1. Introduction

  1. This Benthic Subtidal Ecology Technical Report provides a full benthic subtidal ecology baseline for the Array, using the most contemporary desktop data sources and the results of the 2022 site-specific surveys. This Benthic Subtidal Ecology Technical Report is part of the Array Environmental Impact Assessment (EIA) Report, which has been commissioned for Ossian Offshore Wind Farm Limited (Ossian OWFL).
  2. For the purposes of this Array EIA Report, the Array refers collectively to the offshore components of the wind farm (i.e. wind turbines, Offshore Substation Platforms (OSPs), and inter-array and interconnector cables). The Array is situated within the site boundary ( Figure 2.1   Open ▸ ).
  3. The site boundary is located in the North Sea, approximately 80 km south-east of Aberdeen, and comprises an approximate area of 859 km2.

2. Study Area

  1. Two study areas have been defined for the purpose of the benthic subtidal ecology assessment:
  • The Array benthic subtidal ecology study area, which is defined as the area encompassed by the site boundary ( Figure 2.1   Open ▸ ). The site-specific benthic subtidal ecology surveys were undertaken within this area, the results of which were used to inform the baseline characterisation and identify benthic receptors which could potentially be impacted by the Array.
  • The regional benthic subtidal ecology study area, which is defined as the area encompassing the wider North Sea ( Figure 2.1   Open ▸ ). The boundaries for this regional benthic subtidal ecology study area were adapted from the Sectoral Marine Plan (SMP) Assessment region: East Region. The regional benthic subtidal ecology study area boundary to the south of the site boundary was extended to take account of feedback received during the Scoping Workshop (14 November 2022) requesting that the regional benthic subtidal ecology study area was not large enough to account for both direct and indirect effects. Desktop data sources have been used to characterise the regional benthic subtidal ecology study area, which will, overall, provide wider context to the site-specific data collected within the Array benthic subtidal ecology study area.

Figure 2.1:
Benthic Subtidal Ecology Study Areas

Figure 2.1: Benthic Subtidal Ecology Study Areas


3. Baseline

3.1. Methodology

  1. A desktop review and the results of site-specific surveys have been used to inform the baseline for benthic subtidal ecology in the Array EIA Report. These desktop data sources include academic literature, reports from other developments within the regional benthic subtidal ecology study area (e.g. Berwick Bank Offshore Wind Farm, Seagreen Offshore Wind Farm, etc.) and governmental reports. Two site-specific surveys were undertaken for the Array in 2022: a geophysical survey and an environmental survey ( Table 3.5   Open ▸ ). Detailed methodologies are provided in sections 3.2.1 and 3.2.2.

3.2. Desktop Study

  1. A detailed desktop review of existing studies and datasets was undertaken to gather information on benthic subtidal ecology within the regional benthic subtidal ecology study area. Table 3.1   Open ▸ .summarises the studies and datasets used.
  2. Whilst there is an abundance of data sources highlighted in Table 3.1   Open ▸ , many of them target areas further inshore or other offshore wind farms, with less site-specific information on the Array benthic subtidal ecology study area available. However, the site-specific geophysical and environmental surveys are sufficient to characterise the Array benthic subtidal ecology study area in the absence of extensive desktop data sources.

 

Table 3.1:
Summary of Key Desktop Reports

Table 3.1: Summary of Key Desktop Reports

 

