• Latest Version, 2024 Assessment

Extent of Physical Disturbance to Benthic Habitats: Aggregate Extraction

GES was achieved for broad scale habitats and OSPAR Threatened and/or Declining habitats in the four assessment units of the Greater North Sea and Celtic Seas sub-regions where commercial aggregate extraction occurred. 

Background

This analysis used outputs from the most recent assessment of the OSPAR Indicator ‘Extent of Physical Disturbance to Benthic Habitats: Aggregate Extraction’ (BH3b) to assess Good Environmental Status (GES) for reporting against Part 1 of the UK Marine Strategy (UKMS) in 2024. To assess GES for broad scale habitats, a dual value threshold was used to account for varying sensitivities of biotopes within broad habitat definitions: no greater than 15% of habitat area in High disturbance alone, and no greater than 25% of habitat area in High and Moderate disturbance collectively. To assess GES in OSPAR Threatened and/or Declining (T&D) habitats (those considered to need priority protection), a more restrictive 5% threshold was applied to the proportion of habitat area in High and Moderate disturbance collectively. Threshold values were agreed through expert consultation, with aims of maximising interoperability with wider policy drivers and applied to the total area of habitats within assessment units, within UKMS sub-regions.  

Commercial aggregate extraction activity is a licensed activity and was therefore confined to discrete areas within UKMS sub-regions. In the four assessment units where commercial aggregate extraction occurred throughout the Greater North Sea and Celtic Seas sub-regions, all broad scale habitats achieved the dual threshold value. Furthermore, commercial aggregate extraction activity only overlapped OSPAR T&D habitats in the Southern North Sea assessment unit. Within the Southern North Sea, commercial aggregate extraction activity only occurred in Sabellaria spinulosa reefs. However, High and Moderate disturbance covered less than 5% of the total area of S. spinulosa reefs in the Southern North Sea and the threshold was, therefore, achieved.  

Broad Scale Habitat Thresholds   

Broad scale habitats encompass several habitat types, which have varying levels of sensitivity to physical pressures from human activities, based on their ability to withstand and recover from impacts. Therefore, a dual extent threshold was designed to account for the varying degrees of disturbance possible within broad scale habitats, based on the underlying sensitivity. A higher tolerance threshold value (25%) was applied to the area of broad scale habitats in Moderate and High disturbance groups collectively, and a stricter threshold value (15%) was applied to the area in High disturbance alone, as this disturbance group only occurred where the underlying sensitivity was in the two highest sensitivity categories used in the BH3 methodology (please see the methodology section of this report for further details). The BH3 disturbance groups agreed in the OSPAR Quality Status Report (QSR) 2023 (Zero, Low, Moderate, and High), were selected to represent the pressure and sensitivity elements of disturbance (Matear and others, 2023).  

OSPAR Threatened and Declining Habitat Thresholds 

OSPAR T&D Habitats need priority protection, therefore, a more restrictive threshold of no more than 5% of habitat area in Moderate and High disturbance was applied. The 5% extent threshold corresponded with the methodology implemented by the UK in 2019 Article 17 reporting (EU Habitats Directive), where no greater than 5% of habitat area could be in unfavourable conditions for the habitat to be assessed as in a favourable condition overall (JNCC, 2019).  

The use of thresholds that reflected underlying sensitivity within broad scale habitats both maximised detail of resolution in assessments, whilst simultaneously aligning with wider marine policy at an international scale. In 2023, the EU agreed that for a broad scale habitat to achieve GES, no more than 25% of the habitat should be adversely affected by human pressures under Descriptor 6 in the EU Marine Strategy Framework Directive (MSFD) ‘Seabed Integrity’ (European Commission, 2023; TG Seabed, 2023). In addition, the 25% threshold aligned with the methodology implemented by the UK in 2019 Article 17 assessments, under the EU Habitats Directive, to determine ‘Unfavourable-Bad’ condition for habitats (JNCC, 2019). The 15% threshold was derived from the initial threshold trialled in 2018 reporting against UKMS and enabled a stricter threshold to be applied to the highest levels of disturbance in the most sensitive species and habitats.  

