Significance of the effect
  1. For all IEFs, the magnitude of the impact is deemed to be low, and the sensitivities of the receptors are considered to be low. Based on Table 8.16   Open ▸ , the effect will, therefore, be of negligible to minor adverse significance. Based on expert judgement and adopting a precautionary approach, the effect has been concluded to be of minor adverse significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary benthic subtidal ecology mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.

                        Operation and maintenance phase

                        Magnitude of impact
  1. A large proportion of the length of each mooring line will remain largely static on the seabed during operation and maintenance with movement predominately around the touchdown point. The greatest potential for the increase in SSCs due to mooring lines will be from catenary moorings which have the greatest length in contact with the seabed. Two approaches to the MDS were considered in the assessment of benthic subtidal ecology, in contrast to one considered in the physical processes assessment (volume 2, chapter 7). In line with the physical processes assessment, the first MDS was considered to be the number of foundations with the greatest length of mooring line on the seabed per foundation as this has the potential to result in the greatest increase in SSC within the radius of the mooring footprint of a single turbine mooring footprint. The effects are considered to be very localised, with no interactions in plumes or deposition between adjacent turbine locations. This was assumed as up to 130 semi-submersible turbine foundations with up to 9 catenary mooring lines each ( Table 8.12   Open ▸ ). This first MDS is hereafter referred to as the ‘130 turbine MDS’ for clarity. The second MDS considered was based on up to 265 semi-submersible turbine foundations with up to 6 catenary mooring lines each ( Table 8.12   Open ▸ ) and is hereafter referred to as the ‘265 turbine MDS’ for clarity. This was included in the assessment for benthic subtidal ecology as the 130 turbine MDS represents a potentially higher impact to benthic IEFs and habitats at a localised level (due to a higher number of mooring lines per foundation), but it does not consider the overall footprint of impact over the Array benthic subtidal ecology study area as a whole. Thus, the 265 turbine MDS represents a higher overall length of mooring lines, and therefore area of deposition from increased SSC over the Array benthic subtidal ecology study area as a whole, but a lower potential for impact associated with benthic IEFs and habitats in the immediate vicinity of individual turbines.  
  2. The mooring line radius for both MDSs is 700 m, with a touchdown distance of between 25 m and 150 m from the foundation, and overall length of 750 m. During operation, approximately 680 m of the catenary mooring line will be in contact with the seabed which amounts to up to 6,120 m per foundation for the 130 turbine MDS and up to 4,080 m per foundation for the 265 turbine MDS ( Table 8.12   Open ▸ ). Overall, up to 795,600 m of mooring line may be in contact with the seabed under the 130 turbine MDS, and up to 1,081,200 m under the 265 turbine MDS ( Table 8.12   Open ▸ ). The tidal range at the Array benthic subtidal ecology study area is less than 4 m, therefore it is not anticipated that tidal movements will result in substantial horizontal and vertical movements of floating substructures. As a result, the mooring lines are not considered to notably increase the SSCs under standard operating conditions for both the MDSs.
  3. Under harsher weather conditions, the dynamic interaction between the mooring lines and the seabed will increase with intensity and direction of the storm. Horizontal movement of the floating foundations may result in the lifting of the mooring lines located on the windward side of the turbine, as tension on these mooring lines increases. Mooring lines on the leeward side would experience the opposite effect, whereby the length of mooring line in contact with the seabed increases as they slacken, up to a maximum of 710 m for some mooring lines in the most extreme storm conditions. The length where disturbance is likely to occur will be less, as this will be greater closer to the touchdown point and negligible towards the anchor point. Furthermore, the dimensions of the mooring lines are considered to be small, with a chain thickness of 185 mm, and horizontal diameter of 620 mm, which will limit the volumes of seabed material they have the potential to disturb, even if they were to become completely embedded.
  4. Movement on the seabed by inter-array cables will be limited to a small section between the touch down point and the point where the cable becomes static, resulting in minor increases to SSCs in the vicinity of the touchdown point only. With regard to inter-array cables, the total length of the dynamic inter-array cables will be 116 km with a maximum external cable diameter of 300 mm for both MDSs considered. Movement of the inter-array cables may be reduced through the use of buoyancy modules and clump weights (subject to engineering design) thus limiting movement on the seabed to a very small proportion of the total dynamic cable length between the touchdown point and where it transitions to a static cable. Static inter-array and interconnector cables on the seabed will be buried or fixed with cable protection where target burial depths cannot be achieved. Thus, the potential disturbance area is restricted to small areas in the vicinity of up to two dynamic cable touchdown points per turbine. Increased SSCs would therefore be spatially limited, smaller, and adjacent to any disturbance resulting from the mooring lines.
  5. The spacing between the floating foundations is a minimum 1.4 km for the 130 turbine MDS and a minimum of 1 km for the 265 turbine MDS ( Table 8.12   Open ▸ ). These spacings are large enough for any impacts to SSCs to be considered as isolated, considering the low current speeds and sediment transport rates in the physical processes study area. Any dynamic interactions between the seabed and mooring lines or dynamic cables will likely be experienced similarly at adjacent foundations under tidal and storm conditions, with the foundations moving in the same direction and orientated the same way as their neighbouring foundations. Thus, storm conditions will not impact upon minimum foundation spacing and seabed disturbance areas from mooring lines are considered sufficiently far apart to be isolated even under storm conditions for both MDSs considered.
  6. Variation in seabed composition is limited across the Array benthic subtidal ecology study area, with sand accounting for most of the seabed substrate, with small amounts of mud and gravel. Disturbed materials are more likely to move along the seabed, rather than becoming fully suspended in the water column and due to the low nearbed current speeds, will not be transported for any significant distance before being re-deposited on the seabed. The baseline dominant current direction within the site boundary is to the south or south-south-west, with dominant wind directions also from the south-west. Therefore, disturbed sediments from mooring lines and cabling are likely to move towards the north-east, however, there may also be some effect from littoral currents produced by the dominant wave direction from the north.
  7. As discussed within the physical processes technical report (volume 3, appendix 7.1), movement would only occur during a small proportion of the tidal cycle, due to the reduction in current speeds, therefore material will settle within a few minutes to hours, depending on tidal state and be deposited close to the area of disturbance. Therefore, the potential for changes to the overall sediment transport regime in the Array benthic subtidal ecology study area is unlikely, particularly considering the small quantities of material with potential to be disturbed. There is a low potential to directly impact benthic subtidal ecology from the increase in SSCs, however due to the isolated volumes of potential materials to be disturbed and the low sediment transport rates in the area, the impact can be considered to be relevant within the Array benthic subtidal ecology study area only. For both MDSs considered, direct impact would occur intermittently for short durations of the tidal cycle and would be greatest during storm conditions. Baseline Total Suspended Sediment (TSS) levels were assessed as likely below 10 mg/l during a winter storm, and any increase as a result of the mooring lines and cabling are not expected to exceed this. Seabed scouring from movement of mooring lines and cabling on the seabed during storm events will be limited due to the ongoing sediment transport processes.
  8. Overall, for all IEFs, impact is predicted to be of local spatial extent, short term duration, intermittent, and of high reversibility. The magnitude is therefore considered to be low.
                        Sensitivity of the receptor
  1. The sensitivities of all IEFs are considered to be as previously described for the site preparation and construction phases (see Table 8.20   Open ▸ and paragraphs 117 to 120) and have not been repeated here.
                        Significance of the effect
  1. For all IEFs, the magnitude of the impact is deemed to be low, and the sensitivities of the receptors are considered to be low. Based on Table 8.16   Open ▸ , the effect will, therefore, be of negligible to minor adverse significance. Based on expert judgement and adopting a precautionary approach, the effect has been concluded to be of minor adverse significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary benthic subtidal ecology mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.

