Regional assessment of status and trends in metal concentrations in sediment

Overview

The 2020 assessment of CSEMP data describes the status and trends of contaminant concentrations in biota and sediment at monitoring stations around the UK. Assessments are made for a large number of time series, each of a single contaminant in a single species and tissue (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 biogeographic regional level. In particular, it considers metal concentrations in sediment, where the metals are AS, CD, CR, CU, PB, HG, NI, ZN.

The regional assessment only considers coastal and offshore stations and excludes estuarine stations. The results are therefore based on the monitoring data collected within the waters assessed under the UK Marine Strategy (MS). For simplicity, the term ‘region’ is used throughout to describe biogeographic regions. MS sub-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 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 2014 to 2019
• 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 2014 to 2019
• 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. 8 of the 374 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 areas delineated by the thinner black lines are the biogeographic regions; those delineated by the thicker black lines are the MS sub-regions.

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)
• orange: there is no ERL and the mean concentration is not significantly below the BAC
• 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) or there are too few years to test for trends
• 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 and MS sub-region
• the same information presented as proportions
• the number of time series with blue, green, orange or red status in each biogeographic region and MS sub-region
• the same information presented as proportions

Note that:

• chromium time series are only assessed against the ERL (as the BAC equals the ERL)
• arsenic and nickel time series are only assessed against the BAC; the ERL is not used because it is lower than the BAC and is regarded as too precautionary

Stations

AS

CD

CR

CU

PB

HG

NI

ZN

Trend summary (numbers)

Number of time series with upwards, downwards or no trend by biogeographic region and MS sub-region

MS sub-region	biogeographic region	status	AS	CD	CR	CU	PB	HG	NI	ZN	total
Greater North Sea	Northern North Sea	upward trend	3	1	2	0	1	0	2	1	10
		no trend	7	9	8	8	10	7	9	10	68
		downward trend	0	0	2	3	1	2	1	1	10
	Southern North Sea	upward trend	0	0	0	0	0	0	0	0	0
		no trend	5	4	5	5	4	2	5	5	35
		downward trend	0	0	0	0	1	0	0	0	1
	E Channel	upward trend	1	2	2	1	2	0	2	1	11
		no trend	1	0	0	1	0	2	0	1	5
		downward trend	0	0	0	0	0	0	0	0	0
	total	upward trend	4	3	4	1	3	0	4	2	21
		no trend	13	13	13	14	14	11	14	16	108
		downward trend	0	0	2	3	2	2	1	1	11
Celtic Seas	Minches & W Scotland	upward trend	0	1	0	0	0	0	0	0	1
		no trend	4	2	4	4	4	3	4	4	29
		downward trend	0	0	0	0	0	1	0	0	1
	Irish Sea	upward trend	3	2	1	0	0	0	0	0	6
		no trend	9	9	8	8	10	9	11	10	74
		downward trend	0	0	3	4	2	2	1	2	14
	W Channel & Celtic Sea	upward trend	0	1	1	0	1	0	1	1	5
		no trend	2	1	1	2	1	2	1	1	11
		downward trend	0	0	0	0	0	0	0	0	0
	total	upward trend	3	4	2	0	1	0	1	1	12
		no trend	15	12	13	14	15	14	16	15	114
		downward trend	0	0	3	4	2	3	1	2	15

Trend summary (proportions)

Proportion of time series with upwards, downwards or no trend by biogeographic region and MS sub-region