Assessment method

Input Data Overview 

Estimates of physical disturbance from commercial aggregate extraction, between 2016 and 2020, within UKMS sub-regions were obtained from the outputs of the QSR 2023 (Matear and others, 2023). Data were available and analysed at the scale of OSPAR Extent of Physical Disturbance to Benthic Habitats Assessment Units (hereafter referred to as assessment units) (OSPAR, 2022; Matear and others, 2023). Disturbance was categorised from 0-9, at the resolution of habitat polygons within 50 x 50m grid cells. Assessments considered the most detailed underlying species and habitat information available in the EUNIS 2007 classification (resolution of available sensitivity assessments), and the intensity of extraction pressure from commercial aggregate extraction.  

EUNIS 2007 habitats were translated in the QSR to EU MSFD Benthic Broad Habitat Types (BBHT) to facilitate reporting against national policy commitments. To report against the UKMS, BBHTs from the EU MSFD classification (hereafter referred to as ‘broad scale habitats’) were used for GES assessments to standardise UKMS results and ensure interoperability with the OSPAR BH3 QSR 2023 assessment. To demonstrate the variety of EUNIS codes and, therefore, variety of sensitivity values that can be represented within a single broad scale habitat, an example of EUNIS codes associated with Circalittoral mixed sediment across UKMS sub-regions is provided in Table 1.  

Table 1: Unique EUNIS 2007 codes present within UKMS sub-regions that translated to the EU MSFD Broad Benthic Habitat Type “Circalittoral mixed sediment” and unique surface and subsurface sensitivity categories observed within the habitat (where category 5 is the highest sensitivity).

Broad Scale Habitat	EUNIS Codes	Surface Sensitivity Categories	Subsurface Sensitivity Categories
Circalittoral mixed sediment	A5.4, A5.44, A5.443, A5.445, A5, A5.42, A5.444, A5.441, A5.4411, A5.442	2, 3, 4, 5	3, 4, 5

In the QSR, OSPAR T&D habitats were assessed separately using the OSPAR Habitats in the North-East Atlantic Ocean – 2020 Polygons layer (OSPAR, 2020) available to download from the European Marine Observation and Data Network product catalogue [https://emodnet.ec.europa.eu/geonetwork/srv/eng/catalog.search#/home]. These data were considered the most accurate representation of the distribution of OSPAR T&D habitats at the time of assessment.  

QSR 2023 BH3b data were obtained from the following assessment units (Figure 1): 

• UKMS sub-region Greater North Sea: Southern North Sea, and Channel assessment units 

• UKMS sub-region Celtic Seas: Northern Celtic Sea, Southern Celtic Sea 

Commercial aggregate extraction activity did not occur within any other assessment units in UKMS sub-regions.  

Figure 1: Assessment unit boundaries within UK Marine Strategy sub-regions. Labels indicate spatial location of assessment units where BH3 is assessed. Dots indicate assessment units where commercial aggregate extraction activity did not occur, and therefore thresholds for Good Environmental Status were not tested. The spatial extent of the BH3 assessment in the OSPAR QSR 2023 did not cover the entire area of UK Marine Strategy sub-regions; areas where no BH3 assessment was available are hashed out.  

Assessment of GES  

Data analyses were conducted in ESRI ArcGIS v10.1, QGIS v3.16.5, and R statistical software (v3.6.1); spatial analysis in R was carried out using the ‘sf’ (Pebesma, 2018) and ‘tidyverse’ packages (Wickham and others, 2019). BH3b disturbance estimates were spatially intersected with the UKMS sub-regional boundaries. Disturbance categories, based on aggregated extraction pressure data between 2016 to 2020, were separated into the four disturbance groups defined in the OSPAR QSR 2023: ‘Zero’ = no reported extraction data, ‘Low’ = disturbance categories 1 to 4, ‘Moderate’ = disturbance categories 5 to 7, and ‘High’ = disturbance categories 8 and 9 (Table 2). 