                        Decommissioning phase

                        Magnitude of impact
  1. Decommissioning of infrastructure associated with the Array may lead to increases in SSCs and associated deposition. The MDS is represented by the removal of all infrastructure, as this represents the largest potential for increased SSCs and associated deposition ( Table 8.12   Open ▸ ). It should be noted that the decommissioning strategy is not defined, and cables, cable protection, and scour protection may potentially be left in situ. In reality, if some infrastructure remains in situ, the MDS presented here will be an overestimation, and SSCs will be lower as a result.
  2. Decommissioning activities are assumed to result in increased SSCs and associated deposition that are lesser than or equal to those produced during construction. The impacts of decommissioning activities are therefore predicted to be no greater than those presented in paragraphs 109 et seq. for the site preparation and construction activities. In actuality, the release of sediment in the decommissioning phase will be lower as it does not include activities such as seabed preparation and DEA installation.
  3. Therefore, this impact is predicted to be of local spatial extent, short term duration, intermittent, and of high reversibility. The magnitude is therefore considered to be low.
                        Sensitivity of the receptor
  1. The sensitivities of all IEFs are considered to be as previously described for the site preparation and construction phases (see Table 8.20   Open ▸ and paragraphs 117 to 120) and have not been repeated here.
                        Significance of the effect
  1. For all IEFs, the magnitude of the impact is deemed to be low, and the sensitivities of the receptors are considered to be low. Based on Table 8.16   Open ▸ , the effect will, therefore, be of negligible to minor adverse significance. Based on expert judgement and adopting a precautionary approach, the effect has been concluded to be of minor adverse significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary benthic subtidal ecology mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.

Effects to benthic subtidal ecology from EMF from subsea electrical cabling

  1. There is potential for EMFs to be produced by the subsea electrical cabling throughout the 35 year lifetime of the Array. There were no relevant MarESA pressures and benchmarks available to inform the assessment on any of the IEFs, due to the limited available information on the impacts of EMFs on benthic species (Tillin and Tyler-Walters, 2014a, Tillin and Tyler-Walters, 2014b).

