The evaluation is carried out using an agreed R-script (MIME) that applies the statistical analysis.
To evaluate the contamination status of the Baltic Sea, the ratio of the concentration of a metal to the specified concentration (threshold) levels is used for each biotic and abiotic elements (matrix) of the marine environment. A ratio above 1 therefore indicates non-compliance (failure to meet threshold). Taking into account the scope of monitoring programmes implemented by the EU MS regarding heavy metals, and the target concentrations of individual elements, the appropriate measurement matrices were recommended to allow the use of results in Descriptor 8 (Assessment protocol table 1).
All available data on cadmium (in seawater, biota-mussels and bottom sediments), mercury (in biota-fish muscle) and lead (in seawater, fish liver and bottom sediments) concentrations up to 2016, reported by HELCOM Contracting Parties to the HELCOM COMBINE database, were used to assess the state of the Baltic Sea environment.
The assessment of the present environmental status in respect of heavy metal content has been carried out in all assessment units at scale 4, where data availability was sufficient.
The basis for the assessment carried out in the sub-basins was the determination of the concentrations of individual metals in the respective matrices for each station, which were then compared with threshold values to determine the contamination ratio (CR). Good status in respect of single element is scored if CR ≤1.
A two-way approach was used to determine the representative concentrations of the individual metals in the individual matrices. In the case of stations where long-term data series exist, the agreed script (MIME Script) was used. This method allows determination of the upper value of the 95% confidence level which is regarded as a representative concentration. In the case of stations where data are from 1-2 years only or 'less-than' values make the correct assignment of the above statistical procedures impossible then data are treated as 'initial' data. All initial data is handled in a highly precautionary manner to further ensure that the risk of false positives is minimalised. For all initial data the 95% confidence limit on the mean concentration, based on the uncertainty seen in longer time series throughout the HELCOM area, is used. Applying a precautionary approach, the 90% quantile (psi value, Ψ) of the uncertainty estimates in the longer time series from the entire HELCOM region are used. The same approach is used for time series with three or more years of data, but which are dominated by less-than values (i.e. no parametric model can be fitted). The mean concentration in the last monitoring year (meanLY) is obtained by: restricting the time series to the period 2011-2016 (the last six monitoring years), calculating the median log concentration in each year (treating 'less-than' values as if they were above the limit of detection), calculating the mean of the median log concentrations, and then back-transforming (by exponentiating) to the concentration scale. The upper one-sided 95% confidence limit (clLY) is then given by: exp(meanLY + qnorm(0.95) * Ψ / sqrt(n)), where n is the number of years with data in the period 2011-2016 (HELCOM 2018).
The detailed description of MIME Script method can be found: for biota http://dome.ices.dk/osparmime/help_methods_biota_metals.html
for sediment http://dome.ices.dk/osparmime/help_methods_sediment_metals.html
In order to ensure comparability of the measurements to the core indicator threshold value, the data to be extracted from the HELCOM COMBINE database has been defined in a so called 'extraction table'. Relevant sections of the extraction table are presented in Assessment protocol Table 1.
Assessment protocol Table 1. HELCOM COMBINE 'extraction table' relevant to the Metals core indicator. Overview table of the parameters, matrices, basis and supporting parameters selected for extraction from the COMBINE database to evaluate the core indicators
Supporting parameters or parameters underlined within the table are those used for the current indicator assessment.
WT = water, MU = muscle, SB = soft body, LI = liver, MU&EP = muscle and epidermis, WW = wet weight, DW = dry weight.
NOTE 1: Filtered and unfiltered samples for Cd and Pb are currently used. Filtered and unfiltered data are never combined and, when pressent in the same assessmnet two assessments are run in parallel. When aggregating data from different assessments (i.e. aggregating multiple data sets for a single HELCOM assessment unit) the most abundant data set (i.e. filtered or unfiltered) is used, and the other excluded. This process results in minimal exclusion of samples when aggregating at the assessment unit level.
NOTE 2: Metals should be normalized to 5% Al.
The assessment of the present environmental status in respect of heavy metal content should be carried out, if possible – regarding data availability, in all assessment units (assessment units at scale 4).
The core indicator evaluates the status with regard to concentrations of metals using HELCOM assessment unit scale 4 (division of the Baltic Sea into 17 sub-basins division into coastal and offshore areas, and the coastal areas further divided into WFD water types or bodies).
The assessment units are defined in the HELCOM Monitoring and Assessment Strategy Annex 4.