3.2.1. Regional Benthic Subtidal Ecology Study Area

                        Subtidal sediments

  1. The EUSeaMap broadscale substrate data indicates that the regional benthic subtidal ecology study area is dominated by deep circalittoral sand (A5.27; biotope classification: SS.SSa.OSa) and is interspersed with deep circalittoral coarse sediment (A5.15; SS.SCS.OCS) (EMODnet, 2019) ( Figure 3.1   Open ▸ ). Deep circalittoral mud (A5.37; SS.SMu.OMu) and circalittoral mixed sediments (A5.44; SS.SMx.CMx) are mainly present along the coast and within the Firth of Forth. These four sediment classifications are identified as low energy habitats and are considered characteristic of the North Sea (EMODnet, 2019).
  2. There are a range of designated sites within the regional benthic subtidal ecology study area which also provide valuable baseline information (refer to section 3.2). For example, the Firth of Forth Banks Complex Marine Protected Area (MPA) displays a mosaic of different sands and gravels, which are influenced by strong currents. Despite these sand and gravel sediments being common in the northern North Sea, the dynamic currents within this MPA influence their distribution to create a unique range of habitats (JNCC, 2014; JNCC, 2021). Video and still photography surveys within the MPA identified three broad habitat types: soft sediments with ripples, mixed sediment, and coarse sediments with some rocky outcrops (Axelsson et al., 2014). The Wee Bankie moraine formation within the Firth of Forth Banks Complex MPA comprises prominent (20 m high) submarine glacial ridges with poorly sorted sediments, such as boulders, gravels, clays and sands (JNCC, 2021).
  3. Similarly, site-specific surveys conducted for other offshore wind farm projects within the regional benthic subtidal ecology study area also provide information to support the baseline. An overview of the subtidal sediments identified in these surveys is illustrated in Figure 3.1   Open ▸ . It should be noted that since conducting their site-specific surveys in 2011, the Seagreen Alpha and Seagreen Bravo projects were combined to form Seagreen 1 and Seagreen 1A. The boundary of the whole Seagreen area was changed in the process, so Table 3.2   Open ▸ refers to the superseded Seagreen Alpha and Seagreen Bravo areas to more accurately portray the spatial variation in the data that were collected at the time.

Figure 3.1:
Subtidal Sediments within the Regional Benthic Subtidal Ecology Study Area (Source: EMODnet, 2019)

Figure 3.1: Subtidal Sediments within the Regional Benthic Subtidal Ecology Study Area (Source: EMODnet, 2019)


Table 3.2:
Overview of Benthic Subtidal Sediments from Other Offshore Wind Farm Projects within the Regional Benthic Subtidal Ecology Study Area

Table 3.2: Overview of Benthic Subtidal Sediments from Other Offshore Wind Farm Projects within the Regional Benthic Subtidal Ecology Study Area

 