Disturbance groups were selected in the QSR to represent the pressure and sensitivity elements of the physical disturbance values (Matear and others, 2023). Both pressure and sensitivity were categorised prior to calculating disturbance in the QSR. In both instances, category 5 represents highest intensity of extraction pressure and the highest level of sensitivity to extraction pressure. Therefore, the disturbance groups enabled GES thresholds to be set according to the greatest levels of disturbance in the most sensitive areas; High disturbance only occurred in areas of sensitivity categories 4 or 5 and Moderate disturbance only occurred in areas of sensitivity category 3 or greater (Table 2). For further details on how pressure and sensitivity were assessed and categorised in the QSR please see Matear and others (2023). 

Table 2: Disturbance matrix with summary groups; ‘Low’ (1-4), ‘Moderate’ (5-7), and ‘High’ (8-9). Note ‘Null / 0*’ pressure = No reported extraction data

Disturbance Matrix		Sensitivity
		1	2	3	4	5
Pressure	Null / 0	0	0	0	0	0
	1	1	2	3	4	6
	2	1	2	4	6	7
	3	1	3	5	7	9
	4	1	4	6	8	9
	5	2	4	7	9	9

GES Assessments in Broad Scale Habitats:  

The proportion of the total area of each broad scale habitat, within assessment units, in each disturbance group was calculated and assessed against the following dual or two parts threshold:  

• No greater than 15% of broad scale habitat area in High disturbance (categories 8 and 9) alone and; 

• No greater than 25% of broad scale habitat area in High and Moderate disturbance (categories 5-9) collectively.  

For broad scale habitats to achieve GES, the proportion of the total habitat area, within assessment units would have to meet both the 15% and the 25% threshold.  

GES Assessment in Threatened and Declining Habitats  

The proportion of the total area of each OSPAR T&D habitat, in each assessment unit, in disturbance categories 5-9 (equivalent to High and Moderate disturbance groups combined) was calculated and assessed against the following threshold:  

• No greater than 5% of OSPAR T&D habitat area in High and Moderate disturbance groups (disturbance categories 5-9).  

The full methodology of the BH3b indicator and the results from the QSR can be found here: 

https://oap.ospar.org/en/ospar-assessments/quality-status-reports/qsr-2023/indicator-assessments/phys-dist-habs-agg-ex/ 

Results

Broad Scale Habitats  

Commercial aggregate extraction activity is a licensed activity, therefore its distribution was discrete in nature throughout UKMS sub-regions. In the four assessment units where commercial aggregate extraction occurred throughout the Greater North Sea and Celtic Seas sub-regions, all broad scale habitats achieved the dual threshold value (Table 3, Table 4 and Figure 2). The greatest proportion of broad scale habitat area in High and Moderate disturbance was observed in Offshore circalittoral rock and biogenic reef in the Southern North Sea (<5%) (Figure 2). However, disturbance in Offshore circalittoral rock and biogenic reef should be considered in the context of its constituent habitat types (e.g., Sabellaria spp.). Commercial aggregate extraction activity often takes place in areas of mixed sediments, where tube building polychaetes such as S. spinulosa can occur (OSPAR, 2010). The EUNIS level 4 habitat type ‘Sublittoral polychaete worm reefs on sediment’ (A5.61), which may include S. spinulosa reefs on mixed sediment, corresponds to the Offshore circalittoral rock and biogenic reef broad scale habitat, which may explain the greater proportion of High and Moderate disturbance observed in this habitat type. 

Table 3: Percentage of broad scale habitats in each assessment unit to achieve the dual threshold: no greater than 15% of habitat area in High disturbance (categories 8 and 9) alone and no greater than 25% of habitat area in High and Moderate disturbance (categories 5-9).  

