Regional assessment of status and trends in metal concentrations in sediment
Overview
This document describes a regional assessment of the status and trends of metal concentrations in sediment in Scottish marine waters. It builds on the assessment of data from the UK’s Clean Seas Environmental Monitoring Programme conducted by the Clean Safe Seas Evidence Group. In this, assessments of status and trends were made for a large number of metal time series, each of a single metal at a single monitoring station. The regional assessment synthesises the results of the individual time series to assess status and trends at the biogeographic regional level.
The regional assessment considers the following metals: cadmium, lead and mercury.
The regional assessment synthesises data from monitoring stations that are in estuarine, coastal and offshore waters. The results might therefore differ from regional assessments for Marine Strategy Framework Directive purposes, which exclude estuarine stations.
The following sections:
- summarise the individual time series results
- describe the methods used to combine the individual time series results to estimate regional status and trends
- present the estimates of regional trends
- present the estimates of regional status
Summary of individual time series results
A time series of metal concentrations is assessed for status if:
- there is at least one year with data in the period 2013 to 2018
- there are at least three years of data over the whole time series
- a parametric model can be fitted to the data and used to estimate the mean concentration in the final monitoring year (or, occasionally, if a non-parametric test of status is applied)
The conditions are more stringent for trends. Specifically, a time series is assessed for trends if:
- there is at least one year with data in the period 2013 to 2018
- there are at least five years of data over the whole time series
- a parametric model can be fitted to the data and used to estimate the trend in mean concentration
Note that all trend assessments for individual time series and most status assessments are based on the fit of a parametric model. This is important because only the parametric results are passed into the regional assessments in the following sections. 4 of the 84 metal time series were assessed for status using a non-parametric test.
The first tab below shows all the monitoring stations where there are individual time series assessments of trend or status for metals. The purple circles are stations where there is a trend assessment for at least one metal, and the light blue cirles are stations where there are only status assessments.
The next set of tabs show the trend and status assessments for each metal in turn. The colours have the following meaning:
- blue: the mean concentration is significantly (p < 0.05) below the Background Assessment Concentration (BAC)
- green: the mean concentration is significantly (p < 0.05) below the Effects Range Low (ERL)
- red: the mean concentration is not signficantly below the ERL
And the shapes have the following meaning:
- downward triangle: the mean concentration is significantly decreasing (p < 0.05)
- circle: there is no significant change in mean concentration (p > 0.05)
- upward triangle: the mean concentration is significantly increasing (p < 0.05)
The final set of tabs give:
- the number of time series with upwards, downwards or no trend in each biogeographic region
- the same information presented as proportions
- the number of time series with blue, green or red status in each biogeographic region
- the same information presented as proportions
Stations
HG
CD
PB
Trend summary (numbers)
Number of time series with upwards, downwards or no trend by biogeographic region
| region | status | Cadmium | Lead | Mercury | total |
|---|---|---|---|---|---|
| Northern North Sea | upward trend | 0 | 0 | 0 | 0 |
| no trend | 10 | 13 | 8 | 31 | |
| downward trend | 1 | 2 | 4 | 7 | |
| Scottish Continental Shelf | upward trend | 0 | 0 | 0 | 0 |
| no trend | 1 | 1 | 0 | 2 | |
| downward trend | 0 | 0 | 0 | 0 | |
| Minches & W Scotland | upward trend | 1 | 0 | 0 | 1 |
| no trend | 3 | 5 | 4 | 12 | |
| downward trend | 0 | 0 | 1 | 1 | |
| Irish Sea | upward trend | 0 | 0 | 0 | 0 |
| no trend | 4 | 2 | 2 | 8 | |
| downward trend | 0 | 2 | 2 | 4 | |
| total | upward trend | 1 | 0 | 0 | 1 |
| no trend | 18 | 21 | 14 | 53 | |
| downward trend | 1 | 4 | 7 | 12 |