                        Operation and maintenance phase

                        Magnitude of impact
  1. The MDS accounts for up to 1,261 km of 66 kV inter-array cables, with up to 116 km within the water column (i.e. ‘dynamic cables’) and the rest buried at a minimum target depth of 0.4 m ( Table 8.12   Open ▸ ). There will also be up to 236 km of interconnector cables buried to a minimum target depth of 0.4 m ( Table 8.12   Open ▸ ). Final cable burial depths will be subject to a Cable Burial Risk Assessment (CBRA). It has been estimated in the MDS that up to 20% of these buried cables will require cable protection, with up to 24 cable crossings also requiring protection.
  2. EMFs comprise both the electrical fields, measured in volts per metre (V/m), and the magnetic fields, measured in microtesla (µT), millitesla (mT), or milligauss (mG). Within the North Sea, background magnetic field measurements field are approximately 50 μT, and background electric field measurements are approximately 25 μV/m (Tasker et al., 2010). Subsea cables are constructed using magnetic outer sheathing materials, which can partially block the direct electrical field (E-field), meaning that the only EMFs that are emitted into the marine environment are the magnetic field (B-field) and the resultant induced electrical field (iE-field). Dynamic cables are typically double armoured to increase stability and manage weight, which may inadvertently reduce losses of EMFs (Hervé, 2021). By design, alternating current (AC) and direct current (DC) cables typically contain three and two conductor bundles, respectively, which are superimposed and twisted around each other. This design feature creates partial self-cancellation of the total B-field (CSA Ocean Sciences Inc and Exponent, 2019, Hervé, 2021). At the seabed, cable burial and cable protection are common industry practice measures, which can reduce EMF levels at the seabed surface (Chapman et al., 2023, CSA Ocean Sciences Inc and Exponent, 2019, Gill et al., 2005, Gill et al., 2009). Overall, EMF levels in the vicinity of subsea cables are influenced by a variety of design and installation factors, including distance between cables, cable sheathing, number of conductors, and internal cable configuration.
  3. Although there will be up to 116 km of dynamic cabling, a large portion of this will be higher within the water column itself, and the length of cabling in the vicinity of the seabed will be much lower. At this stage of the Array design, it is not possible to refine this value further. However, the intensity of EMF from subsea cables decreases at approximately the inverse square/power of the distance away from the cable (Hutchison et al., 2021). This attenuation is the same for buried, unburied, and dynamic cables (Hutchison et al., 2021). Therefore, this impact is likely to be highly localised to the vicinity of dynamic cabling and therefore only the portion of cable close to the seabed will potentially impact benthic species.
  4. The impact is predicted to be of local spatial extent, long term duration, continuous, and of high reversibility (as cables will be removed after the operation and maintenance phase). It is predicted that the impact will affect the receptors directly. This impact presents some measurable, long term minor loss of and alteration to the affected areas of seabed within the Array benthic subtidal ecology study area. The magnitude is therefore considered to be low.
                        Sensitivity of the receptor
  1. While there is a growing evidence base on the impacts of EMFs on fish species (Armstrong et al., 2015, Cresci et al., 2022, CSA Ocean Sciences Inc and Exponent, 2019, Gill et al., 2009, Gill and Taylor, 2001, Hutchison et al., 2018, Normandeau Associates Inc et al., 2011, Orpwood et al., 2015, Snyder et al., 2019), studies on benthic invertebrates are limited, with research primarily focussing on crustaceans (Harsanyi et al., 2022, Hutchison et al., 2020b, Hutchison et al., 2018, Scott et al., 2021, Scott et al., 2018). Therefore, there is a knowledge gap surrounding the ability of benthic species to detect EMFs and any associated physiological or behavioural impacts (Albert et al., 2020). As a result, there was no MarESA available for any of the benthic IEFs identified within this assessment to impacts associated with EMFs (Tillin and Tyler-Walters, 2014a, Tillin and Tyler-Walters, 2014b).
  2. Recently, Chapman et al. (2023) presented the findings of a study on the behavioural and physiological responses of two echinoderms (common starfish and common sea urchin Echinus esculentus), velvet swimming crab Necora puber, and common periwinkle Littorina littorea to EMFs from subsea power cables. This represents the first study on the effects of EMF on common sea urchin, although previous studies have demonstrated developmental delay in the embryos of painted urchin Lytechinus pictus and purple urchin Strongylocentrotus purpuratus due to EMF exposure between 10 µT to 100,000 µT (Cameron et al., 1993, Levin and Ernst, 1997, Zimmerman et al., 1990). Chapman et al. (2023) exposed common starfish, common sea urchin, velvet swimming crab, and common periwinkle to EMFs of 500 μT for 24 hours and reported no significant behavioural or physiological responses in any of the species (Chapman et al., 2023). Similarly, Bochert and Zettler (2006) found that an artificial static EMF of approximately 2,700 μT had no effect on common starfish distribution in laboratory settings over 22 hours. Bochert and Zettler (2006) also exposed ragworm Hediste diversicolor and the isopod crustacean Saduria entomon to the same environmental conditions, with ragworm distribution unaffected by the EMF levels, as per their results on common starfish. However, only one third of S. entomon individuals were recorded in the vicinity of the EMF source after 22 hours, while the control group population was equally distributed in the enclosure, suggesting a potential avoidance to EMFs in this species (Bochert and Zettler, 2006).
