Regional assessment of status and trends in imposex (VDS) in whelks
Overview
This document describes a regional assessment of the status and trends of imposex (VDS) in dogwhelks in Scottish marine waters. It builds on the 2020 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 imposex time series, each consisting of VDS measurements in dogwhelks 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 synthesises data from monitoring stations that are in estuarine, coastal and offshore waters. The results might therefore differ from regional assessments for Marine Strategy 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 VDS measurements is assessed for status if:
- there is at least one year with data in the period 2014 to 2019
- there are at least three years of data over the whole time series
A time series is also assessed for trends if:
- there are at least three years of data with individual measurements or four years of data with pooled measurements
The first tab below shows all the monitoring stations where there are individual time series assessments of trend or status for VDS. The purple circles are stations where there is a trend assessment and the light blue cirles are stations where there is only a status assessment,
The next set of tabs show the trend and status assessments for VDS. The colours have the following meaning:
- blue: the mean VDS is significantly (p < 0.05) below the Background Assessment Concentration (BAC)
- green: the mean VDS is significantly (p < 0.05) below the Environmental Assessment Criterion (EAC)
- red: the mean VDS is not signficantly below the EAC
And the shapes have the following meaning:
- downward triangle: the mean VDS is significantly decreasing (p < 0.05)
- circle: there is no significant change in mean VDS (p > 0.05) or there are too few years to test for trends
- upward triangle: the mean VDS 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
VDS
Trend summary (numbers)
Number of time series with upwards, downwards or no trend by biogeographic region
| region | status | VDS |
|---|---|---|
| Northern North Sea | upward trend | 0 |
| no trend | 5 | |
| downward trend | 4 | |
| Scottish Continental Shelf | upward trend | 0 |
| no trend | 7 | |
| downward trend | 13 | |
| Minches & W Scotland | upward trend | 0 |
| no trend | 2 | |
| downward trend | 0 | |
| Irish Sea | upward trend | 0 |
| no trend | 2 | |
| downward trend | 1 | |
| total | upward trend | 0 |
| no trend | 16 | |
| downward trend | 18 |
Trend summary (proportions)
Proportion of time series with upwards, downwards or no trend by biogeographic region
| region | status | VDS |
|---|---|---|
| Northern North Sea | upward trend | 0 |
| no trend | 56 | |
| downward trend | 44 | |
| Scottish Continental Shelf | upward trend | 0 |
| no trend | 35 | |
| downward trend | 65 | |
| Minches & W Scotland | upward trend | 0 |
| no trend | 100 | |
| downward trend | 0 | |
| Irish Sea | upward trend | 0 |
| no trend | 67 | |
| downward trend | 33 | |
| total | upward trend | 0 |
| no trend | 47 | |
| downward trend | 53 |
Status summary (numbers)
Number of time series with each status by biogeographic region
| region | status | VDS |
|---|---|---|
| Northern North Sea | blue | 0 |
| green | 9 | |
| red | 2 | |
| Scottish Continental Shelf | blue | 7 |
| green | 12 | |
| red | 1 | |
| Minches & W Scotland | blue | 1 |
| green | 0 | |
| red | 1 | |
| Irish Sea | blue | 0 |
| green | 3 | |
| red | 0 | |
| total | blue | 8 |
| green | 24 | |
| red | 4 |
Status summary (proportions)
Proportion of time series with each status by biogeographic region
| region | status | VDS |
|---|---|---|
| Northern North Sea | blue | 0 |
| green | 82 | |
| red | 18 | |
| Scottish Continental Shelf | blue | 35 |
| green | 60 | |
| red | 5 | |
| Minches & W Scotland | blue | 50 |
| green | 0 | |
| red | 50 | |
| Irish Sea | blue | 0 |
| green | 100 | |
| red | 0 | |
| total | blue | 22 |
| green | 67 | |
| red | 11 |
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 odds ratio of the VDS of an individual whelk being above the EAC in one year relative to the previous year. Values of 1 indicate no trend; i.e. the odds of an individual being above the EAC in one year are the same as in the year before. Values < 1 indicate that the odds of being above the EAC in one year are lower than in the year before, so there is a decline in the level of imposex. Conversely, values > 1 indicate an increase in the level of imposex. Regional trends are then estimated by fitting the following linear mixed model by restricted maximum likelihood:
- response: trend (odds ratio)
- fixed model: region
- random model: trend estimation variation + residual variation
The fixed model means that a trend is estimated for each region. The random model has two terms:
- 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’ or ‘impacted’ (close to a point source). ‘Baseline’ stations (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 or impacted stations. Note that the inclusion of impacted stations differs from the approach used for contaminants (where they are excluded). Many imposex monitoring stations were chosen because they were close to harbours (i.e. impacted). However, the ban of TBT (which causes imposex) will have been the major driver of trends in imposex throughout , so it seems plausible that trends in VDS might be similar (on the logistic scale) at both impacted and representative stations.
