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Geophysics
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Methods
Date: 07/07/2018
Location: Penlee Point Headland
Conditions: 0/8 octants; clear; calm water; sunny; LW: 05:25, 1.9m; HW: 11:44, 4.4m LW: 17:56, 2.0m
Vessel: MV Xplorer
Penlee Point headland, Plymouth Sound, was the surveyed area because Plymouth Sound is a Special Area of Conservation (SAC).
A side scan sonar was carried out on the MV Xplorer at 100KHz and a track plot was
produced and printed out. In the lab, the track plot was cut at the start and end
of each transect by looking at the corresponding times recorded in the log book.
The individual transect track plots were then rearranged into a mosaic (not true
mosaic -
Penlee Point headland is the south eastern corner of Plymouth Sound, located in the Looe Basin. At Penlee, the rock is composed of a dolerite sill with the headland being situated in the Lower Devonian and the remaining formation intruding into the Mylor Slate. The dolerite is comprised of mainly fine grains of amphibole, plagioclase and chlorite.
Our aim was to create a benthic habitat map of Penlee Point Headland in Plymouth Sound, using a combination of four side scan sonar transects and footage from three video deployments. The transects were approximately 2 km long and 0.1 km apart from each other.
Side Scan Sonar
Large areas of high return were observed along transect 1. This substrate was identified as rock due to a higher level of reflection, producing a darker image on the ship track. The rocky substrate shared a similar composition to that of the headland at Penlee Point, where the transects were carried out. This suggests that the rock could be an extension of the headland.
Along transects 2 and 3, areas of lower reflection (weaker return) were identified as areas of finer sediment. A clear rocky outcrop was observed (Figure x) by areas of high return to the scanner, which can be seen covered in depressions and elevations in the footage.
Along transect 2 there is a clear boundary where the substrate changes from a rocky substrate to finer sediment demonstrated by the switch to lesser return. This boundary continues along transect 2 through transect 3 and continues off the transect.
A pattern in the side scan was identified as a boat and the consequent bow waves, as the shape occurs through the water column and on the sediment.
Camera Deployment
From the video transects carried out, one species was identified clearly. This species was Marthasterias glacialis. This species was identified on the substrates rock and sand, and sometimes in large groupings as shown on a video. This could be due to the behaviour of the species as in summer they tend to congregate for spawning to take place and tend to be found from low intertidal to nearly 200m depth. The data showed that this species was only in transect one, were the majority of the substrate was sand, suggesting that this could be its preferred habitat. Phaeophyta and red algae was also observed in both habitats Phaeophyta are the most plentiful of all seaweeds so this is as expected, they tend to be found on shores with rocky substrates rather than sediment as they require a hard substrata to anchor onto which is why the seaweed was found in transects 2 and 3 with rocks and shell/pebble/gravel areas. Only red and brown seaweeds were identified and no green seaweeds, an explanation for this could be the type of light available in the water, as red and brown seaweeds can utilise the green and blue light whereas green seaweeds can only make use of red light, which is attenuated faster, therefore suggesting that there is less red light.
(Above) John’s state of the art camera deployment equipment
(Above) Screen-
(Above) Ships track and desired transects
Summary
While carrying out the side scan sonar, there were numerous occasions where the boat
encountered wake from a passing vessel. As a result, there was interference on the
track. On one occasion, the vessel veered from the planned transects due to a sailing
boat in the way and consequently the ship track was not always as planned. During
the transects the side scan sonar recording was liable to artefacts and distortions,
showing potential fish and lobster plots in addition to the boats own wake. Due to
height fluctuations, the visual representation of surfaces changed throughout which
made discerning between sand, rock and mixed. more difficult. The video survey method
had a change in resolution and quality as the camera was lifted and lowered throughout
the transect in order to dodge rocky substrates on the seafloor. The recording lacked
a scale reference making it hard to grasp an accurate sense of perspective. Another
problem with this survey method is that turbidity in the water column severely affected
the image quality. When analysing the video footage, a screenshot was taken every
minute in order to minimise bias. However, in-
The results are not a true representation of the seabed communities because only a small section of Plymouth Sound was surveyed which is not large enough to be represented. The one minute screenshots were potluck as to whether the screenshot was representative of the area and many species were missed and large aggregations may have been ignored. The abundance of features and species was calculated as an average so it took into account the extremities which could skew the data.
(Above) Habitat map produced by the side scan sonar data (Mouse over to expand)