  3. Effects of EMF on ragworm were also investigated by Jakubowska et al. (2019) and Stankevičiūtė et al. (2019). The former study assessed the effect of EMF levels of 1 mT from a cable of 50 Hz for eight days on the avoidance behaviour, burrowing, and physiology (food consumption, respiration, and extraction of ammonia) of ragworm (Jakubowska et al., 2019). No avoidance or attraction behaviour to the EMF source was reported, and there were no changes in food consumption and respiration rates (Jakubowska et al., 2019). Similarly, Albert et al. (2022) observed no alteration in feeding behaviour of blue mussels Mytilus edulis exposed to artificial B-field treatment of 300 µT. However, ragworm burrowing activity increased and ammonia excretion was significantly lower when exposed to the EMF conditions, although the mechanisms behind these observations remain unclear (Jakubowska et al., 2019). In addition, genotoxic and cytotoxic effects of 50 Hz 1 mT EMFs over 12 days were investigated for ragworm and the Baltic tellin Macoma balthica (i.e. effects which could cause DNA and cellular damage, respectively) (Stankevičiūtė et al., 2019). Exposure to EMF did not induce any significant cytotoxic responses in ragworm, however a significant elevation in frequencies of cells with 8-shaped nuclei, apoptotic cells, and binucleated cells was recorded for Baltic tellin, which suggest cytotoxic effects (Stankevičiūtė et al., 2019). Both ragworm and Baltic tellin displayed genotoxic effects as a result of EMF exposure, measured by increased formation of micronuclei and nuclear buds, which are markers of DNA damage such as chromosomal loss and mitotic disruption (Stankevičiūtė et al., 2019).
  4. Although there are no studies to date on ocean quahog sensitivity to EMF, the results above for the Baltic tellin could be applicable, given that both species are North Sea burrowing bivalves. Further, Jakubowska-Lehrmann et al. (2022) assessed the effects of 50 Hz 1 mT EMFs over eight days on another bivalve, the lagoon cockle Cerastoderma glaucum. As recorded for ragworm (Jakubowska et al., 2019), there were no changes in respiration rate of the lagoon cockle, but significantly lower ammonia excretion was recorded after EMF exposure (Jakubowska-Lehrmann et al., 2022). Lagoon cockle showed no changes in antioxidant enzyme activity or lipid peroxidation (indicators of oxidative stress), however increased protein carbonylation and inhibition of acetylcholinesterase activity were observed after EMF exposure (indictors of oxidative stress and neurotoxicity) (Jakubowska-Lehrmann et al., 2022). The latter finding suggests that EMF exposure could have oxidative and neurotoxic impacts on lagoon cockle (i.e. damage to cells, proteins, DNA, and the nervous system) (Jakubowska-Lehrmann et al., 2022). Furthermore, increased oxidative stress was recorded in the mollusc Elysia leucolegnote after exposure to EMFs in laboratory conditions (Fei et al., 2023), although it should be noted that E. leucolegnote is a species of sea slug, only recorded in Hong Kong, thus not be representative of the IEFs defined in this assessment.
  5. Overall, there is limited literature available on the potential impacts of EMF on benthic invertebrates (Albert et al., 2020, Hervé, 2021) and none of the existing studies described in paragraphs 145 to 148 include any of the IEFs defined as part of this assessment. While echinoderms, bivalves, and polychaetes are key components of the representative biotopes of the Offshore subtidal sands and gravels IEF and the Subtidal sands and gravels IEF, impacts of IEFs on these taxa are varied throughout the literature (as described in paragraphs 146 to 148). To date, there have been no studies on the impact of EMFs on hydroids (such as sea tamarisk) or anthozoans (such as dead man’s fingers or phosphorescent sea pen), and assessing sensitivity is challenging as a result. There have been several recent studies on the impact of EMFs on bivalves (Albert et al., 2020, Jakubowska-Lehrmann et al., 2022, Jakubowska et al., 2019, Stankevičiūtė et al., 2019). The results are varied, but cytotoxic, genotoxic, and neurotoxic impacts of EMFs were recorded, which could also occur in the ocean quahog, given its relative taxonomic similarity to the bivalve species assessed. However, it should be noted that these studies recorded results at considerably higher levels than would be expected to occur several metres away from subsea power cables: 300 µT  (Albert et al., 2022) and 1 mT (Jakubowska-Lehrmann et al., 2022, Jakubowska et al., 2019, Stankevičiūtė et al., 2019).
  6. Overall, on a precautionary basis, all IEFs are deemed to be of medium vulnerability, medium recoverability, and regional to national value. In reality, this is likely to be over precautionary, based on the results of the literature summarised in the preceding paragraphs, which largely suggests minimal impacts at EMF levels likely to be present at the cables associated with the Array. Further, the literature typically considers EMF levels which are much higher than those that would be associated within several metres of the cables associated with the Array. Further, the results of some studies (such as Chapman et al. (2023) and Bochert and Zettler (2006)) found little to no impact of EMF on the species in their assessments. The sensitivities of the receptors are, therefore, considered to be medium.
                        Significance of the effect
  1. For all IEFs, the magnitude of the impact is deemed to be low, and the sensitivities of the receptor are considered to be medium. The effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary benthic subtidal ecology mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.