Trend summary (proportions)
Proportion of time series with upwards, downwards or no trend by biogeographic region
| region | status | Cadmium | Lead | Mercury | total |
|---|---|---|---|---|---|
| Northern North Sea | upward trend | 0 | 0 | 0 | 0 |
| no trend | 91 | 87 | 67 | 82 | |
| downward trend | 9 | 13 | 33 | 18 | |
| Scottish Continental Shelf | upward trend | 0 | 0 | 0 | |
| no trend | 100 | 100 | 100 | ||
| downward trend | 0 | 0 | 0 | ||
| Minches & W Scotland | upward trend | 25 | 0 | 0 | 7 |
| no trend | 75 | 100 | 80 | 86 | |
| downward trend | 0 | 0 | 20 | 7 | |
| Irish Sea | upward trend | 0 | 0 | 0 | 0 |
| no trend | 100 | 50 | 50 | 67 | |
| downward trend | 0 | 50 | 50 | 33 | |
| total | upward trend | 5 | 0 | 0 | 2 |
| no trend | 90 | 84 | 67 | 80 | |
| downward trend | 5 | 16 | 33 | 18 |
Status summary (numbers)
Number of time series with each status by biogeographic region
| region | status | Cadmium | Lead | Mercury | total |
|---|---|---|---|---|---|
| Northern North Sea | blue | 15 | 14 | 11 | 40 |
| green | 1 | 0 | 3 | 4 | |
| red | 0 | 2 | 2 | 4 | |
| Scottish Continental Shelf | blue | 0 | 1 | 1 | 2 |
| green | 1 | 0 | 0 | 1 | |
| red | 0 | 0 | 0 | 0 | |
| Minches & W Scotland | blue | 5 | 3 | 4 | 12 |
| green | 0 | 2 | 1 | 3 | |
| red | 0 | 0 | 0 | 0 | |
| Irish Sea | blue | 5 | 0 | 0 | 5 |
| green | 1 | 0 | 3 | 4 | |
| red | 0 | 6 | 3 | 9 | |
| total | blue | 25 | 18 | 16 | 59 |
| green | 3 | 2 | 7 | 12 | |
| red | 0 | 8 | 5 | 13 |
Status summary (proportions)
Proportion of time series with each status by biogeographic region
| region | status | Cadmium | Lead | Mercury | total |
|---|---|---|---|---|---|
| Northern North Sea | blue | 94 | 88 | 69 | 83 |
| green | 6 | 0 | 19 | 9 | |
| red | 0 | 12 | 12 | 8 | |
| Scottish Continental Shelf | blue | 0 | 100 | 100 | 67 |
| green | 100 | 0 | 0 | 33 | |
| red | 0 | 0 | 0 | 0 | |
| Minches & W Scotland | blue | 100 | 60 | 80 | 80 |
| green | 0 | 40 | 20 | 20 | |
| red | 0 | 0 | 0 | 0 | |
| Irish Sea | blue | 83 | 0 | 0 | 28 |
| green | 17 | 0 | 50 | 22 | |
| red | 0 | 100 | 50 | 50 | |
| total | blue | 89 | 64 | 57 | 70 |
| green | 11 | 7 | 25 | 14 | |
| red | 0 | 29 | 18 | 16 |
Regional assessment - methods
Tabulating the number of time series with each status category by region provides a quick summary of the individual time series results. However, it does not provide an objective regional assessment of status. Similarly, tabulating the number of time series with an upward or downward trend does not provide an objective regional assessment of trend. This section describes how the individual time series results can by synthesised in a meta-analysis to assess both status and trend at the regional level.
Trends
For a regional trend assessment, the trend in each time series is summarised by the estimated change in log concentration over the last twenty years (or shorter if the time series doesn’t extend that far back). Regional trends are then estimated by fitting the following linear mixed model by restricted maximum likelihood:
- response: trend (yearly change in log concentration)
- fixed model: region * metal
- random model: station + trend estimation variation + residual variation
The fixed model means that a separate regional trend is estimated for each metal. The random model has three terms:
- station allows for variation in trend between stations common to all metals
- trend estimation variation is the variance of the trend estimates from the individual time series analysis, assumed known and fixed
- residual variation is the variation that cannot be explained by any of the fixed effects or the other random effects
The meta-analysis is restricted to time series from monitoring stations that are classified as ‘representative’. ‘Baseline’ stations (those with near pristine conditions or only subject to very diffuse inputs) are omitted because any trends there will likely be caused by different processes to those at representative stations. Similarly ‘impacted’ stations (those close to a point source) are omitted because trends there will likely reflect changes due to the point source, and will be ‘unrepresentative’ at the regional level.
The meta-analysis is further restricted to region and metal combinations with at least three trend stations with good geographic spread. Three stations is considered the minimum required to provide some sort of evidence base at the regional level.