UK Marine Strategy Sub-region	Assessment Unit	Total Number of Broad Scale Habitats	Total Number of Broad Scale Habitats to Achieve Dual Threshold	Percentage of Broad Scale Habitats to Achieve Dual Threshold
Greater North Sea	Southern North Sea	15	15	100%
	Channel	15	15	100%
Celtic Seas	Northern Celtic Sea	20	20	100%
	Southern Celtic Sea	19	19	100%

Table 4: Threshold assessment for broad scale habitats within assessment units where: Achieved = no greater than 15% of habitat area in High disturbance (categories 8 and 9) alone and no greater than 25% of habitat area in disturbance High and Moderate disturbance (categories 5-9); Not achieved = more than 15% of habitat area in disturbance High disturbance (categories 8 and 9) alone and / or more than 25% of habitat area in High and Moderate disturbance (categories 5-9); Not present = the habitat was not observed in the assessment unit. 

Broad scale habitat	Channel	Southern North Sea	Northern Celtic Sea	Southern Celtic Sea
Infralittoral rock and biogenic reef	Achieved	Achieved	Achieved	Achieved
Infralittoral coarse sediment	Achieved	Achieved	Achieved	Achieved
Infralittoral mixed sediment	Achieved	Achieved	Achieved	Achieved
Infralittoral sand	Achieved	Achieved	Achieved	Achieved
Infralittoral mud	Achieved	Achieved	Achieved	Achieved
Circalittoral rock and biogenic reef	Achieved	Achieved	Achieved	Achieved
Circalittoral coarse sediment	Achieved	Achieved	Achieved	Achieved
Circalittoral mixed sediment	Achieved	Achieved	Achieved	Achieved
Circalittoral sand	Achieved	Achieved	Achieved	Achieved
Circalittoral mud	Achieved	Achieved	Achieved	Achieved
Offshore circalittoral rock and biogenic reef	Achieved	Achieved	Achieved	Achieved
Offshore circalittoral coarse sediment	Achieved	Achieved	Achieved	Achieved
Offshore circalittoral mixed sediment	Achieved	Achieved	Achieved	Achieved
Offshore circalittoral sand	Achieved	Achieved	Achieved	Achieved
Offshore circalittoral mud	Achieved	Achieved	Achieved	Achieved
Upper bathyal rock and biogenic reef	Not present	Not present	Achieved	Achieved
Upper bathyal sediment	Not present	Not present	Achieved	Achieved
Lower bathyal rock and biogenic reef	Not present	Not present	Achieved	Not present
Lower bathyal sediment	Not present	Not present	Achieved	Achieved
Abyssal	Not present	Not present	Achieved	Achieved

Figure 2: The percentage of total broad scale habitat area, within assessment units, within UKMS sub-regions, predicted to be subject to the following disturbance groups: High = disturbance categories 8 and 9; and Moderate = disturbance categories 5-7. 

Threatened and Declining Habitats 

Within UKMS sub-regions, commercial aggregate extraction activity only overlapped OSPAR T&D habitats in the Southern North Sea assessment unit, therefore GES assessments were only conducted on the T&D habitats that were observed in this assessment unit (Table 5 and Figure 3). Within the Southern North Sea, commercial aggregate extraction activity only occurred in S. spinulosa reefs (Figure 3). However, High and Moderate disturbance covered less than 5% of the total area of S. spinulosa reefs in the Southern North Sea and the threshold was, therefore, achieved by all observed T&D habitats (Table 5 and Figure 3).  

Table 5: Threshold assessment for OSPAR Threatened and / or Declining Habitats within assessment units where: Achieved = no greater than 5% of habitat area in High and Moderate disturbance (categories 5-9).  

OSPAR T&D Habitat	Southern North Sea
Littoral chalk communities	Achieved
Intertidal mudflats	Achieved
Intertidal Mytilus edulis beds on mixed and sandy sediments	Achieved
Sabellaria spinulosa reefs	Achieved

Figure 3: Percentage of the total area of OSPAR Threatened and/or Declining Habitats predicted to be subject to High and Moderate disturbance (categories 5-9). Percentages were assessed against the threshold of ≤5% of area in High and Moderate disturbance (categories 5-9).   