Colonisation of hard substrates

  1. The introduction of the hard substrates on the seabed and the foundations of floating, mooring lines and dynamic cables of wind turbines within the water column may potentially affect the established benthic community by providing new habitat and ecosystem function. These hard substrates include:
  • mooring lines and anchors on the seabed;
  • OSP foundations;
  • inter-array and interconnector cable protection and cable crossing protection;
  • subsea junction boxes;
  • scour protection for mooring lines, anchors, OSP foundations, and subsea junction boxes; and
  • floating wind turbine foundations in the water column.
  1. These artificial hard structures are expected to be colonised by a range of organisms, which could lead to local biodiversity increases. The relevant MarESA pressure associated with this impact is the same as assessed above for ‘Long term habitat loss and disturbance’:
  • Physical change (to another seabed type): the benchmark for which is change in sediment type from sedimentary or soft rock substrata to hard rock or artificial substrate or vice-versa.

                        Operation and maintenance phase

                        Magnitude of impact
  1. The MDS for this impact is similar to as described above for ‘Long term habitat loss and disturbance’ which assumes that up to 19.27 km2 of artificial hard substrate will be installed on the seabed within the Array benthic subtidal ecology study area (2.25% of the entire area) ( Table 8.12   Open ▸ ). This comprises mooring lines and anchors on the seabed, OSP foundations, inter-array and interconnector cable protection and cable crossing protection, subsea junction boxes, and scour protection for mooring lines, anchors, OSP foundations, and subsea junction boxes. In addition, the presence of floating wind turbine foundations, anchor mooring lines, and dynamic cables represent hard substrate which may be colonised within the water column ( Table 8.12   Open ▸ ).
  2. It is expected that these artificial hard structures will be colonised by epifaunal species local to the Array benthic subtidal ecology study area. However, this impact will represent a shift in the baseline seabed conditions from soft to hard substrate in the areas where the infrastructure is installed. This could result in beneficial effects. For example, a 12 year monitoring study on the artificial foundations installed at the Lysekil research site in Sweden reported increased biodiversity, abundance of reef species, and total number of species over time (Bender et al., 2020). Colonising communities on offshore installations are typically dominated by mussels, macroalgae, and barnacles near the water surface, which essentially creates a new intertidal zone, while the community is dominated by filter feeding arthropods at intermediate depths, and by anemones in deeper locations (De Mesel et al., 2015, Karlsson et al., 2022). Colonisation of the hard substrates associated with the Array is therefore likely to result in an increase in biodiversity and a change compared to the baseline if no hard substrates were present (Lindeboom et al., 2011). In addition, the structural complexity of artificial substrates such as OSP foundations and floating wind turbine foundations may provide refuge as well as increasing feeding opportunities for larger and more mobile species. For example, Mavraki et al. (2020), demonstrated higher food web complexity associated with zones which had high accumulation of organic material (such as soft substrate or scour protection), suggesting potential reef effect benefits from the presence of artificial hard structures.
  3. Although this impact is expected to be beneficial in terms of increasing biodiversity and enhancing reef effects, the installation of hard structures will result in habitat loss for the Offshore subtidal sands and gravels IEF and the Subtidal sands and gravels IEF. However, given the wide availability of both of these habitats over the Array benthic subtidal ecology study area and regional benthic subtidal ecology study area, and the localised nature of this impact (2.25% of the Array benthic subtidal ecology study area), this impact is only expected to result in minor loss or alteration to the soft bottom sediments of these IEFs as a whole.
  4. Overall, for all IEFs, the impact is predicted to be of local spatial extent (2.25% of the Array benthic subtidal ecology study area), long term duration, continuous, and of low reversibility. It is predicted that the impact will affect the receptors directly. The magnitude is therefore considered to be low.
                        Sensitivity of the receptor
  1. Introduction of hard structures within the Array benthic subtidal ecology study area will represent a shift in seabed type and species assemblage. In terms of the MarESA, the sensitivity of the IEFs to this impact are as previously described for physical change (to another seabed type) in the assessment of ‘Long term habitat loss and disturbance’ (see Table 8.19   Open ▸ and paragraphs 94 to 97). The MarESA sensitivities were high for all IEFs except dead man’s fingers and sea tamarisk, which were assessed as low.
  2. Colonisation of hard structures may have indirect adverse effects on the baseline communities and habitats identified within the Array benthic subtidal ecology study area due to increased predation on and competition for the existing soft sediment species. Nonetheless, these effects are difficult to predict on large scales and timelines, especially as monitoring to date has focussed on the colonisation and aggregation of species close to wind turbine foundations rather than broad scale studies. Introducing hard structures on the seabed not only creates new habitat but also modifies or removes existing, sandy and soft bottom habitats. Often it replaces an essentially two-dimensional sedimentary seabed with a complex three-dimensional structure, thereby increasing surface area, surface complexity and number of niches Dannheim et al. (2020). Increased biodiversity and connectivity of populations is dependent on suitable artificial hard substrates being created at the right location and distances from source populations (Chase, 2015). Substrates may also only be suitable for colonisation after being suitably weathered, through the loss of any surface contaminants, the production of biofilms, and the sequence of development of the community after settlement (Chase, 2015; Thompson et al., 1998). Rougher textures may facilitate greater microhabitat diversity than smoother ones (Anderson and Underwood, 1994) and could have greater potential for colonisation.