MS sub-region	biogeographic region	status	AS	CD	CR	CU	PB	HG	NI	ZN	total
Greater North Sea	Northern North Sea	upward trend	30	10	16	0	9	0	17	9	12
		no trend	70	90	67	73	83	78	75	83	77
		downward trend	0	0	17	27	8	22	8	8	11
	Southern North Sea	upward trend	0	0	0	0	0	0	0	0	0
		no trend	100	100	100	100	80	100	100	100	97
		downward trend	0	0	0	0	20	0	0	0	3
	E Channel	upward trend	50	100	100	50	100	0	100	50	69
		no trend	50	0	0	50	0	100	0	50	31
		downward trend	0	0	0	0	0	0	0	0	0
	total	upward trend	24	19	21	5	16	0	21	11	15
		no trend	76	81	68	78	74	85	74	84	77
		downward trend	0	0	11	17	10	15	5	5	8
Celtic Seas	Minches & W Scotland	upward trend	0	33	0	0	0	0	0	0	3
		no trend	100	67	100	100	100	75	100	100	94
		downward trend	0	0	0	0	0	25	0	0	3
	Irish Sea	upward trend	25	18	8	0	0	0	0	0	6
		no trend	75	82	67	67	83	82	92	83	79
		downward trend	0	0	25	33	17	18	8	17	15
	W Channel & Celtic Sea	upward trend	0	50	50	0	50	0	50	50	31
		no trend	100	50	50	100	50	100	50	50	69
		downward trend	0	0	0	0	0	0	0	0	0
	total	upward trend	17	25	11	0	6	0	5	6	8
		no trend	83	75	72	78	83	82	89	83	81
		downward trend	0	0	17	22	11	18	6	11	11

Status summary (numbers)

Number of time series with each status by biogeographic region and MS sub-region

MS sub-region	biogeographic region	status	AS	CD	CR	CU	PB	HG	NI	ZN	total
Greater North Sea	Northern North Sea	blue	6	9	0	10	9	7	8	10	59
		green	0	4	8	2	0	1	0	1	16
		orange	6	0	0	0	0	0	6	0	12
		red	0	1	6	1	5	6	0	1	20
	Southern North Sea	blue	0	0	0	0	0	0	0	0	0
		green	0	6	0	2	0	0	0	0	8
		orange	9	0	0	0	0	0	9	0	18
		red	0	3	9	7	9	9	0	5	42
	E Channel	blue	0	0	0	0	0	0	0	0	0
		green	0	3	0	1	0	0	0	0	4
		orange	4	0	0	0	0	0	4	0	8
		red	0	1	4	3	4	4	0	3	19
	total	blue	6	9	0	10	9	7	8	10	59
		green	0	13	8	5	0	1	0	1	28
		orange	19	0	0	0	0	0	19	0	38
		red	0	5	19	11	18	19	0	9	81
Celtic Seas	Minches & W Scotland	blue	4	4	0	4	3	4	0	4	23
		green	0	0	0	0	1	0	0	0	1
		orange	0	0	0	0	0	0	4	0	4
		red	0	0	4	0	0	0	0	0	4
	Irish Sea	blue	12	8	0	7	4	2	2	3	38
		green	0	7	0	2	0	5	0	4	18
		orange	3	0	0	0	0	0	13	0	16
		red	0	0	15	6	11	8	0	8	48
	W Channel & Celtic Sea	blue	0	0	0	0	0	0	0	0	0
		green	0	2	0	0	0	0	0	0	2
		orange	2	0	0	0	0	0	2	0	4
		red	0	0	2	2	2	2	0	2	10
	total	blue	16	12	0	11	7	6	2	7	61
		green	0	9	0	2	1	5	0	4	21
		orange	5	0	0	0	0	0	19	0	24
		red	0	0	21	8	13	10	0	10	62

Status summary (proportions)

Proportion of time series with each status by biogeographic region and MS sub-region

MS sub-region	biogeographic region	status	AS	CD	CR	CU	PB	HG	NI	ZN	total
Greater North Sea	Northern North Sea	blue	50	64	0	77	64	50	57	83	55
		green	0	29	57	15	0	7	0	9	15
		orange	50	0	0	0	0	0	43	0	11
		red	0	7	43	8	36	43	0	8	19
	Southern North Sea	blue	0	0	0	0	0	0	0	0	0
		green	0	67	0	22	0	0	0	0	12
		orange	100	0	0	0	0	0	100	0	26
		red	0	33	100	78	100	100	0	100	62
	E Channel	blue	0	0	0	0	0	0	0	0	0
		green	0	75	0	25	0	0	0	0	13
		orange	100	0	0	0	0	0	100	0	26
		red	0	25	100	75	100	100	0	100	61
	total	blue	24	33	0	39	33	26	30	50	29
		green	0	48	30	19	0	4	0	5	14
		orange	76	0	0	0	0	0	70	0	18
		red	0	19	70	42	67	70	0	45	39
Celtic Seas	Minches & W Scotland	blue	100	100	0	100	75	100	0	100	72
		green	0	0	0	0	25	0	0	0	3
		orange	0	0	0	0	0	0	100	0	13
		red	0	0	100	0	0	0	0	0	12
	Irish Sea	blue	80	53	0	47	27	14	13	20	32
		green	0	47	0	13	0	33	0	27	15
		orange	20	0	0	0	0	0	87	0	13
		red	0	0	100	40	73	53	0	53	40
	W Channel & Celtic Sea	blue	0	0	0	0	0	0	0	0	0
		green	0	100	0	0	0	0	0	0	13
		orange	100	0	0	0	0	0	100	0	25
		red	0	0	100	100	100	100	0	100	62
	total	blue	76	57	0	52	33	28	10	33	36
		green	0	43	0	10	5	24	0	19	13
		orange	24	0	0	0	0	0	90	0	14
		red	0	0	100	38	62	48	0	48	37