Status
For a regional status assessment, the status of each time series is summarised by the difference between the estimated mean log concentration in the final monitoring year and the log assessment concentration. This ensures that status is always measured on the same scale, even though the assessment concentration might vary between metals and regions. Essentially the same linear mixed model as for trends is then fitted:
- response: status (mean log concentration - log assessment concentration)
- fixed model: region * metal
- random model: station + status estimation variation + residual variation
where status estimation variation is the variation in the status estimates from the individual time series analysis, assumed known and fixed.
There are no restrictions on the time series used in the status meta-analysis based on the classification of the monitoring station; time series from baseline, representative and impacted stations are all included. However, the few time series with a non-parametric assessment of status must be excluded, because there is no summary measure of status to use in the mixed model.
Again, the meta-analysis is restricted to regions and metal combinations with at least three status stations with good geographic spread.
Presentation
The results of both the trend and status regional assessments are back-transformed for presentation. The estimated regional trend is then interpreted as the percentage yearly change in concentration and the estimated regional status as the ratio of the mean regional concentration to the assessment concentration. To illustrate the latter, a value of 1 indicates that the mean regional concentration is equal to the assessment concentration; a value of 0.5 indicates that it is half the assessment concentration, and a value of 2 indicates that it is twice the assessment concentration.
Regional assessment - availability of time series
This section provides more detail on the number and geographic spread of the time series available for the meta-analyses. The map shows the stations with parametric trend and status assessments having excluded trend assessments at baseline and impacted stations. The purple circles are stations where there is a trend assessment for at least one metal, and the light blue cirles are stations where there are only status assessments.
The following regions have an insufficient number or time series or spread of stations and are excluded from the regional assessment:
- trend: Scottish Continental Shelf
- status: Scottish Continental Shelf
Regional trends
The first 3 tabs show:
- Regional trends by metal: regional trend estimates for each metal with pointwise 95% confidence intervals plotted by metal
- Regional trends by region: regional trend estimates for each metal with pointwise 95% confidence intervals plotted by region
- Individual time series: the trend estimates from the individual time series on which the regional assessment is based. The estimates for regions with insufficient geographic spread (and excluded from the meta-analysis) are also shown.
The symbols in all these plots have the following interpretation:
- downward triangle: the mean concentration is significantly decreasing (p < 0.05)
- circle: there is no change in mean concentration (p > 0.05)
- upward triangle: the mean concentration is significantly increasing (p < 0.05)
The final tab shows the estimates of the regional trend by metal.
Regional trends by metal
Regional trends by region
Individual time series
Estimates by region and metal
This table shows the estimates of the regional trend by determinand:
- trend: the estimated mean yearly change in log concentration across the region (multiplied by 100 for presentation)
- se: the corresponding standard error
- lower, upper: the corresponding pointwise 95% confidence limits
- % yearly change: the estimated mean % yearly change in concentration across the region
- %yc lower, %yc upper: the corresponding pointwise 95% confidence limits
| region | metal | trend | se | lower | upper | % yearly change | %yc lower | %yc upper |
|---|---|---|---|---|---|---|---|---|
| Northern North Sea | Cadmium | 0.82 | 1.20 | -1.52 | 3.17 | 0.83 | -1.51 | 3.22 |
| Lead | -1.28 | 0.68 | -2.62 | 0.06 | -1.28 | -2.59 | 0.06 | |
| Mercury | -4.03 | 0.98 | -5.95 | -2.10 | -3.95 | -5.77 | -2.08 | |
| Minches & W Scotland | Cadmium | 3.85 | 1.88 | 0.16 | 7.53 | 3.92 | 0.17 | 7.83 |
| Lead | -0.87 | 1.25 | -3.32 | 1.59 | -0.86 | -3.27 | 1.61 | |
| Mercury | -4.57 | 1.49 | -7.50 | -1.64 | -4.47 | -7.22 | -1.63 | |
| Irish Sea | Cadmium | -0.00 | 1.76 | -3.45 | 3.44 | -0.00 | -3.39 | 3.50 |
| Lead | -2.44 | 1.26 | -4.91 | 0.04 | -2.41 | -4.79 | 0.04 | |
| Mercury | -3.83 | 1.48 | -6.73 | -0.93 | -3.76 | -6.51 | -0.93 |
Regional status compared to the Effects Range Low (ERL)
The first 3 tabs show:
- Regional status by metal: regional status estimates for each metal with pointwise upper one-sided 95% confidence intervals plotted by metal
- Regional status by region: regional status estimates for each metal with pointwise upper one-sided 95% confidence intervals plotted by region
- Individual time series: the status estimates from the individual time series on which the regional assessment is based. The estimates for regions with insufficient geographic spread (and excluded from the meta-analysis) are also shown.