  3. With regards to the Offshore subtidal sands and gravels IEF and the Subtidal sands and gravels IEF, several studies have also shown that the installation of artificial habitat have no significant effect on the soft sediment environments. For example, the soft sediment benthic community underwent no drastic changes eight to nine years after the installation of C-power and Belwind Offshore Wind Farms in Belgium (De Backer et al., 2020). Furthermore, the species originally inhabiting the sandy substrate were still present and remained dominant in both Offshore Wind Farms (De Backer et al., 2020). Additionally, post-construction monitoring at the Block Island Wind Farm in the USA showed no strong gradients of change in sediment grain size, enrichment, or benthic macrofauna within 30 m to 90 m distance bands of the wind turbines (Hutchison et al., 2020a). Recent post-construction monitoring of the Beatrice Offshore Wind Farm in the Moray Firth demonstrated extensive biofouling on all the wind turbines with signs of zonation and successional development (APEM, 2022). Across all the wind turbines, plumose anemones Metridium senile and tube worms S. triqueter were the most abundant species, with the highest biomass at 40 m depth (APEM, 2022). The hermit crab Pagurus bernhardus, various flatfish species, and common sea urchin were found at the bases of the wind turbines with decreasing abundance further from the foundations, indicating a source of food although no biological matter could be seen (APEM, 2022). Similarly, at Hywind Scotland off the coast of Aberdeenshire, plumose anemones and tube worms Spirobranchus sp. dominated the bottom and mid-section of floating wind turbine substructures, and a general increase in epifouling growth between 2018 and 2020 was recorded (Karlsson et al., 2022).
  4. Larval distribution can be influenced by the introduction of hard substrates, which could have potential impacts on species distribution and population connectivity. Research from the oil and gas sector has examined the potential impact of infrastructure in the interception and production of larvae (McLean et al., 2022). Sound, chemical cues, light, and vibrations can all trigger larval settlement. Where artificial hard structures exist in offshore waters far from natural reefs, their influence on larval dispersal and settlement may be comparatively high, in relation to platforms in more naturally connected environments, therefore influencing geographic and population connectivity (McLean et al., 2022). As species become established on and around the artificial hard structures, they can start producing larvae, with one study demonstrating that networks of oil and gas infrastructure in the North Sea could facilitate ecological connectivity by acting as stepping stones for larval connectivity (Henry et al., 2018). Similarly, another North Sea study found interannual variability in the North Atlantic Oscillation results in cold-water coral Lophelia pertusa larvae being dispersed from oil and gas structures across distances of ~300 km (Fox et al., 2016). The influence of oceanographic features in species dispersal and distribution emphasises the importance of characterising the hydrodynamics underpinning potential connectivity (Boschetti et al., 2020). Potential barriers to settlement, growth, reproduction and survival of larvae on offshore infrastructure also exist, such as cleaning regimes, surface coatings (e.g. antifoulant), and operational discharges.
  5. Finally, artificial hard substrates can often support higher densities of INNS than natural environments, due to reduced competition from established native species, more vacant habitat, and year-round settlement allowing opportunistic colonisation of vacant space (Mineur et al., 2012). However, increased risk and spread of INNS has been assessed separately in paragraphs 180 to 207.
  6. The ocean quahog IEF and phosphorescent sea pen IEF require a soft sedimentary habitat, and physical change to hard artificial or rock substratum would represent habitat loss for these species, which are therefore highly vulnerable to this impact (Hill and Tyler-Walters, 2018, Tyler-Walters and Sabatini, 2017). Overall, all IEFs except dead man’s fingers and sea tamarisk are deemed to be of high vulnerability, low recoverability, and national and regional value. The sensitivities of the receptors are, therefore, considered to be high ( Table 8.19   Open ▸ ).
  7. In contrast however, the dead man’s fingers IEF and the sea tamarisk IEF naturally live on hard substrates, including bedrock, rocks, boulders, shells, and man-made artificial hard structures (Budd, 2008, Wilson, 2002). Therefore, this impact does not represent a change from a preferred habitat to an unsuitable one for these IEFs, in comparison to the other IEFs. In addition, hydroids (such as sea tamarisk) are typically one of the first taxa to colonise new substrates (Boero, 1984). For example, a study on marine growth on the North Sea oil platform Montrose Alpha recorded eight species of hydroid (although none were sea tamarisk), present on the hard structures associated with the platform (Forteath et al., 1982). Therefore, the dead man’s fingers IEF and sea tamarisk IEF are deemed to be of low vulnerability, medium recoverability, and regional value. The sensitivities of the receptors are, therefore, considered to be low ( Table 8.19   Open ▸ ).
                        Significance of the effect
  1. Overall, for the dead man’s fingers IEF and sea tamarisk IEF, the magnitude of the impact is deemed to be low, and the sensitivities of the receptor are considered to be low. Based on Table 8.16   Open ▸ , the effect will, therefore, be of negligible to minor beneficial significance. Based on expert judgement and adopting a precautionary approach, the effect has been concluded to be of minor beneficial significance, which is not significant in EIA terms.
  2. For all other IEFs, the magnitude of the impact is deemed to be low, and the sensitivities of the receptors are considered to be high. As per Table 8.16   Open ▸ , the effect will, therefore, be of minor to moderate significance. The potential for increased biodiversity as a result of this impact could be considered to be beneficial, however introduction of hard substrates would represent some small-scale habitat loss for these IEFs. Given the low footprint of long term habitat loss with respect to both the Array benthic subtidal ecology study area and the North Sea as a whole, and the widespread availability of alternative suitable habitat, the effect is concluded to be of minor adverse significance, which is not significant in EIA terms.
Secondary mitigation and residual effect
  1. No secondary benthic subtidal ecology mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.