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’ or ‘baseline’ (near pristine conditions or only subject to very diffuse inputs). ‘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: E Channel, W Channel & Celtic Sea
• status: E Channel, W Channel & Celtic Sea

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 significant 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

biogeographic region	metal	trend	se	lower	upper	% yearly change	%yc lower	%yc upper
Northern North Sea	Arsenic	2.15	0.70	0.77	3.52	2.17	0.77	3.58
	Cadmium	1.15	1.11	-1.03	3.32	1.16	-1.02	3.38
	Chromium	-0.72	0.66	-2.01	0.58	-0.71	-1.99	0.58
	Copper	-1.33	0.81	-2.92	0.25	-1.32	-2.88	0.26
	Lead	-0.25	0.62	-1.47	0.97	-0.25	-1.46	0.97
	Mercury	-1.87	0.93	-3.69	-0.05	-1.85	-3.62	-0.05
	Nickel	-0.13	0.72	-1.53	1.28	-0.13	-1.52	1.29
	Zinc	-1.33	0.68	-2.67	0.00	-1.33	-2.64	0.00
Southern North Sea	Arsenic	0.90	1.48	-1.99	3.79	0.90	-1.97	3.87
	Cadmium	3.45	2.35	-1.15	8.06	3.51	-1.15	8.39
	Chromium	0.60	1.17	-1.69	2.89	0.60	-1.68	2.93
	Copper	0.48	1.38	-2.22	3.18	0.48	-2.19	3.23
	Lead	-0.33	1.32	-2.91	2.26	-0.33	-2.87	2.29
	Nickel	1.11	1.26	-1.35	3.58	1.12	-1.34	3.65
	Zinc	-0.05	1.20	-2.41	2.31	-0.05	-2.38	2.34
Minches & W Scotland	Arsenic	1.74	1.22	-0.64	4.13	1.76	-0.64	4.22
	Cadmium	4.75	1.66	1.50	7.99	4.86	1.51	8.32
	Chromium	-0.10	1.07	-2.20	2.00	-0.10	-2.17	2.02
	Copper	-0.39	1.32	-2.98	2.20	-0.39	-2.94	2.22
	Lead	-0.44	1.19	-2.78	1.89	-0.44	-2.74	1.91
	Mercury	-3.95	1.33	-6.55	-1.35	-3.87	-6.34	-1.34
	Nickel	-0.05	1.12	-2.25	2.16	-0.05	-2.23	2.18
	Zinc	-0.36	1.28	-2.88	2.15	-0.36	-2.84	2.18
Irish Sea	Arsenic	0.81	0.60	-0.36	1.98	0.82	-0.35	2.00
	Cadmium	2.06	0.92	0.26	3.85	2.08	0.26	3.93
	Chromium	-0.53	0.60	-1.70	0.65	-0.53	-1.69	0.65
	Copper	-3.24	0.69	-4.59	-1.88	-3.18	-4.49	-1.86
	Lead	-1.42	0.67	-2.73	-0.12	-1.41	-2.69	-0.12
	Mercury	-2.17	0.80	-3.73	-0.61	-2.15	-3.66	-0.61
	Nickel	-1.07	0.65	-2.33	0.20	-1.06	-2.31	0.20
	Zinc	-1.49	0.66	-2.79	-0.20	-1.48	-2.75	-0.20

