AIM
To survey a section of Plymouth Sound’s sea bed using side sonar scan and an underwater video camera to map the benthic environment and identify species.
Habitat Mapping
INTRODUCTION
Weather: NNW wind 7mph (7:00 UTC)
NNW wind 2mph (10:00 UTC)
Temperature: 26 degrees celcius
High water: 4.54m (13.41 UTC)
Low water: 1.88m (7.28 UTC )
Start Time: 08.52 UTC
Length of boat: 14m
Length of tow rope: 6m
Underwater habitat mapping can be used to look at vulnerable communities and track the changes of habitats and species over time. It can be used as support to set up conservation areas or in the development of underwater structures.
Sampling took place on the RV Xplorer. We left the station on a flood tide at 08.20. The Principal Scientific Officer (PSO), Mia Green chose a sampling area south of the Plymouth Breakwater along the edge of the Special Area of Conservation SAC. A dual beam sidescan sonar tow fish was dragged from the stern on a 6m length of rope and used to image 75m of sea floor on each side of the SAC. Four 2km transect lines were taken, travelling south from the breakwater, with 100m intervals between each line. On one of the lines, we ventured more than 2km in order to cross the SAC boundary. The images from the sonar were collected on a print out which were combined to create the habitat map.
In response to the sonar images, we used an underwater camera for ground truthing. Ground truthing examines the seafloor composition which is used to support the findings of the image output of the sonar backscatter. The camera was deployed in three separate locations; close to the breakwater, 1km south of the breakwater and across of the SAC boundary. Due to an equipment malfunction, the video did not record however, pictures were taken of the video shown on the television screen.
FINDINGS
Close to the breakwater showed a flat sandy plane, relatively bare of visible biota
aside from sparse patches of sea grass. The other two locations, 1km south of the
breakwater and across the SAC boundary, comprised of a littoral and sub-
Seagrass beds are present in the estuary. However, evidence suggests that they are impoverished or declining. Shoot density and macrofaunal abundance is lower than elsewhere in the UK.
PHYSICAL FEATURES SURVEY
Heybrook Bay and Renney Rocks
4/07/18. The survey area included rock outcrops and intervening unconsolidated sands and gravels. The Renney Rocks formation is a medium to thin bedded with alluvial (river‐deposited) sandstones and mudstones, from the Lower Devonian age that have been deformed by folding and faulting. This coastal outcrop has the same rock formations as above the high water mark. Our group made a small structural map of the folded rocks, by taking the strike and dip of the rocks over the feature, using clinometer.
Geology of Plymouth Area
The rocks found at Renney have origins in the Devonian through to the Carboniferous periods. Continental river plain and shallow marine sediments were intruded by granites and deformed in Hercynian orogeny that was caused by closure of Rheic ocean. Plymouth is in the South Devon Basin and so the geology is variable and includes Plymouth limestone, Devonian slates and Staddon Grits.
Renney Rocks are part of a bedding where continental and oceanic crust meet. The sediments here were deposited in a distal alluvial setting on the coastal plain of the Devonian ‘Old Red Sandstone’ continent. Sands and muds were laid down by meandering rivers; some freshwater fish remains were discovered and then the currents have been reconstructed from cross bedding.
We saw features of folding, faulting and the development of cleavage on the outcrop.
Multi-