Effects to benthic subtidal ecology due to removal of hard substrates

  1. The removal of artificial hard substrates in the decommissioning phase may affect the established benthic community associated with the Array benthic subtidal ecology study area, with the seabed returning to its current sandy sediments. These artificial hard structures are expected to have been colonised by a range of organisms over the 35 year lifecycle of the Array, which has been previously assessed above in ‘Colonisation of hard structures’. The relevant MarESA pressure associated with this impact is the same as assessed above for ‘Long term habitat loss and disturbance’ and ‘Colonisation of hard structures’:
  • Physical change (to another seabed type): the benchmark for which is change in sediment type from sedimentary or soft rock substrata to hard rock or artificial substrate or vice-versa.
  1. In this case, however, the physical change to another seabed type refers to a change from artificial hard substrata to soft sandy sediments.

                        Decommissioning phase

                        Magnitude of impact
  1. The MDS accounts for up to a total of 19.27 km2 of artificial hard substrates to be removed from the seabed during the decommissioning phase, which represents up to 2.25% of the total Array benthic subtidal ecology study area ( Table 8.12   Open ▸ ). In addition, MDS accounts for the removal of hard substrate in the water column, such as floating wind turbine foundations, dynamic cables, and anchor mooring lines ( Table 8.12   Open ▸ ). As per the justification presented in Table 8.12   Open ▸ , the MDS for this impact is the complete removal of all infrastructure installed on the seabed and in the water column in the Array benthic subtidal ecology study area, as this represents the largest potential impact. It should be noted that the decommissioning strategy is not yet defined, and cable protection, cable crossing protection, and scour protection may potentially be left in situ. Anchors will also be removed or cut on or at the seabed and left in situ, however are considered unlikely to contribute to this impact as they will be a significant depth below the seabed. Leaving cable protection, cable crossing protection, and scour protection in situ represents the MDS in the decommissioning phase for ‘Long term habitat loss and disturbance’ and has been assessed as such in paragraphs 101 to 103. In reality, if this infrastructure remains in situ, the MDS presented here will be an overestimation in the area of hard substrates removed.
  2. Overall, for all IEFs, the impact is predicted to be of local spatial extent (2.25% of the Array benthic subtidal ecology study area), long term duration, continuous, and of low reversibility. It is predicted that the impact will affect the receptors directly. The magnitude is therefore considered to be low.
                        Sensitivity of the receptor
  1. Removal of hard structures within the Array benthic subtidal ecology study area will represent a shift in seabed type and species assemblage. In terms of the MarESA, the sensitivity of the IEFs to this impact are as previously described for physical change (to another seabed type) in the assessment of ‘Long term habitat loss and disturbance’ (see Table 8.19   Open ▸ and paragraphs 94 to 97). All IEFs except for dead man’s fingers and sea tamarisk were assessed as highly sensitive to the introduction of hard structures as these species and biotopes are dependent on soft, sandy, and/or muddy sediments (see Table 8.19   Open ▸ and paragraphs 94 to 97). However, the removal of hard substrates in the decommissioning phase would allow sandy and soft bottom sediments to gradually return at the former footprints of the installed artificial hard infrastructure. Therefore, this impact would result in an increase in available habitat for these IEFs.
  2. Conversely, the dead man’s fingers IEF and sea tamarisk IEF were assessed as having low sensitivity to the introduction of hard structures as these species could potentially colonise the installed hard substrates (see Table 8.19   Open ▸ and paragraphs 94 to 97). Therefore, a gradual return to soft, sandy bottom substrates following the removal of all infrastructure may represent a small loss of habitat for these IEFs.
  3. Overall, all IEFs except dead man’s fingers and sea tamarisk are deemed to be of low vulnerability, high recoverability, and national and regional value. The sensitivities of the receptors are, therefore, considered to be low.
  4. In contrast however, as the dead man’s fingers IEF and the sea tamarisk IEF naturally live on hard substrates, including bedrock, rocks, boulders, shells, and man-made artificial hard structures (Budd, 2008, Wilson, 2002), this impact represents a change from a preferred habitat to a less suitable one for these IEFs. Therefore, the dead man’s fingers IEF and sea tamarisk IEF are deemed to be of high vulnerability, medium recoverability, and regional value. The sensitivities of the receptors are, therefore, considered to be high.
                        Significance of the effect
  1. Overall, for the dead man’s fingers IEF and sea tamarisk IEF, the magnitude of the impact is deemed to be low, and the sensitivities of the receptors are considered to be high. As per Table 8.16   Open ▸ , the effect will, therefore, be of minor to moderate significance. Given the low footprint of hard substrates to be removed during the decommissioning phase (2.25% of the Array benthic subtidal ecology study area) and the widespread availability of alternative suitable habitat, the effect is concluded to be of minor adverse significance, which is not significant in EIA terms.
  2. For all other IEFs, the magnitude of the impact is deemed to be low, and the sensitivities of the receptors are considered to be low. Based on Table 8.16   Open ▸ , the effect will, therefore, be of negligible to minor adverse significance. Based on expert judgement and adopting a precautionary approach, the effect has been concluded to be of minor adverse significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary benthic subtidal ecology mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.

Increased risk of introduction or spread of INNS

  1. Vessels used during the construction, operation and maintenance, and decommissioning phases of the Array could inadvertently transport INNS. INNS could also be transported during turbine ballasting in the construction and decommissioning phases. The relevant MarESA pressure and its benchmark which has been used to inform this impact assessment is:
  • introduction or spread of INNS: the benchmark for which is the introduction of one or more INNS.
  1. This impact is related to the impact of ‘Colonisation of hard substrates’, which may lead to an increased risk of potential habitat that could be colonised by INNS.