The meta-analysis is further restricted to regions 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 ratio between the estimated mean VDS in the final monitoring year and the assessment critrion. This ensures that status is always measured on the same scale, even though the assessment criterion might vary between time series (because VDS is measured in a different species). The ratio is transformed to the square root scale (to better satisfy modelling distributional assumptions) and essentially the same linear mixed model as for trends is then fitted:
- response: status (sqrt(mean VDS / assessment criterion))
- fixed model: region
- random model: 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.
Again, the meta-analysis is restricted to regions with at least three status stations with good geographic spread.
Presentation
The results of the status regional assessment are back-transformed for presentation. A value of 1 indicates that the regional mean VDS is equal to the assessment criterion; a value of 0.5 indicates that it is half the assessment criterion; and a value of 2 indicates that it is twice the assessment criterion. There is no need to back-transform the estimated regional trend, which is on the odds ratio scale.
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 stations. The purple circles are stations where there is a trend assessment and the light blue cirles are stations where there is only a status assessment,
The following regions have an insufficient number or time series or spread of stations and are excluded from the regional assessment:
- trend: Irish Sea, Minches & W Scotland, Scottish Continental Shelf
- status: Irish Sea, Minches & W Scotland, Scottish Continental Shelf
Regional trends
The first 2 tabs show:
- Regional trends: estimates of the regional trend for VDS with pointwise 95% confidence intervals
- 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 VDS is significantly decreasing (p < 0.05)
- circle: there is no significant change in mean VDS (p > 0.05)
- upward triangle: the mean VDS is significantly increasing (p < 0.05)
The final tab shows the estimates of the regional trend in VDS.
Regional trends
Individual time series
Estimates by region
This table shows the estimates of the regional trend:
- trend: the estimated mean odds ratio of the VDS of an individual whelk being above the EAC in one year relative to the previous year
- se: the corresponding standard error
- lower, upper: the corresponding pointwise 95% confidence limits
| region | trend | se | lower | upper |
|---|---|---|---|---|
| Northern North Sea | 0.79 | 0.05 | 0.69 | 0.90 |
Regional status compared to the Environmental Assessment Criterion (EAC)
The first 2 tabs show:
- Regional status: estimates of the regional status for VDS with pointwise upper one-sided 95% confidence intervals
- 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 VDS is significantly (p < 0.05) below the Background Assessment Concentration (BAC)
- green: the mean VDS is significantly (p < 0.05) below the EAC
- red: the mean VDS is not significantly below the EAC
Note that to see why some points might be 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 EAC with the assessment of status relative to the BAC because the ratio of the BAC to the EAC can vary between species.
The final tab shows the estimates of the regional status in VDS.
Regional status
Individual time series
Estimates by region
This table shows the estimates of the regional status:
- status: the estimated mean sqrt (VDS / EAC)
- se: the corresponding standard error
- upper: the corresponding pointwise upper one-sided 95% confidence limit
- VDS: the estimated mean VDS relative to the EAC
- VDS upper: the corresponding pointwise upper one-sided 95% confidence limit
| region | status | se | upper | VDS | VDS upper |
|---|---|---|---|---|---|
| Northern North Sea | 0.50 | 0.13 | 0.71 | 0.25 | 0.51 |
Regional status compared to the Background Assessment Concentration (BAC)
The first 2 tabs show:
- Regional status: estimates of the regional status for VDS with pointwise upper one-sided 95% confidence intervals
- 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 VDS is significantly (p < 0.05) below the BAC
- green: the mean VDS is significantly (p < 0.05) below the Environmental Assessment Criterion (EAC)
- red: the mean VDS is not significantly below the EAC
The final tab shows the estimates of the regional status in VDS.
Regional status
Individual time series
Estimates by region
This table shows the estimates of the regional status:
- status: the estimated mean sqrt (VDS / BAC)
- se: the corresponding standard error
- upper: the corresponding pointwise upper one-sided 95% confidence limit
- VDS: the estimated mean VDS relative to the BAC
- VDS upper: the corresponding pointwise upper one-sided 95% confidence limit
| region | status | se | upper | VDS | VDS upper |
|---|---|---|---|---|---|
| Northern North Sea | 1.30 | 0.33 | 1.84 | 1.68 | 3.37 |