Conclusions

The OSPAR QSR 2023 assessment was the first OSPAR-wide analysis of commercial aggregate extraction activity via the BH3 indicator. Therefore, UKMS 2024 will be the first UK-scale assessment of GES in relation to extraction. Of the two activities assessed by BH3 (bottom-contact fisheries and commercial aggregate extraction), extraction-related pressure data were available at a higher spatial resolution than fishing pressure (50 x 50 m grid cells), which enabled the extent of disturbance to be determined with greater accuracy than fishing, whilst still protecting commercially sensitive information. Commercial aggregate extraction is a licensed activity and therefore, was confined to discrete locations within UKMS sub-regions. The activity occurred across four assessment units in the Greater North Sea and Celtic Seas and all broad scale habitats achieved the dual threshold. Furthermore, extraction only overlapped with OSPAR T&D habitats in the Southern North Sea assessment unit, specifically S. spinulosa reefs. High and Moderate disturbance covered less than 5% of the total area the habitat in the Southern North Sea, resulting in GES being achieved for S. spinulosa reefs. 

Knowledge gaps

The following recommendations will help to refine the thresholds as well as provide an indication of steps that could be taken to achieve GES:  

Improved extraction data 

Commercial aggregate extraction commonly involves two types of dredging: trailer suction hopper dredging, where the vessel moves slowly across a dredging zone with the dredge gear deployed; and static dredging while the dredger is anchored (BMAPA and The Crown Estate, 2017). In addition, at the time of assessment, UK-scale pressure data available were solely available at the resolution of dredging duration in 50 x 50 m grid cells, without information indicative of the type of dredging that occurred. At a North-East Atlantic scale, trailer suction hopper dredging is the most common method for extracting marine aggregates (OSPAR, 2009). Therefore, all data were assumed to be mobile, and the intensity of extraction pressure was quantified as the area dredged, per grid cell, per year.  

The parameters for draghead width (part of dredge gear in contact with the seabed) and vessel speed were obtained via literature review and verified by industry experts. However, estimates of the intensity of extraction pressure could be improved with additional data, such as the type of extraction (static or mobile), the vessel speed, the volume of sediment extracted, and the exact footprint of the draghead used (Matear and others, 2023). Therefore, GES assessments and associated management measures would be greatly improved through increased data availability. Data sharing agreements with industry would further facilitate our understanding of where GES has been achieved and inform interventions to ensure it is maintained.    

Improved monitoring 

The use of the dual value threshold enabled detailed assessments of GES in broad scale habitats that accounted for the sensitivities of component habitat types. In addition to improved accuracy, this approach ensured that assessments used standardised habitat classifications (broad scale habitats), facilitating interoperability with wider assessments, such as the OSPAR QSR 2023. The accuracy of outputs was influenced by the resolution of habitat and sensitivity data. Therefore, future assessments would benefit from improved habitat maps (increased in-situ data) to understand spatial relationships between pressure and sensitive habitats to improve the accuracy of GES determination.   

References

BMAPA and the Crown Estate (2017). Good Practice Guidance: Extraction by Dredging of Aggregates from England’s Seabed. 

European Commission (2023). Descriptors under the Marine Strategy Framework Directive. Available online: https://environment.ec.europa.eu/topics/marine-environment/descriptors-under-marine-strategy-framework-directive_en#descriptor-6-seabed-integrity. (Accessed 21/11/2023).  

Defra (2019). Marine Strategy Part One: UK updated assessment and Good Environmental Status.  

Matear, L., Vina-Herbon, C., Woodcock, K.A., Duncombe-Smith, S.W., Smith, A.P., Schmitt, P., Kreutle, A., Curtis, E.J., and Wade, K. (2023). Extent of Physical Disturbance to Benthic Habitats: Aggregate Extraction. In: OSPAR, 2023: The 2023 Quality Status Report for the Northeast Atlantic. OSPAR Commission, London. Available at: https://oap.ospar.org/en/ospar-assessments/quality-status-reports/qsr-2023/indicator-assessments/phys-dist-habs-agg-ex/ 

OSPAR Commission (2009). Summary assessment of sand and gravel extraction in the OSPAR maritime area.  