Subtidal benthic communities

  1. The results of eight Ministry of Agriculture, Fisheries and Food (now DEFRA, Department for Environment, Food and Rural Affairs) surveys conducted between 1980 and 1985, and the 1986 North Sea Benthos Survey were analysed by Heip and Craeymeersch (1995). The benthic macrofaunal species with the highest frequencies of occurrence in the North Sea are the bristleworm Spiophanes bombyx, polychaetes Pholoe sp., spotted chevron worm Goniada maculata, brittlestar Amphiura filiformis, and catworm Nephtys hombergii (Heip and Craeymeersch, 1995). The diversity of molluscs and crustaceans in the North Sea was highest at a latitude of 56º North (thus within the regional benthic subtidal ecology study area and in line with the Array benthic subtidal ecology study area) (Heip and Craeymeersch, 1995). Conversely, the diversity of annelids and echinoderms in the North Sea was highest from 57º to 60º North, and thus partly overlapping with the regional benthic subtidal ecology study area.
  2. More recently, as part of the Regional Seabed Monitoring Programme (RSMP), Cooper and Barry (2017) describe results of the baseline assessment of the UK’s macrobenthic infauna. Although the study was focussed on the aggregates industry, a “big data” approach was taken which collated data from across UK waters, including within the regional benthic subtidal ecology study area. This data was collated from various industries, including offshore wind farms, oil and gas, nuclear and port and harbour sectors. Cooper and Barry (2017) categorised benthic macrofaunal communities into broad groups, based on similarities in their community composition.
  3. No RSMP infaunal samples have been taken within the Array boundary itself, however there were some approximately 30 km to the west of it (Cefas 2023; Cooper and Barry 2017). The sample from the dataset were characterised by deep circalittoral sands and deep circalittoral coarse sediment and associated benthic infaunal communities of polychaetes (D2b, D2c and D2d faunal groups: Spionidae, Nephtydae, Lumbrineridae, Oweniidae, Cirratulidae, Capitellidae, Ampharetidae, Opheliidae and Magelonidae), bivalve molluscs (D2b and D2d faunal groups: Semelidae and Tellinidae) and nemerteans (D2b faunal group). Elsewhere in the regional benthic subtidal ecology study area, the Cooper and Barry (2017) dataset presents a mixture of deep circalittoral sands with deep circalittoral coarse sediment and associated benthic infaunal communities of polychaetes (C1a and C1b faunal groups: Spionidae, Glyceridae, Terebellidae, Capitellidae and Phyllodocidae) and nemerteans (D2a faunal group). Within the Firth of Forth, deep circalittoral mud is present, which is associated with infaunal Nephtyidae communities (faunal group D2c: Nephtyidae, Spionidae and Ophelidae).
  4. These findings are in line with those of Sotheran and Crawford-Avis (2014) who identified Marine Nature Conservation Review (MNCR) habitats from samples within the regional benthic subtidal ecology study area. S. bombyx aggregations in offshore sands (SS.SSa.OSa.[Sbom]) were identified in deep circalittoral sand. S. bombyx aggregations in offshore coarse sands (SS.SCS.OCS.[Sbom]) and polychaete-rich Galathea communities with encrusting bryozoans and other epifauna on offshore coarse sediments (SS.SCS.OCS.[PoGintBy]) were identified in deep circalittoral coarse sediments.
  5. Similarly, Pearce et al. (2014) identified Ross worm Sabellaria spinulosa on stable circalittoral mixed sediment (SS.SBR.PoR.SspiMx), polychaete-rich Galathea communities with encrusting bryozoans and other epifauna on offshore circalittoral mixed sediment (SS.SMx.OMx.[PoGintBy]), and both SS.SSa.OSa.[Sbom] and SS.SCS.OCS.[Sbom], within the regional benthic subtidal ecology study area, albeit further inshore. In a more recent study, Pearce and Kimber (2020) idenitifed the presence of Ross worm off the coasts of Peterhead and Fraserburgh (Aberdeenshire), which are approximately 100 km north of the Array. However, given that the majority of the data presented in Pearce and Kimber (2020) are out with the regional benthic subtidal ecology study area, this study has not been used further to characterise the Array.
  6. As stated in paragraph 9, there are a range of designated sites within the regional benthic subtidal ecology study area which provide information about the benthic environment. A 2011 survey within the Firth of Forth Banks Complex MPA described 12 biotopes with a circalittoral muddy sand (SS.SSa.CMuSa) biotope complex being the most commonly recorded (Axelsson et al., 2014). The SS.SSa.CMuSa biotope complex was comprised of two biotopes, one dominated by the bivalves Abra alba and Nucula nitidosa, and the other by brittlestar Acronida brachiata and sea star Astropecten irregularis (Axelsson et al., 2014). Mixed sediment habitats (consisting of gravel, sand, mud and some shell material) were typically dominated by brittlestars Ophiothrix fragilis and Ophiocomina nigra, bryozoan Flustra foliacea, or the horse mussel Modiolus. The soft coral, dead man’s fingers Alcyonium digitatum, and ascidians were typically present in coarser sediments (a mixture of gravel, pebbles and cobble overlying finer sediments) (Axelsson et al., 2014).
  7. Furthermore, site-specific surveys conducted for other offshore wind farm projects within the regional benthic subtidal ecology study area also provide information to support the baseline. An overview of the subtidal benthic communities recorded in these surveys is presented in Table 3.3   Open ▸ . As per paragraph 10, the superseded names for Seagreen 1 and Seagreen 1A (Seagreen Alpha and Seagreen Bravo) are referred to, in order to more accurately portray the spatial distribution of the data that were collected at the time.
Table 3.3:
Overview of Benthic Subtidal Communities from Other Offshore Wind Farm Projects within the Regional Benthic Subtidal Ecology Study Area

