability from the sediments, which may be a side-effect of ocean acidification.

Further information

Mercury was found to be problematic in all areas for shellfish and fish, being above both the BAC for mussels and the QSsp assessment criteria. Mercury is the only metal with a defined EQS value for fish and shellfish adjusted in the EU Water framework Directive. In sediments, mercury was above the ERL assessment criteria in two of the four regions with data, with Minches and Western Scotland below background assessment concentrations.

For lead and cadmium, levels for shellfish and fish were above background in all of the regions. In sediments, lead was similar to mercury compared to ERL being above it in Southern North Sea and Irish Sea. For cadmium, no areas were above the ERL, and three areas were below BAC: Northern North Sea, Minches and Western Scotland and Irish Sea. Copper and arsenic were also below BAC in these three regions. Zinc and copper concentrations in shellfish and fish are also above the background concentration in all areas, with sediment concentrations in copper only above ERL in the Southern North Sea. For many other metals, no assessment criteria are available (arsenic, cobalt, nickel, selenium and silver).

The trends for mercury were either upwards (Southern North Sea) or not significant in fish and shellfish. For cadmium and copper, the Southern North Sea also exhibited increasing trends, with copper also increasing in Irish Sea. Lead showed no significant trends in any region. In the three areas with time trends for sediments, all mercury trends were decreasing. Cadmium was increasing in Minches and Western Scotland but showing no trends elsewhere. Lead was decreasing in the Irish Sea. The discrepancy between the direction of mercury time-trends in biota and sediment could be an indication that mercury is being released from the sediment e.g. due to warming trends increasing oxygen deficit and increasing sediment methylation, or increased influx from the atmosphere, not yet reaching the sediment. More detailed studies of mercury circulation in the North Sea are needed to conclude on this, though. In the Southern North Sea, mercury, copper and cadmium are showing no trend in the sediment but increasing in biota. Time-trends in biota are both showing upward and downward trends, varying from metal to metal and area to area. In the Irish Sea, decreasing trends of zinc, copper, cadmium and lead in sediment is observed, whereas the only significant trend found for biota there is an increase in copper. Only arsenic (Northern North Sea) and cadmium (Minches and Western Scotland) showed increasing trends in sediments, noting that only four areas had enough data for time-trend analysis.

Shipping traffic and heavy industry could be the sources of pollution, but for sediments, copper and zinc which would be expected to correlate with shipping intensity are only showing downward trends, indicating more local studies and assessments are probably needed to explain the results. Most sediment data are from more offshore areas, whereas shellfish samples are coastal, and fish can move between the two areas. The time-trends could therefore also signify a difference between coastal, more directly polluted waters, and open water with large dilution to land-based sources. Despite the global convention on mercury, and much legislation on reducing or banning mercury and other metals being in place, time trends are showing increasing levels in some subregions for biota, whilst decreasing in sediments. As the input to the marine environment seems to be decreasing, the increasing biota trends could suggest that bioavailability of metals in sediments is increasing. This has been predicted to be a side-effect of ocean acidification (Stockdale and others, 2016).

Knowledge gaps

There is a lack of ecotoxicological data for developing new assessment criteria based on the European Union Water Framework Directive or OSPAR Environmental Assessment Criteria (EAC) principles, to replace the current ERL criteria for sediment. Thresholds for trace metals in sediments, fish and shellfish should be developed or updated. There are also no environmental criteria for cadmium and lead in biota.

The reasons for the increasing concentrations of trace metals in biota, particularly in the Southern North Sea need to be investigated to identify the sources or environmental processes. Links between metal concentrations and confounding factors like the size of shellfish and fish, the condition of the species analysed, trophic level and dry weight/lipid concentrations should be investigated further.

Further information

Of the additional metals in fish and shellfish, more data for silver, nickel, selenium, arsenic and chromium has been collected than has been submitted to MERMAN for inclusion in assessments. If this data was made available, trends for more metals in more regions could be investigated.

Missing agreed assessment criteria and conversion factors between organs are the main knowledge gap to improve the assessment and OSPAR area status:

Development and agreement on assessment criteria (e.g. EQS (QSsp) values in biota for cadmium, lead, and other metals to be used as environmental thresholds in future assessments. Establishing standard distribution factors between liver, muscle, and whole fish for metals in different species for recalculation of concentrations measured in existing monitoring to comparison with the environmental thresholds.

Development of QSsediment criteria based on the WFD/MSFD principles for reliable assessment of the impact of sediment concentrations on the marine environment, to replace the current ERL thresholds.

Development of a better understanding of why concentrations of some metals in sediment, fish and shellfish are increasing at some sites and decreasing at others, despite perceived decreasing inputs to the marine environment.

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Authors

Jon Barber¹

¹Centre for Environment, Fisheries and Aquaculture Science

Assessment Metadata

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