At the end of 2019, marine acoustic data analysts and marine ecologists from MarineSpace and ABPmer discovered some interesting seabed habitat features in bathymetric and drop-down video (DDV) survey data.
The data were acquired by ABPmer offshore of Anglesey and Pembrokeshire, Wales, during the 2019 summer for Welsh Government as part of the ‘Sustainable Management of Marine Natural Resources’ project. The project is funded by the European Maritime and Fisheries Fund (an EU and Welsh Government fund) with the aim to expand the environmental evidence base with which decisions will be made relating to tidal stream, wave energy and aquaculture development: to inform the Welsh National Marine Plan.
Interesting Seabed Features
Analyses of the Pembrokeshire multibeam (backscatter) bathymetric data revealed numerous bedrock reef outcroppings. However, a series of outcrop features were detected offshore, amongst areas previously thought to just be seabed sediments (sands and gravels).
Multibeam bathymetry off Pembrokeshire
Analyses of DDV ground-truthing data by MarineSpace and ABPmer ecologists revealed some very interesting features. Many of the outcroppings within the sediment were associated with distinct ‘aggregations’ of the biogenic reef-building polychaete, ross worm Sabellaria spinulosa. This is not an unusual discovery as the species is well recorded in Welsh waters. In fact, sites within Welsh waters are one of only a handful of locations within UK waters where both native species of Sabellariid worm are known to co-exist in the sublittoral: S. spinulosa and the honeycomb worm S. alveolata (the latter a predominantly eulittoral species).
Sabellaria spp. reef offshore from Anglesey
So what makes the offshore outcrop features interesting? The features consist of discrete mosaics of S. spinulosa and the bivalve mollusc, blue mussel Mytilus edulis. Both species are known to form biogenic reef structures, but usually these are species-specific and associated with shallow waters. The Pembrokeshire features are located at water depths around 45 m below Chart Datum, extensive in tide-parallel lines, and are formed of both species together.
Sabellaria spp. and Mytilus edulis reef offshore from Pembrokeshire
The potential reefs appear to be formed from initial settlement of M. edulis spat that has subsequently been ‘colonised’ by S. spinulosa. Both live M. edulis and dead shells form the base of the features which then have an associated ‘matrix’ of S. spinulosa tubes. The latter appear to consolidate the structures. Growth rings apparent on the M. edulis show that the seabed features have been present for at least 2-3 years.#
Sabellaria spp. and Mytilus edulis mosaic reef offshore from Pembrokeshire
The occurrence of the features in deeper waters is likely a reflection of the high energy tidal streams and associated nearbed sediment transport environment. A supply of sediment particles is critical for the evolution of Sabellariid reefs: the worms using the particles to build their tubes. It is also likely that front systems associated with the survey areas cause mixing of water column and transportation of food from the surface waters: important for both S. spinulosa and M. edulis.
Significance of the Discovery
Biogenic reefs are important as they are known to stabilise sediment systems, promulgate settlement of epizooitic species, alter local biodiversity, and underpin increased food-web energetics, supporting higher trophic levels/systems. For these reasons they represent features of potential nature conservation significance.
Also important is their sensitivity to certain pressures from anthropogenic activity. An understanding of their presence, extent, evolution and functioning within the local environment will benefit an informed and evidence-based Welsh National Marine Plan. This shows the benefit of high resolution surveys to inform sustainable use of our seas.