Table 3.3: Overview of Benthic Subtidal Communities from Other Offshore Wind Farm Projects within the Regional Benthic Subtidal Ecology Study Area

 

3.2.2. Array Benthic Subtidal Ecology Study Area

                        Subtidal sediments

  1. The EUSeaMap broadscale substrate data indicate that the sediments within the Array benthic subtidal ecology study area are almost entirely represented by deep circalittoral sand (A5.27; SS.SSa.OSa). There is one small area comprised of deep circalittoral coarse sediment (A5.15; SS.SCS.OCS) located within the north-west of the area ( Figure 3.2   Open ▸ ).

Figure 3.2:
Broadscale Subtidal Sediments within the Array Benthic Subtidal Ecology Study Area (Source: EMODnet, 2019)

Figure 3.2: Broadscale Subtidal Sediments within the Array Benthic Subtidal Ecology Study Area (Source: EMODnet, 2019)


                        Subtidal benthic communities

  1. The NBN Atlas collates species records and presents them spatially on an interactive map for users to explore. Using the NBN Atlas (2021) to explore the Array benthic subtidal ecology study area, the following species were recorded:
  • ocean quahog Arctica islandica;
  • gastropod Colus gracilis;
  • red whelk Neptunea antiqua;
  • prickly cockle Acanthocardia echinata;
  • dark necklace shell Euspira fusca; and
  • horse mussel.
  1. These six molluscs were all recorded during dredging surveys conducted by the Conchological Society of Great Britain and Ireland. These surveys have been conducted since 1876, with these particular recordings within the Array benthic subtidal ecology study area dating from 1980 to 2016 (Conchological Society of Great Britain and Ireland, 2023).
  2. Otherwise, there are currently few data sources characterising the subtidal benthic communities within the Array subtidal ecology study area itself, with the majority of sources focussed on elsewhere in the regional benthic subtidal ecology study area (as presented in section 2). Therefore, the subtidal benthic communities within the Array subtidal ecology study area will largely be informed by the results of the site-specific environmental survey (refer to section 2).

3.2.3. Designated Sites

  1. While the Array benthic subtidal ecology study area does not overlap with any protected sites that have been designated for benthic subtidal features, numerous sites occur within the regional benthic subtidal ecology study area. These include an MPA and three Special Areas of Conservation (SACs). Sites of Special Scientific Interest (SSSIs) have not been listed here due to their distance from the Array benthic subtidal ecology study area and their intertidal features which would not be impacted by the Array. For example, the closest SSSIs with benthic ecological designations are the Tayport Tentsmuir Coast SSSI (124.03 km), Berwickshire Coast Intertidal SSSI (126 km), and the Firth of Forth SSSI (126.99 km), which are designated for mudflats, saline lagoons and rocky shores.
  2. Using publicly available data supplied by the JNCC, EMODnet, and the Marine Scotland NMPi maps, there are no known Annex I sandbanks, mudflats, reefs, shallow inlets or bays, submerged or partially submerged sea caves, estuaries, or Oslo and Paris Convention (OSPAR) listed threatened and declining habitats overlapping with the Array benthic subtidal ecology study area. As indicated in Figure 3.3   Open ▸ , the closest designated site with benthic subtidal ecology features is the Firth of Forth Banks Complex MPA, which is located at least 25 km from the Array benthic subtidal ecology study area at its closest point. All other designated sites are much further away (i.e. >100 km) and are therefore unlikely to be affected by the Array ( Table 3.4   Open ▸ ).

 

Table 3.4:
Summary of Designated Sites with Relevant Benthic Qualifying Features

Table 3.4: Summary of Designated Sites with Relevant Benthic Qualifying Features

Figure 3.3:
Designated Sites in Proximity to the Array Benthic Subtidal Ecology Study Area

Figure 3.3: Designated Sites in Proximity to the Array Benthic Subtidal Ecology Study Area