OSPAR Commission (2010). Case Reports for the OSPAR List of threatened and/or declining species and habitats – Update. Sabellaria spinulosa reefs. Available at: https://qsr2010.ospar.org/media/assessments/Species/p0010_supplements/CH10_04_Sabellaria_spinulosa.pdf 

OSPAR Commission (2020). OSPAR Habitats in the North-East Atlantic Ocean – 2020 Polygons. Available to download from: https://emodnet.ec.europa.eu/geonetwork/emodnet/eng/catalog.search#/metadata/1e7ed77a-ced4-40f5-b0be-e907c0a8f29e  

OSPAR Commission (2022) OSPAR CEMP Guidelines - Common Indicator: BH3 Extent of Physical Disturbance to Benthic Habitats OSPAR Agreement 2017-09 (Update 2023) – Technical Specifications Version 6 October 2022 

Pebesma, E. (2018). Simple Features for R: Standardized Support for Spatial Vector Data. The R Journal 10 (1), 439-446, https://doi.org/10.32614/RJ-2018-009 

Technical Group on seabed habitats and sea-floor integrity (TG Seabed) (2023). Setting of EU Threshold Values for extent of loss and adverse effects on seabed habitats. Marine Strategy Framework Directive Common Implementation Strategy guidance document n.19. Seabed_D6C4_D6C5_Threshold_Values_Endorsed_2023.pdf 

Wickham, H., Averick, M., Bryan, J., Chang, W., McGowan, L.D., François, R., Grolemund, G., Hayes, A., Henry, L., Hester, J., Kuhn, M., Pedersen, T.L., Miller, E., Bache, S.M., Müller, K., Ooms, J., Robinson, D., Seidel, D.P., Spinu, V., Takahashi, K., Vaughan, D., Wilke, C., Woo, K., Yutani, H. (2019). “Welcome to the tidyverse.” Journal of Open Source Software, 4(43), 1686. doi:10.21105/joss.01686. 

Authors

Lead authors: Kirsty Woodcock¹, Liam Matear², Stephen Duncombe-Smith¹, Cristina Vina-Herbon¹  

¹Joint Nature Conservation Committee  

²Department for Environment, Food & Rural Affairs   

Contributors: this assessment built upon the Extent of Physical Disturbance to Benthic Habitats: Aggregate extraction indicator from the OSPAR QSR 2023 (Matear and others, 2023). Thresholds for assessing Good Environmental Status were developed and applied to the maritime area relevant to UKMS reporting.   

Supported by:  The Benthic Sub-Group of the Healthy and Biologically Diverse Seas Evidence Group  

Assessment metadata

Assessment Type	UK Marine Strategy
	Benthic Habitats
Point of contact email	marinestrategy@defra.gov.uk
Metadata date	Saturday, March 1, 2025
Title	Extent of Physical Disturbance to Benthic Habitats: Aggregate Extraction
Resource abstract
Linkage	Matear, L., Vina-Herbon, C., Woodcock, K.A., Duncombe-Smith, S.W., Smith, A.P., Schmitt, P., Kreutle, A., Curtis, E.J., and Wade, K. 2023. Extent of Physical Disturbance to Benthic Habitats: Aggregate Extraction. In: OSPAR, 2023: The 2023 Quality Status Report for the Northeast Atlantic. OSPAR Commission, London. Available at: https://oap.ospar.org/en/ospar-assessments/quality-status-reports/qsr-2023/indicator-assessments/phys-dist-habs-agg-ex/
Conditions applying to access and use	Please contact marinestrategy@defra.gov.uk
Assessment Lineage	UK Marine Strategy Part 1 Assessment in 2019: https://moat.cefas.co.uk/previous-assessments/2018-assessment/biodiversity-food-webs-and-marine-protected-areas/benthic-habitats/physical-damage/
Dataset metadata	Information on metadata is provided here: OSPAR Extent of Physical Disturbance to Benthic Habitats - Data Snapshot: https://odims.ospar.org/en/submissions/ospar_phys_dist_habs_dsnap_2023_06/
Dataset DOI	Data files used in the assessment can be found here: OSPAR Extent of Physical Disturbance to Benthic Habitats - Data Snapshot: https://odims.ospar.org/en/submissions/ospar_phys_dist_habs_dsnap_2023_06/

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