The indicator measures the extent to which levels of metals, polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) are sufficiently low that they are unlikely to harm marine organisms in Scottish waters.

Source of Data:

Contaminant data are collected as part of the UK Clean Seas Environmental Monitoring Programme (CSEMP) to:

a)       cover five contaminant groups – cadmium, mercury, lead, PAHs and PCBs;

b)       in biota (fish and shellfish) and sediments; and,

c)        for four biogeographic regions around Scotland – Northern North Sea, Scottish Continental Shelf, Minches and Western Scotland, and Irish Sea (Clyde & Solway).

The data are submitted to the UK Marine Environment Monitoring and Assessment National (MERMAN) database. 

Annual assessments of the data are made by the UK Clean Safe Seas Evidence Group (CSSEG).  The most recent assessment can be found here [link to be inserted].  The assessments consist of the analysis of a large number of time series, each of the concentration of a single contaminant in a single species (for biota) at a single monitoring station. 

The Clean Seas Indicator 2020 is constructing by synthesising the results of the 2020 CSEMP assessment at the biogeographic regional level.  For biota and sediment in turn, the mean concentration of each contaminant group in each region is compared to the OSPAR Environmental Assessment Criterion (EAC), or an equivalent threshold when the EAC has not been established.  If the mean concentration is statistically significantly below the EAC, then it is unlikely to cause harm to marine life and is acceptably low. 

Combining the contaminant groups (5), the regions (4) and biota or sediment gives 40 possible comparisons of mean concentrations against EACs.  However, it has proved difficult to make regional assessments in the Scottish Continental Shelf due to inadequate spatial coverage.  Therefore, only three regions – Northern North Sea, Minches and Western Scotland, and Irish Sea (Clyde & Solway) – are used in the Clean Seas Indicator.  With these three regions, there are 30 possible comparisons of mean concentrations against EACs. The Indicator is the proportion of these comparisons in which concentrations are acceptably low. 

Note that the Indicator for 2015 – 2017 was originally calculated using all four regions (i.e. including the Scottish Continental Shelf).  However, it has been recalculated (using three regions) so the Indicator is consistent across years.

The data used to produce the 2020 Indicator can be downloaded here.  The methods and detailed results for the 2020 Indicator are given here.


Biogeographic region Scottish waters are divided into four biogeographic regions for contaminant assessments: Northern North Sea, Scottish Continental Shelf, Minches and Western Scotland, and Irish Sea (Clyde & Solway).  Of these, only assessments in the Northern North Sea, Minches and Western Scotland, and Irish Sea (Clyde & Solway) are used in the Clean Seas Indicator. (A fifth region, the Atlantic North West Approaches, is rarely sampled because of its remoteness and depth.)

Polycyclic aromatic hydrocarbons occur naturally, but also arise from incomplete combustion and crude oil and refinery products.  There are many PAHs but the Indicator only uses data for nine of them.

Polychlorinated biphenyls are man-made and have been used as coolants, lubricants, plasticizers and sealing agents.  They were banned over 30 years ago but are extremely persistent in the environment and can still be found in old electrical equipment.  There are many PCBs but the Indicator only used data for seven of them. 

Environment Assessment Criterion are thresholds developed by the Oslo and Paris Commission for assessing the status of chemical concentrations.  If concentrations are below the EAC then they are unlikely to cause adverse effects in marine organisms.  EAC are only available for some of the contaminant groups (PAHs and PCBs in biota and PCBs in sediment).  Other thresholds are used when EACs are unavailable.

Criteria for Change:

This evaluation is based on:

  • any difference in the Indicator of 3% or more over last year's figure suggests that the position has changed. 
  • An increase of 3% or more suggests the position is improving
  • a decrease of 3% or more suggests the position is worsening.

Future issues or reviews:


Data are collected annually and the contaminant assessment is updated accordingly.

A more appropriate threshold for mercury in biota is now available and the Indicator needs to be revised to take this into account. Other developments in international (OSPAR) contaminant assessments can be incorporated at the same time. The Indicator will be revised in 2021 and all previous versions recalculated. The revised Indicator will be more realistic and will show that the environment is less healthy than suggested by the current Indicator. The revised Indicator will also be more sensitive to change.

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