4. Summary

4.1. Regional Benthic Subtidal Ecology Study Area

103. The regional benthic subtidal ecology study area was mainly characterised through a desktop review of key literature sources (presented in Table 3.1   Open ▸ ). Broadscale substrate data indicates that the regional benthic subtidal ecology study area is dominated by deep circalittoral sand and is interspersed with deep circalittoral coarse sediment, which is characteristic of the North Sea (EMODnet, 2019). Other low energy habitats, such as deep circalittoral mud and circalittoral mixed sediments are recorded along the coast and within the Firth of Forth (EMODnet, 2019). Finer sediments, moderate energy circalittoral rock, circalittoral mixed sediments, and circalittoral sandy mud were recorded further inshore (EMODnet, 2019).
104. There were a diverse range of benthic species and communities identified within the regional benthic subtidal ecology study area by Pearce et al. (2014), Sotheran and Crawford-Avis (2014), Cooper and Barry (2017), and site-specific surveys undertaken for other offshore wind farms ( Table 3.3   Open ▸ ). However, it should be noted that these datasets were based on areas further inshore and with more heterogenous sediment composition than that of the Array benthic subtidal ecology study area. Species and communities that were also identified in the site-specific survey for the Array include polychaetes (particularly S. bombyx), soft coral A. digitatum, and various echinoderms and bryozoans (such as F. foliacea).
105. There are a range of designated sites within the regional benthic subtidal ecology study area (refer to section 2), however none overlap with the Array benthic subtidal ecology study area or are likely to be impacted by the Array. For example, the closest designated site with qualifying benthic ecological features is the Firth of Forth Banks Complex MPA, which is located a minimum of 25.06 km from the Array benthic subtidal ecology study area. Due to this large distance and the lack of mobile qualifying interest features, this MPA is unlikely to be affected by the Array.

4.2. Array Benthic Subtidal Ecology Study Area

106. Overall, the results from the site-specific geophysical and environmental surveys concluded that the Array benthic subtidal ecology study area was dominated by sand, classified as MC521 – Faunal communities of Atlantic circalittoral sand. Mixed sediments were present predominantly in the north-west and were classified as MC421 – Faunal communities of Atlantic circalittoral mixed sediment. This MC421 habitat classification decreased towards the south-east, only occurring occasionally and often associated with ripple features. The majority of sampling sites shared components of MC521 and MC421, albeit to a varying degree. Higher mud content, gravel, and diamicton were observed in the central and south-eastern sections, however the Array benthic subtidal ecology study area was largely homogenous. The widespread presence of megaripples and sand waves, however, indicated some sediment mobility, while occasional furrows, mainly in the west, were indicative of erosion. These results are in line with the EUSeaMap broadscale substrate data, which indicate that the Array benthic subtidal ecology study area is significantly dominated by deep circalittoral sand (EMODnet, 2019).
107. Regarding sediment contamination, levels of PCBs, organotins were below the limit of detection at all sampled sites. Similarly, metals were generally low, except for arsenic at sample site S002. This value marginally exceeded the NEA Good 2 threshold and the CCM ISQG threshold but was within the various other thresholds tested (such as the Cefas action levels) therefore is not considered of concern. THC concentrations varied across the Array benthic subtidal ecology study area and were generally higher in the southern and eastern areas. They did not exceed any of the Dutch RIVM intervention levels at any of the sampling sites, and were lower than values considered background levels for the North Sea. Similarly, PAH concentrations were low overall, with concentrations higher in the southern and eastern areas in the same trend as THC. There were no threshold values exceeded for individual PAHs but the sum of the EPA16 compounds exceeded the lower threshold value for NEA Good 2 at sampling site S051.
108. Biomass between grab sampling sites was varied, with six major phyla identified: Echinodermata, Mollusca, Annelida, Arthropoda, Cnidaria and Bryozoa. Echinoderms comprised the majority of the biomass within the grab samples (65%), which is due to S. purpureus and Echinocardium cordatum occurring at several grab sampling sites. The phyletic composition was dominated by annelids, mainly L. conchilega and S. bombyx. The phyletic composition of sessile colonial fauna was dominated by cnidarians and bryozoans, with cnidarians representing the highest number of taxa and bryozoans the highest number of colonies.
109. The most abundant non-colonial fauna identified in the DDV and photography survey were annelids, and cnidarians, with total abundances of 39% and 28%, respectively. Cnidarians covered the largest surface area within the imagery, with a total contribution of 48%, followed by bryozoans and bryozoans/cnidarians at 47% and 5%, respectively.
110. Within the trawl samples, arthropods dominated the phyletic composition of non-colonial fauna, and cnidarians represented the highest number of individuals and colonies of the sessile colonial fauna. The total faunal biomass was dominated by chordates, with the most abundant chordate being the long rough dab Hippoglossoides platessoides (discussed further in the volume 3, appendix 8.1).
111. Species richness, diversity, and evenness were relatively low between grab sampling sites, which could be explained by the limited variation in sediment composition. The number of taxa and the number of individuals ranged between 14 to 34 taxa and 28 to 143 individuals per 0.1 m2. There were two statistical groups produced in the SIMPROF analysis on untransformed macrofaunal data, with the majority of grab samples sites within group b. The similarity explored in the NMDS plot presented a stress value of 0.21. The SIMPROF analysis conducted with a square root transformation resulted in five statistically distinct groups, with the majority of sample sites in group e, and a stress value of 0.27. These results were indicative of homogeneity between sampling sites, with the gravel and mud proportions being the main driver for faunal community diversity. The results of the BEST test indicated that mud and gravel were the variables that best explained the spatial distribution of fauna (rho = 0.29, P = 0.01), and were statistically significant variables.

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