MIME regional assessment of status and trends in metal concentrations in sediment

Overview

The 2017 MIME assessment describes the trends and status of contaminant concentrations in biota and sediment at monitoring stations in the OSPAR area. Assessments are made for a large number of time series, each of a single contaminant in a single species (for biota) at a single monitoring station. This document is one of a series that synthesises the results of the individual time series to assess status and trends at the MIME regional level. In particular, it considers metal concentrations in sediment, where the metals are HG, CD, PB. For simplicity, the term ‘region’ is used throughout to describe MIME regions. OSPAR regions are always referred to as such.

The following sections:

  • summarise the individual time series results
  • describe the methods used to combine the individual time series results to estimate regional trends and status
  • 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 2010 to 2015
  • 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 2010 to 2015
  • 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 concentrations

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. Only 11 of 479 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 magenta circles are stations where there is a trend assessment for at least one metal, and the cyan 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 Effects Range Low (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 MiME and OSPAR region
  • the same information presented as proportions
  • the number of time series with blue, green, or red status in each MIME and OSPAR 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 MIME and OSPAR region

OSPAR region MIME region status HG CD PB total
2 Northern North Sea upward trend 0 0 0 0
no trend 16 12 20 48
downward trend 3 3 2 8
Southern North Sea upward trend 0 0 1 1
no trend 25 30 42 97
downward trend 23 17 13 53
Channel upward trend 0 0 1 1
no trend 4 4 4 12
downward trend 1 0 0 1
total upward trend 0 0 2 2
no trend 45 46 66 157
downward trend 27 20 15 62
3 Irish and Scottish West Coast upward trend 0 1 0 1
no trend 6 5 5 16
downward trend 1 0 2 3
Irish Sea upward trend 0 2 1 3
no trend 11 8 12 31
downward trend 6 2 4 12
Celtic Sea upward trend 0 1 1 2
no trend 2 1 0 3
downward trend 0 0 1 1
total upward trend 0 4 2 6
no trend 19 14 17 50
downward trend 7 2 7 16
4 Iberian Sea upward trend 1 0 0 1
no trend 13 13 12 38
downward trend 1 2 3 6
Gulf of Cadiz upward trend 0 1 1 2
no trend 8 13 13 34
downward trend 6 0 0 6
total upward trend 1 1 1 3
no trend 21 26 25 72
downward trend 7 2 3 12


Trend summary (proportions)

Proportion of time series with upwards, downwards or no trend by MIME and OSPAR region

OSPAR region MIME region status HG CD PB total
2 Northern North Sea upward trend 0 0 0 0
no trend 84 80 91 86
downward trend 16 20 9 14
Southern North Sea upward trend 0 0 2 1
no trend 52 64 75 64
downward trend 48 36 23 35
Channel upward trend 0 0 20 7
no trend 80 100 80 86
downward trend 20 0 0 7
total upward trend 0 0 2 1
no trend 62 70 80 71
downward trend 38 30 18 28
3 Irish and Scottish West Coast upward trend 0 17 0 5
no trend 86 83 71 80
downward trend 14 0 29 15
Irish Sea upward trend 0 16 6 7
no trend 65 67 71 67
downward trend 35 17 23 26
Celtic Sea upward trend 0 50 50 33
no trend 100 50 0 50
downward trend 0 0 50 17
total upward trend 0 20 8 8
no trend 73 70 65 70
downward trend 27 10 27 22
4 Iberian Sea upward trend 7 0 0 2
no trend 86 87 80 85
downward trend 7 13 20 13
Gulf of Cadiz upward trend 0 7 7 5
no trend 57 93 93 81
downward trend 43 0 0 14
total upward trend 4 3 4 3
no trend 72 90 86 83
downward trend 24 7 10 14


Status summary (numbers)

Number of time series with each status by MIME and OSPAR region

OSPAR region MIME region status HG CD PB total
2 Northern North Sea blue 11 15 12 38
green 3 9 2 14
red 10 1 11 22
Southern North Sea blue 2 6 1 9
green 11 53 3 67
red 50 5 61 116
Channel blue 0 1 0 1
green 2 8 1 11
red 13 5 14 32
total blue 13 22 13 48
green 16 70 6 92
red 73 11 86 170
3 Irish and Scottish West Coast blue 5 6 5 16
green 2 1 1 4
red 1 1 1 3
Irish Sea blue 4 13 5 22
green 6 4 2 12
red 7 1 11 19
Celtic Sea blue 0 0 0 0
green 1 2 1 4
red 1 0 1 2
total blue 9 19 10 38
green 9 7 4 20
red 9 2 13 24
4 Iberian Sea blue 0 0 0 0
green 0 14 0 14
red 15 1 15 31
Gulf of Cadiz blue 0 3 1 4
green 8 11 1 20
red 6 0 12 18
total blue 0 3 1 4
green 8 25 1 34
red 21 1 27 49


Status summary (proportions)

Proportion of time series with each status by MIME and OSPAR region

OSPAR region MIME region status HG CD PB total
2 Northern North Sea blue 46 60 48 51
green 12 36 8 19
red 42 4 44 30
Southern North Sea blue 3 9 1 5
green 18 83 5 35
red 79 8 94 60
Channel blue 0 7 0 2
green 13 57 7 25
red 87 36 93 73
total blue 13 21 12 15
green 16 68 6 30
red 71 11 82 55
3 Irish and Scottish West Coast blue 63 75 72 70
green 25 13 14 17
red 12 12 14 13
Irish Sea blue 24 72 28 41
green 35 22 11 23
red 41 6 61 36
Celtic Sea blue 0 0 0 0
green 50 100 50 67
red 50 0 50 33
total blue 34 68 37 46
green 33 25 15 25
red 33 7 48 29
4 Iberian Sea blue 0 0 0 0
green 0 93 0 31
red 100 7 100 69
Gulf of Cadiz blue 0 21 7 9
green 57 79 7 48
red 43 0 86 43
total blue 0 10 4 5
green 28 86 3 39
red 72 4 93 56


MIME 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 regions 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 criterion might vary between metals and time series. 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 tabs show the stations with parametric trend and status assessments for each OSPAR region in turn, having excluded trend assessments at baseline and impacted stations. The magenta circles are stations where there is a trend assessment for at least one metal, and the cyan 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: Celtic Sea
  • status: Celtic Sea, Iberian Sea


OSPAR region 1


OSPAR region 2


OSPAR region 3


OSPAR region 4


Regional trends

The first 3 tabs show:

  • Regional trends by metal: regional trend estimates with pointwise 95% confidence limits by metal
  • Regional trends by region: regional trend estimates with pointwise 95% confidence limits by region
  • Individual time series: the trend estimates from the individual time series on which the regional assessment is based. The estimates from 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 show the estimates of the regional trend by metal.


Regional trends by metal