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Existing multibeam echosounder bathymetry data were used to assist in the planning of surveys and interpretation of seabed habitats. The data were collected between 26th August 2011 and 6 th April 2012 as part of the MCAÔÇÖs CHP. The bathymetric data were collected and processed in accordance with the International Hydrographic Organisation (IHO) Standards for Hydrographic Surveys - Order 1a (Special Publication 44, Edition 4). The bathymetry data are also archived by the United Kingdom Hydrographic Office (UKHO). The data were provided to Cefas as fully processed and cleaned bathymetry and backscatter data from the MCA and the Channel Coastal Observatory, the data management delivery team of the Strategic Regional Coastal Monitoring Programme. Both bathymetry and backscatter were gridded at 1 m resolution for analysis (see Appendix 2 for images derived from acoustic data). Further details on the acquisition and processing of multibeam echosounder bathymetry data can be found in the Report of Survey for HI1366 (UKHO, 2012).
Ground truth samples were collected during the Cefas-commissioned groundtruthing survey undertaken by the EAÔÇÖs MMS. The spatial location and descriptions of these groundtruthing data and information were used in conjunction with the bathymetry and backscatter datasets. Such information aided interpretation of areas of rock, sediment and areas where the surficial topographic expression of the underlying bedrock appeared to be masked, by a variable thickness of sediment. From the visual descriptions of surficial substrate, the broadscale EUNIS Level 3 habitat types were classified.
The sediment samples collected by EAÔÇÖs MMS were analysed (using the Folk classification), which enabled an iterative interpretation of habitat types to be conducted. Interpreted multibeam bathymetry and backscatter data (acquired and interpreted under the Civil Hydrography Programme) were used, in conjunction with UK Hydrographic Office (UKHO) Admiralty charts, to select appropriate locations and densities of sediment groundtruthing stations. Benthic grabs were used at all stations to collect sediments and infauna, locations of planned grab sample stations are shown in Figure 2. An underwater camera system was deployed at 26 stations to collect video and still images of the seabed, locations shown in Figure 3.
Groundtruthing of the sea floor was carried out using a Hamon Grab deployed from the stern of the survey vessel. The stern gantry was used as the default offset for position fixing. Once recovered, the grab sample was decanted into a suitable container and photographed. If the sample volume was >5 litres a representative Poole Rocks MCZ Post-survey Site Report 8 sub-sample of sediment, approx. 500 ml, was taken for Particle Size Analysis (PSA) and the remaining volume measured and recorded. The sample was washed over a 1 mm sieve, photographed and the retained material transferred into a sample container with buffered formaldehyde solution (8%). If the sample volume was <5 litres, the material was photographed and discarded.
The bathymetric dataset collected by the MCAÔÇÖs CHP, which covers the extent of the MCZ area, was gridded at a 1 m resolution. All new habitat maps and their derivatives have been projected to WGS84 and Ordnance Datum. An indicative BSH map for the site was produced by analysing and interpreting the available acoustic data collected through the MCAÔÇÖs CHP and the additional groundtruthing data collected through the Cefas-commissioned survey of this site. Analysis of the revised photographic stills and video footage enabled the infralittoral and circalittoral boundary to be estimated as 10 mOD for this site.
The software package IVS Fledermaus v7.3 was used to load the bathymetry data and export the ASCII-format files as a single layer for subsequent use in ArcGIS v10. The Spatial Analyst toolbox within ArcGIS enables slope, aspect and hillshade layers to be derived, as detailed in Table 3. Areas with different acoustic signatures and characteristics were identified and manually digitised, and information from the PSA and qualitative groundtruthing data was used to assign substrate descriptions and sediment types. A BSH map of the MCZ has been created through expert visual interpretation of the processed bathymetry, backscatter and groundtruthing data and their derivatives.
The technical quality of the updated habitat map was assessed using the MESH ÔÇÿConfidence AssessmentÔÇÖ Tool2 , originally developed by an international consortium of marine scientists working on the MESH (Mapping European Seabed Habitats) project. This tool considers the provenance of the data used to make a biotope/habitat map, including the techniques and technology used to characterise the physical and biological environment and the expertise of the people who made the map. In its original implementation, it was used to make an auditable judgement of the confidence that could be placed in a range of existing, local biotope maps that had been developed using different techniques and data inputs, but were to be used in compiling a full coverage map for north-west Europe. Where two of the original maps overlapped, that with the highest MESH confidence score would take precedence in the compiled map. |
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