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 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 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

biogeographic region	metal	status	se	upper	concentration	conc upper
Northern North Sea	Arsenic
	Cadmium	-1.97	0.17	-1.68	0.14	0.19
	Chromium	-0.34	0.16	-0.08	0.71	0.92
	Copper	-1.31	0.16	-1.04	0.27	0.36
	Lead	-0.30	0.16	-0.04	0.74	0.96
	Mercury	-0.76	0.17	-0.48	0.47	0.62
	Nickel
	Zinc	-1.00	0.16	-0.73	0.37	0.48
Southern North Sea	Arsenic
	Cadmium	-1.53	0.22	-1.17	0.22	0.31
	Chromium	0.32	0.20	0.64	1.37	1.91
	Copper	-0.22	0.20	0.11	0.80	1.12
	Lead	0.30	0.20	0.63	1.35	1.88
	Mercury	0.16	0.25	0.57	1.17	1.76
	Nickel
	Zinc	-0.10	0.23	0.29	0.91	1.33
Minches & W Scotland	Arsenic
	Cadmium	-2.32	0.32	-1.80	0.10	0.17
	Chromium	0.08	0.29	0.56	1.08	1.75
	Copper	-0.90	0.30	-0.41	0.41	0.66
	Lead	-0.63	0.29	-0.15	0.53	0.86
	Mercury	-1.33	0.30	-0.84	0.27	0.43
	Nickel
	Zinc	-0.60	0.29	-0.12	0.55	0.89
Irish Sea	Arsenic
	Cadmium	-1.79	0.16	-1.53	0.17	0.22
	Chromium	0.32	0.15	0.57	1.38	1.77
	Copper	-0.49	0.15	-0.24	0.61	0.79
	Lead	0.05	0.15	0.30	1.05	1.35
	Mercury	-0.17	0.16	0.09	0.84	1.09
	Nickel
	Zinc	-0.16	0.15	0.09	0.85	1.09

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)
• orange: there is no ERL and the mean concentration is not significantly below the BAC
• 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

biogeographic region	metal	status	se	upper	concentration	conc upper
Northern North Sea	Arsenic	-0.67	0.16	-0.41	0.51	0.67
	Cadmium	-0.61	0.17	-0.33	0.55	0.72
	Chromium
	Copper	-1.08	0.16	-0.81	0.34	0.45
	Lead	-0.09	0.16	0.17	0.92	1.18
	Mercury	0.01	0.17	0.28	1.01	1.33
	Nickel	-0.49	0.16	-0.23	0.61	0.79
	Zinc	-0.79	0.16	-0.52	0.46	0.59
Southern North Sea	Arsenic	0.26	0.20	0.60	1.30	1.82
	Cadmium	-0.16	0.22	0.19	0.85	1.21
	Chromium
	Copper	0.01	0.20	0.34	1.01	1.41
	Lead	0.52	0.20	0.85	1.68	2.33
	Mercury	0.94	0.24	1.34	2.56	3.81
	Nickel	0.33	0.20	0.66	1.39	1.93
	Zinc	0.10	0.23	0.47	1.10	1.60
Minches & W Scotland	Arsenic	-1.20	0.29	-0.72	0.30	0.49
	Cadmium	-0.99	0.31	-0.48	0.37	0.62
	Chromium
	Copper	-0.67	0.29	-0.19	0.51	0.83
	Lead	-0.42	0.29	0.06	0.66	1.07
	Mercury	-0.57	0.30	-0.08	0.57	0.92
	Nickel	0.06	0.29	0.54	1.06	1.71
	Zinc	-0.40	0.29	0.08	0.67	1.09
Irish Sea	Arsenic	-0.63	0.15	-0.38	0.53	0.68
	Cadmium	-0.44	0.16	-0.18	0.65	0.84
	Chromium
	Copper	-0.27	0.15	-0.02	0.76	0.98
	Lead	0.26	0.15	0.51	1.30	1.67
	Mercury	0.59	0.16	0.84	1.80	2.32
	Nickel	0.07	0.15	0.32	1.07	1.38
	Zinc	0.04	0.15	0.29	1.04	1.33