The symbols in all the plots have the following interpretation:
- blue: the mean concentration is significantly (p < 0.05) below the Background Assessment Concentration (BAC)
- green: the mean concentration is significantly (p < 0.05) below the ERL
- red: the mean concentration is not significantly below the ERL
Note that to see why some points are coloured blue, it is necessary to look ahead to the next section, where status is compared to the BAC. Unfortunately, it isn’t easy to present the assessment of status relative to the ERL with the assessment of status relative to the BAC because the ratio of the BAC to the ERL can vary between metals and regions.
The final tab shows the estimates of the regional status by metal.
Regional status by metal
Regional status by region
Individual time series
Estimates by region and metal
This table shows the estimates of the regional trend by determinand:
- status: the estimated mean log concentration minus the log ERL
- se: the corresponding standard error
- upper: the corresponding pointwise upper one-sided 95% confidence limit
- concentration: the estimated mean concentration relative to the ERL
- conc upper: the corresponding pointwise upper one-sided 95% confidence limit
| region | metal | status | se | upper | concentration | conc upper |
|---|---|---|---|---|---|---|
| Northern North Sea | Cadmium | -2.33 | 0.15 | -2.08 | 0.10 | 0.12 |
| Lead | -0.73 | 0.14 | -0.49 | 0.48 | 0.61 | |
| Mercury | -1.19 | 0.15 | -0.95 | 0.30 | 0.39 | |
| Minches & W Scotland | Cadmium | -2.43 | 0.29 | -1.96 | 0.09 | 0.14 |
| Lead | -0.56 | 0.25 | -0.14 | 0.57 | 0.87 | |
| Mercury | -1.31 | 0.26 | -0.88 | 0.27 | 0.42 | |
| Irish Sea | Cadmium | -2.09 | 0.24 | -1.70 | 0.12 | 0.18 |
| Lead | 0.17 | 0.23 | 0.55 | 1.18 | 1.73 | |
| Mercury | 0.01 | 0.23 | 0.40 | 1.01 | 1.49 |
Regional status compared to the Background Assessment Concentration (BAC)
The first 3 tabs show:
- Regional status by metal: regional status estimates for each metal with pointwise upper one-sided 95% confidence intervals plotted by metal
- Regional status by region: regional status estimates for each metal with pointwise upper one-sided 95% confidence intervals plotted by region
- Individual time series: the status estimates from the individual time series on which the regional assessment is based. The estimates for regions with insufficient geographic spread (and excluded from the meta-analysis) are also shown.
The symbols in all the plots have the following interpretation:
- blue: the mean concentration is significantly (p < 0.05) below the BAC
- green: the mean concentration is significantly (p < 0.05) below the Effects Range Low (ERL)
- red: the mean concentration is not significantly below the ERL
The final tab shows the estimates of the regional status by metal.
Regional status by metal
Regional status by region
Individual time series
Estimates by region and metal
This table shows the estimates of the regional trend by determinand:
- status: the estimated mean log concentration minus the log BAC
- se: the corresponding standard error
- upper: the corresponding pointwise upper one-sided 95% confidence limit
- concentration: the estimated mean concentration relative to the BAC
- conc upper: the corresponding pointwise upper one-sided 95% confidence limit
| region | metal | status | se | upper | concentration | conc upper |
|---|---|---|---|---|---|---|
| Northern North Sea | Cadmium | -0.98 | 0.15 | -0.73 | 0.37 | 0.48 |
| Lead | -0.51 | 0.14 | -0.28 | 0.60 | 0.75 | |
| Mercury | -0.43 | 0.15 | -0.18 | 0.65 | 0.83 | |
| Minches & W Scotland | Cadmium | -1.08 | 0.29 | -0.61 | 0.34 | 0.55 |
| Lead | -0.34 | 0.25 | 0.07 | 0.71 | 1.08 | |
| Mercury | -0.55 | 0.26 | -0.11 | 0.58 | 0.89 | |
| Irish Sea | Cadmium | -0.74 | 0.24 | -0.35 | 0.48 | 0.71 |
| Lead | 0.38 | 0.23 | 0.76 | 1.46 | 2.15 | |
| Mercury | 0.77 | 0.23 | 1.16 | 2.16 | 3.18 |