West Hebrides (2004)
The aim of this survey was to investigate and characterise the
habitats and biological communities of five areas of potential
Annex I reef
off the western Outer Hebrides, west and south-west of the island
of Barra. This survey was run in collaboration with Scottish
Natural Heritage, the Agri-Food and Biosciences Institute (AFBI)
and Queen's University Belfast.
Multibeam echosounders were used to generate complete-coverage
bathymetric maps and stratify the video surveys for bedrock
habitats. Bathymetric data collected on this survey revealed
complex topographies at each survey site, with depth ranges of up
to 115m. The acoustic backscatter data collected by the multibeam
echosounders was analysed to produce backscatter mosaics,
which indicate differing acoustic ground-types over each survey
area. In addition and where possible, single-beam acoustic ground
discrimination system data was gathered to complement the multibeam
echosounder data, which enhanced ground-type discrimination.
Video tows were completed at each site using either a towed
sledge or drop-frame camera system. Where coverage was adequate,
habitat maps were produced surrounding the ground-truthed area
within each survey site. Video images were semi-quantitatively
analysed to give SACFOR epifaunal species abundances and detailed
sediment descriptions made to permit biotope or biotope complex
classification. This data was spatially linked to the acoustic
datasets, guiding the interpretation of areas surrounding the video
tows into habitats. Grab samples were used to describe the
sedimentary basins surrounding the bedrock outcrops.
The bedrock reefs at each survey site showed habitats typical of
deep, high energy, exposed environments, with encrusting and erect
sponges dominating the epifauna. Encrusting and erect bryozoans,
cup corals and keel worms were also common members of the reef
epifauna. A total of seven habitats were identified from the reef
sites:
CR.HCR.DpSp,
CR.HCR.DpSp.PhaAxi,
CR.HCR.XFa,
CR.HCR.XFa.ByErSp,
CR.MCR.EcCr.CarSp.Bri, CR.MCR.EcCr.CarSp.PenPor and
CR.MCR.EcCr.CarSwi. The bedrock was generally highly fissured.
In all but one site, deep gullies separated the bedrock topographic
highs, which were often infilled by coarse sands, gravels, cobbles
and boulders. The sloping edges of the reefs typicaly showed the
highest species richness, particularly further from the surrounding
sediment where sand scour may have constrained species richness.
The surrounding sedimentary areas were composed of fine to coarse
sands, often with a gravel component, which were frequently rippled
or megarippled. Boulder fields and extensive areas of coarse sands
with cobbles fringed the base of each reef, characterised by the
habitat
SS.SCS.CCS.PomB and in the south-west Barra site by the habitat
CR.MCR.EcCr.CarSwi, with the seafan Swiftia pallida
common.
Further reading
- Mitchell, A. 2008. Broadscale subtidal biotope mapping to the
west of the Outer Habrides, Scotland, UK. JNCC Report
424.
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work then please contact us.