Abstract
Sabellaria spinulosa is a reef-building polychaete that lives in a tube, usually attached to a hard substrate, or other Sabellaria tubes. When worms gather in large numbers, their tubes may stand upright and create crusts or mounds that span several meters. Unaggregated individuals can reach densities of up to 300 ind. m-2, while in aggregations, densities of 4000 ind. m-2 have been observed (George and Warwick 1985). However, reefs can only be formed in specific geomorphological areas, such as sandy or gravelly seafloors, the edges of sandbanks, channels, and drop-offs, along with conditions, such as high turbidity, elevated sediment levels, moderate currents and a moderate amount of suspended organic particles. This species prefers areas with high bottom shear stress, mobile sand and high gravel content of the sediment (Jackson & Hiscock, 2008; Jackson-Blue et al., 2022; Gaida et al., 2023). In areas where S. spinulosa reefs are present, they can offer a diverse habitat for various invertebrates and juvenile fish. These reefs help stabilise sediments and promote nutrient cycling by offering access to essential resources like food and shelter (De Smet et al. 2013, Rabaut et al. 2013). Offshore anthropogenic structures, such as offshore wind farms (OWFs), may provide such habitats for this species due to the provisioning of artificial hard substrate such as scour protection layers, turbine foundations and cable crossings in combination with local scouring which can lead to high bottom shear stress and availability of mobile sand. Furthermore, OWFs in the Dutch North Sea are closed to bottom disturbing fisheries, which could facilitate the development of S. spinulosa reefs, as reefs experience pressures of abrasion (Baptist et al. 2024).
The reefs that S. spinulosa forms are listed as habitats under threat and/or declining in the North Sea by OSPAR (OSPAR Commission, 2013). This species has been observed multiple times in Dutch waters, both in relation to OWFs (Princess Amalia, Borssele) and oil and gas platforms, as well as natural reefs, such as the Borkum Reef Grounds (Vanagt & Faasse, 2014; Coolen et al. 2020; Bos et al., 2024; Kingma et al., 2024 ) and on ship wrecks (personal observations Joop Coolen). Within the Dutch North Sea, the species has also been observed forming reefs in sandbank troughs in the Brown Bank area (van der Reijden et al. 2019). Furthermore, it has been selected as a species of interest for nature restoration or nature inclusive design target species in OWF development (Sas et al., 2023).
For the suitability of OWF development for Sabellaria spinulosa reefs, further research and knowledge is needed. The Netherlands Enterprise Agency (“Rijksdienst Voor Ondernemend Nederland” or RVO) has commissioned Wageningen Marine Research (WMR), Deltares and TNO to assess the suitability of the Nederwiek II and III wind farm development site for the establishment of Sabellaria spinulosa. This report is part of a series of 3 reports and will focus on the results of WMR on:
(a) Reviewing published reports/articles to identify records of Sabellaria spinulosa within the Dutch North Sea and within offshore windfarms on a wider North Sea scale, and;
(b) Coordinating a data call and search to collect data that we (WMR and Deltares) are not aware of containing observations of Sabellaria spinulosa, leveraging international benthic networks within ICES, such as the ICES Working Group on Marine Benthos and Renewable Energy Developments (WGMBRED), which would provide more data points for both the WMR data analysis and the models created by Deltares.