                        Construction phase

                        Magnitude of impact
  1. The MDS for this impact accounts for up to 7,902 vessel round trips over the course of the site preparation and construction phase, with up to 97 vessels on site at any one time ( Table 8.12   Open ▸ ). These provide vectors for the potential introduction of INNS into the habitats within the Array benthic subtidal ecology study area. In addition, the installation of artificial hard substrate on the seabed and in the water column throughout the construction phase could provide new habitat for INNS to colonise into the operation and maintenance phase. Finally, turbines may be towed to and from ports to the site boundary, which may represent another pathway for introduction of INNS. Up to 265 turbines will be installed, with up to three turbines towed to the site boundary at any one time ( Table 8.12   Open ▸ ). Ballasting may be required during towing, which would be required to comply with the IMO ballast water management guidelines, which will help reduce the risk of potential introduction and spread of INNS as far as practicable.
  2. There are many benthic INNS widespread and established in Scottish waters and the North Sea, including:
  • modest barnacle Austrominius modestus;
  • Japanese skeleton shrimp Caprella mutica;
  • leathery sea squirt Styela clava;
  • orange tipped sea squirt Corella eumyota;
  • orange ripple bryozoan Schizoporella japonica (NatureScot, 2023).
  1. However, there were no INNS recorded during the site-specific surveys for the Array (see volume 3, appendix 8.1, annex A).
  2. Many of the vessels engaged in site preparation and construction activities will utilise ports and harbours in east coast of the Scotland and the UK during the construction phase. Therefore, the potential for introduction of INNS from outside this region is reduced. Some of the established INNS in Scottish waters, however, are known to spread as fouling on ships, such as the modest barnacle, which could introduce them to the Array benthic subtidal ecology study area. Delivery vessels may come directly to site from fabrication yards located in international ports and harbours, however, all vessels will be required to comply with the INNSMP.
  3. As described in Table 8.17   Open ▸ , an INNSMP will be implemented (volume 4, appendix 21, annex B), which aims to manage and reduce the potential risk of introduction and spread of INNS as far as reasonably practicable. In addition, all vessels will be required to comply with the IMO ballast water management guidelines, which will help reduce the risk of potential introduction and spread of INNS as far as practicable.
  4. Overall, for all IEFs, the impact is predicted to be of local spatial extent (with hard structures installed in up to 2.25% of the Array benthic subtidal ecology study area), medium term duration over the site preparation and construction phase, intermittent (in terms of invasions), and of low reversibility. It is predicted that the impact will affect the receptors directly. The magnitude is therefore considered to be low.
                        Sensitivity of the receptor
  1. The sensitivity of the IEFs to increased risk of INNS is presented in Table 8.21   Open ▸ . These sensitivities are based on the MarESA (where available).
  2. The mobile sandy sediments of both the representative biotopes of the Offshore subtidal sands and gravels IEF and Subtidal sands and gravels IEF are typically at low risk of invasion by the INNS currently recorded in the UK, due to high levels of sediment disturbance (Tillin, 2016a, Tillin, 2016b). However, there are two INNS that may be of concern to these biotopes: the slipper limpet Crepidula fornicata and the carpet sea squirt Didemnum vexillum. There are patchy records of both of these INNS in Scottish waters (Begg et al., 2020, Beveridge et al., 2011, NatureScot, 2023). The slipper limpet may settle on stones, hard substrates (artificial or natural, such as bivalve shells), and can form dense carpets which smother the characteristic bivalves of these biotopes (e.g. A. prismatica) (Tillin, 2016a, Tillin, 2016b). Few other bivalves can live amongst stacks of slipper limpets (Blanchard, 1997). The carpet sea squirt typically colonises artificial hard surfaces but could have the potential to colonise and smother other seabed habitats (Tillin, 2016a, Tillin, 2016b). However, the slipper limpet and carpet sea squirt are typically more coastal species, with patchy records in the offshore environment (Gibson-Hall and Bilewitch, 2018, Rayment, 2008), so may not pose a significant threat to these IEFs. Overall, the representative biotopes of these IEFs would typically be too mobile for most INNS, however they are vulnerable to potential biotope reclassification if they were to be colonised by INNS (Tillin, 2016a, Tillin, 2016b). Overall, the Offshore subtidal sands and gravels and Subtidal sands and gravels IEFs are deemed to be of high vulnerability, low recoverability, and regional value. The sensitivities of the receptors are, therefore, considered to be high ( Table 8.21   Open ▸ ).
  3. For the remaining IEFs, there is limited evidence to their vulnerability and recoverability to INNS invasions, and the pressure of ‘Introduction and spread of INNS’ was not assessed in their MarESAs. However, as per the previous paragraph, ocean quahog and phosphorescent sea pen also typically inhabit soft sandy and/or muddy sediments, which are not typically at risk of invasion from most species. Therefore, following the same logic for the Offshore subtidal sands and gravels IEF and the Subtidal sands and gravels IEF, these species’ habitats may be vulnerable to colonisation from slipper limpet or carpet sea squirt. Furthermore, installation of hard structures and subsequent colonisation by INNS would represent a loss of available habitat for these IEFs. However, given the burrowing capabilities of the ocean quahog (Powilleit et al., 2009, Powilleit et al., 2006), it is not likely that individuals’ shells will be colonised by slipper limpets. Overall, the ocean quahog IEF and the phosphorescent sea pen IEF are deemed to be of high vulnerability, low recoverability, and national and regional value, respectively. The sensitivities of the receptors are, therefore, considered to be high ( Table 8.21   Open ▸ ).
  4. Both the dead man’s fingers IEF and sea tamarisk IEF inhabit hard substrates, which are more likely to be colonised by INNS. Therefore, these species are likely to be more vulnerable to a wider range of INNS than the other IEFs. If their substrates are colonised by INNS, the dead man’s fingers and sea tamarisk will be susceptible to a reduction in habitat and may be outcompeted. Overall, the dead man’s fingers IEF and sea tamarisk IEF are deemed to be of high vulnerability, low recoverability, and national value. The sensitivities of the receptors are, therefore, considered to be high ( Table 8.21   Open ▸ ).

 

Table 8.21:
Sensitivity of the IEFs to Increased Risk of INNS

Table 8.21: Sensitivity of the IEFs to Increased Risk of INNS