MIME regional assessment of status and trends in organo-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 organo-metal concentrations in sediment, where the organo-metals are TBTIN, DBTIN, MBTIN, TPTIN, DPTIN, MPTIN. 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 organo-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 0 of 83 organo-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 organo-metals. The magenta circles are stations where there is a trend assessment for at least one organo-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 organo-metal in turn. The colours have the following meaning:

  • black: there are no assessment criteria available

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


Stations


TBTIN


DBTIN


MBTIN


TPTIN


DPTIN


MPTIN


Trend summary (numbers)

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

OSPAR region MIME region status TBTIN DBTIN MBTIN total
2 Southern North Sea upward trend 0 0 0 0
no trend 0 3 10 13
downward trend 13 12 5 30
total upward trend 0 0 0 0
no trend 0 3 10 13
downward trend 13 12 5 30


Trend summary (proportions)

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

OSPAR region MIME region status TBTIN DBTIN MBTIN total
2 Southern North Sea upward trend 0 0 0 0
no trend 0 20 67 30
downward trend 100 80 33 70
total upward trend 0 0 0 0
no trend 0 20 67 30
downward trend 100 80 33 70


MIME regional assessment - methods

Tabulating the number of time series with an upward or downward trend provides a quick summary of the individual time series results. However, it does not provide an objective regional assessment of trends. This section describes how the individual time series results can by synthesised in a meta-analysis to assess trends at the regional level. However, in practice, there are only sufficient time series to do this for the Southern North Sea. As no assessment criteria have yet been adopted for organo-metals in sediment, no formal regional assessment of status is possible. However, instead, a meta-analysis can estimate the mean concentration profile of organo-metals, again in the Southern North Sea.

Regional trend

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). The mean trend for each organo-metal in the Southern North Sea is then estimated by fitting the following linear mixed model by restricted maximum likelihood:

  • response: trend (yearly change in log concentration)
  • fixed model: organo-metal
  • random model: station + trend estimation variation + residual variation

The fixed model means that a separate regional trend is estimated for each organo-metal. The random model has three terms:

  • station allows for variation in trend between stations common to all organo-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.

Status

The profile of concentrations across organo-metals can sometimes be useful for examining sources. Profiles were examined by fitting the following linear mixed model by restricted maximum likelihood:

  • response: mean log concentration
  • fixed model: organo-metal
  • random model: station + estimation variation + residual variation

where, for each time series, the mean log concentration is the estimated mean log concentration in the final monitoring year and the estimation variance is the variance associated with this estimate.

Presentation

The results of both the trend and status regional assessments are back-transformed for presentation. The regional trend is then interpreted as the percentage yearly change in concentration and the regional status is just the mean 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 organo-metal, and the cyan cirles are stations where there are only status assessments.

Note that, although there are no asssessment criteria available, the cyan circles show where there are time series with estimates of the mean concentration in the final montitoring year. These can be used to construct mean concentration profiles.


OSPAR region 1


OSPAR region 2


OSPAR region 3


OSPAR region 4


Mean trends

The first 2 tabs show:

  • Mean trend by organo-metal: mean trend estimates with pointwise upper one-sided 95% confidence limits by organo-metal
  • Individual time series: the status estimates from the individual time series on which the meta-analysis is based. The estimates from other regions (and excluded from the meta-analysis) are also shown.

The symbols in 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 mean trend by organo-metal.


Mean trends by organo-metal


Individual time series


Estimates by organo-metal

This table shows the estimates of the mean trend for each organo-metal:

  • 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


organo-metal trend se lower upper % yearly change %yc lower %yc upper
TBTIN -14.58 0.81 -16.16 -13.00 -13.57 -14.92 -12.19
DBTIN -10.33 0.90 -12.09 -8.57 -9.82 -11.39 -8.21
MBTIN -3.11 0.76 -4.59 -1.63 -3.06 -4.49 -1.61


organo-metal profiles

The estimated organo-metal concentration profiles are shown below with pointwise 95% confidence limits. The units are ug/kg dw. ```