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GEOPHYSICS

TAMAR SAMPLING

OFFSHORE

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The views and opinions expressed are of those of the members of Group 8 and are not representative of University of Southampton or National Oceanography Centre.

Lab Protocol

Home Geophysics Tamar Sampling Offshore

INTRODUCTION

Plymouth Sound is an ocean inlet located in SW Britain. It is supplied by freshwater from the River Tamar. The Sound and Estuaries of Plymouth are under investigation by the Environmental Agency and English Nature with areas of marine conservation. The survey was carried out within the breakwater zone which is identified by the English Nature report on Plymouth Sound and Estuaries (File: ENRR208), published in 1997. Outer sound has a steeper slope than the coastal regions and according to Moore et al. (1999), the breakwater zone is affected by significant turbidity increase and salinity reduction, due to out flow of the River Tamar. Some rare sponge species are observed south of the breakwater such as Axinella damicormis (Langston et al., 2003). Another study carried out by Parry et al. (1999) has contrasted sediment granulometry with macrofaunal assemblages and found out that coarse sediments hosted greater number of species with higher diversity and that body size is non-conservative despite the difference in grain size and heterogeneity. According to the broad scale mapping of the Plymouth Sound, made in 1997 January, the area of the transect was predominantly made of mixed substrate, including sand with slates, cobbles or boulders. Gravels and mega ripples are also included.

Geophysics

SAMPLING METHODS

Side scan sonar:

The start location of the survey was deciphered using the EDCUS navigation system on the vessel. The projections were converted to Northing and Easting system using a coordinate conversion programme on board. There other track lines are generated parallel to the south of the former line with same length and distance of 100 meters using HYDROPro. …… model side scan sonar tow fish is towed behind the vessel R/V Echo Sounder along the pre-determined transects at the speed of approximately 5 knots. Side scan trace is printed out simultaneously on board. The port side of the trace had so much noise/intereference from the electrical system of the boat, hence was not useful. The side scan sonar ran at 100 kHz.

Towed video system:

Similar to side scan sonar, the system is towed behind a vessel approximately a meter above the seabed. The live video of the sea bed is monitored and recorded on board. Towed between the locations 50 ° 19.801N, 004 ° 08.976 and 50 ° 19.849 N 004° 09.334 W.

Creating the map:

To create the map, the side-scan trace (with turns removed) printed on board the Echo Explorer Plymouth were fitted together. Anywhere a change between the colour or texture was recorded, a boundary was drawn to represent this. Points around the boundary were taken, then the location of these along the time stamps and the distance from the tow fish were measured so the structures could be plotted onto the track plot created on surfer 8. Once this was mapped, the grab sites and the video track were also added. This was shaded in different colours as shown by the key; representing the different habitats and substrates seen on the sea bed. These were judged using a mixture of the grab sample sediments, the video track images and the different structures seen on the side-scan trace.

AIM

To collect data to represent the physical set of the sediment and couple it with biological data to form a habitat map using side scan sonar for the sediment structure and towed video system for biological assessment.

RESULT

A brief summary of the research findings and conclusions are given here. For more information, see the full poster.

• Rhodophyta and Chlorophyta were dominant across all three transects especially in regions of exposed rock. However, they also grew within the troughs of ripples present in transect 3 - henceforth suggesting that these bedforms are slow moving.

• 4 different habitat types were observed (1 – kelp covered rock, 2 – gravel waves, 3 – mudflat and 4 – sand covered rock). Screen grabs were taken of each of these, and the percentage cover of each species determined. Site 2 and 4 were most similar in species composition, whereas site 1 and 4 were the most different.

• Of the sites observed, sites 2 and 4 were the most similar. This is most likely due to the absence of L. digitata and Cladophoraceae

Geology

The structure of the seabed is heavily influenced by the geology of the area. Folding of rocks is responsible for some of the outcrops and troughs seen in the side scan trace. The shape of these bedforms influences flow around the boundary layer, affecting how sediment and water is moved and how much energy areas have. Features such as the Rhodophyta growing in troughs of the gravel waves can be explained by this as these are more sheltered static environments where the sediment is rarely disrupted, meaning the algae has time to grow in these troughs. The type of rock present over different areas means erosion occurs at varying rates, creating a number of different sediment types which correspond to different habitats.

(Click the picture to see the full poster)