No. 14 CEND 19x/12: Offshore seabed survey of Braemar Pockmarks SCI and Scanner Pockmark SCI
(014 - 2017)
Rance, J., Barrio Froján, C. & Schinaia, S.
The report describes the presence, location and extent of the pockmark features, along with detail relating to the presence and location of any associated seafloor carbonate structures where they are observed to occur. A physical and biological characterisation of the wider area, surrounding the pockmark features, is also provided.
 

Abstract

 

This report presents the findings from the analyses of the data gathered during the seabed survey of the Braemar Pockmarks and Scanner Pockmark Sites of Community Importance (SCI), as defined in the European Commission Habitats Directive 92/43/EEC.  The pockmark features present within both of the SCIs are a series of crater-like depressions on the seafloor, and can include the Directive’s Annex I habitat ‘Submarine structures made by leaking gases’.  These structures consist of large blocks, pavements, slabs and smaller fragments of carbonate rock, including methane-derived authigenic carbonate (MDAC).

Full-coverage acoustic multibeam bathymetry and backscatter data were acquired at the two SCIs and data quality was found to be good.  Additionally, sidescan sonar data were collected, however the quality of these data was affected by the survey conditions during acquisition, and was considered to be of low quality.  Areas of distinct acoustic signatures from multibeam data were delineated using the semi-automated eCognition software.  Standard univariate and multivariate analyses on the taxa abundance data have been performed to identify distinct faunal assemblages.

Combined analysis of the acoustic and groundtruthing data enabled the identification of two distinct sediment types within both SCIs, namely mud and mixed sediments.  Further interpretation led to the assignment of the biotopes A5.36 Circalittoral Fine Mud and A5.44 Circalittoral Mixed Sediment to these sediment types.

Presence of the Annex I habitat ‘Submarine structures made by leaking gases’ within the Braemar Pockmark SCI was suggested through observations of seabed structures on the video and still image data, and confirmed following laboratory analysis of carbonate samples (MDAC) collected in a number of grab samples.  Sidescan sonar records also provided evidence of gas bubbles in the water column, which appeared to be venting from one of the pockmarks.  No Annex I habitat ‘Submarine structures made by leaking gases’ (MDAC) was observed in either the underwater video footage, still images or grab samples at the Scanner Pockmark SCI, and no evidence of active venting was observed on the sidescan sonar data.

 
 
ISSN 2051-6711
 
Please cite as: Rance, J., Barrio Froján, C. & Schinaia, S., (014 - 2017), No. 14 CEND 19x/12: Offshore seabed survey of Braemar Pockmarks SCI and Scanner Pockmark SCI, ISSN 2051-6711